perineural invasion in cancer

Perineural Invasion in Cancer

A Review of the Literature

Catherine Liebig, MD1; Gustavo Ayala, MD2; Jonathan A. Wilks, MD1; David H. Berger, MD1;

and Daniel Albo, MD, PhD1

Perineural invasion (PNI) is the process of neoplastic invasion of nerves and is an under-recognized route

of metastatic spread. It is emerging as an important pathologic feature of many malignancies, including

those of the pancreas, colon and rectum, prostate, head and neck, biliary tract, and stomach. For many of

these malignancies, PNI is a marker of poor outcome and a harbinger of decreased survival. PNI is a dis-

tinct pathologic entity that can be observed in the absence of lymphatic or vascular invasion. It can be a

source of distant tumor spread well beyond the extent of any local invasion; and, for some tumors, PNI

may be the sole route of metastatic spread. Despite increasing recognition of this metastatic process, there

has been little progress in the understanding of molecular mechanisms behind PNI and, to date, no tar-

geted treatment modalities aimed at this pathologic entity. The objectives of this review were to lay out a

clear definition of PNI to highlight its significance in those malignancies in which it has been studied best.

The authors also summarized current theories on the molecular mediators and pathogenesis of PNI and

introduced current research models that are leading to advancements in the understanding of this meta-

static process. Cancer 2009;115:3379–91. VC 2009 American Cancer Society.

KEY WORDS: perineural invasion, perineural spread, neurotropic carcinoma, cancer, neurotrophins, axon

guidance molecule.

A key feature of malignant cells is their ability to dissociate from the primary tumor and to establish meta-static deposits at distant sites. Vascular and lymphatic channels are well accepted routes of metastaticspread. They are well characterized in the literature and are the focus of much current research on tumorbiology. However, another route of tumor spread that occurs in and along nerves has been described in theliterature since the mid-1800s but has received relatively little research attention. Perineural invasion(PNI) is the process of neoplastic invasion of nerves. It also has been called neurotropic carcinomatousspread and perineural spread. PNI was reported first in the European literature by scientists who describedhead and neck cancers that exhibited a predilection for growth along nerves as they made their way towardthe intracranial fossa.1,2 PNI has emerged since then as a key pathologic feature of many other malignan-cies, including those of the pancreas, colon and rectum, prostate, biliary tract, and stomach. For many ofthese malignancies, PNI is a marker of poor outcome and a harbinger of decreased survival.3-7

Received: September 9, 2008; Revised: December 5, 2008; Accepted: January 7, 2009

Published online: May 29, 2009 VC 2009 American Cancer Society

DOI: 10.1002/cncr.24396, www.interscience.wiley.com

Corresponding author: Daniel Albo, MD, PhD, Michael E. DeBakey VA Medical Center, 2002 Holcombe Boulevard, OCL 112-A, Houston, TX 77030;

Fax: (713) 794-7352; [email protected]

1Department of Surgery, Michael E. DeBakey Veterans Affairs Medical Center, Baylor College of Medicine, Houston, Texas; 2Department of Pathology,

Baylor College of Medicine, Houston, Texas

Cancer August 1, 2009 3379

Review Article

PNI is a distinct pathologic entity that can be

observed in the absence of lymphatic or vascular invasion.

It can be a source of distant tumor spread well beyond the

extent of any local invasion; and, for some tumors, PNI

may be the sole route of metastatic spread. PNI is neither

an extension of lymphatic metastasis nor simply tumor

cell migration through a low-resistance plane. Definitive

studies have proven that there are no lymphatics within

the inner sanctum of the nerve sheath, and several layers

of collagen and basement membrane separate the inside of

the nerve from the surrounding lesion; this is not a low-re-

sistance path.8-11

Despite increasing recognition of this metastatic

process, there has been little progress in the understanding

of molecular mechanisms behind PNI and, to date, no

targeted treatment modalities aimed at this pathologic en-

tity. In fact, the true prevalence of PNI in various malig-

nancies still has not been established. Lack of a concise,

universal definition for PNI across all disciplines has

resulted in significant confusion and probably is 1 reason

for the seemingly slow progress. It is the objective of this

review to lay out a clear definition of PNI and to highlight

its significance in those malignancies in which it has been

characterized best. We also aim to summarize current the-

ories on the molecular mediators and pathogenesis of PNI

and to introduce current research models that are leading

to advancements in our understanding of this metastatic

process. It is our hypothesis that PNI is as significant in

some tumors as lymphovascular invasion. A better under-

standing of PNI may lend insight into tumor metastasis

and recurrence and open doors to improved staging strat-

egies, novel treatment modalities, and perhaps even para-

digm shifts in our treatment of patients with patients.

Definition

Knowledge of the basic structure of the peripheral nerve

sheath is integral to understanding PNI (Fig. 1). The

nerve sheath is composed of 3 connective tissue layers.

From outside to in, these layers are the epineurium, the

perineurium (not to be confused with the perineurum, or

area just outside of the nerve sheath), and the endoneu-

rium.12 The epineurium, which binds 1 or more fascicles

into a single nerve, is composed of 2 distinct layers: an

outer layer of areolar connective tissue and loosely

arranged collagen bundles and an inner layer of densely

organized collagen fibrils and elastin fibers.13 It is within

the areolar connective tissue on the outer portion of the

epineurium that the epineurial component of the vasa

nervorum, the rich vascular network of the peripheral

nerve, and the perineural lymphatic channels reside.

According to the most recent literature, these lymphatics

do not penetrate the epineurium, although this issue his-

torically has been the focus of considerable debate in the

PNI literature.8,14,15

Akert et al., using electron microscopy to delineate

the structural organization of the perineurial sheath of the

peripheral nerve, describe a multilamellated structure of

concentrically arranged endothelial cells.11 Each endothe-

lial cell layer of the perineurium is flanked on either side

by basement membrane.9,11 Junctions between the endo-

thelial cells are formed by dove-tailing, that is, overlap-

ping and tightly fitting projections of adjacent cell

membranes.11 These junctions, or zonulae occludentes,

work together with the multiple basal laminae to confer a

highly selective barrier function to the perineurium. This

effectively separates the intrafascicular compartment of

the nerve from the surrounding epineurium.

The endoneurium, or innermost layer of the nerve

sheath, forms a matrix around individual nerve fibers and

envelops the Schwann cells and individual axons of the

nerve.14 The endothelial lining of endoneurial blood ves-

sels is made up of tight junctions without evidence of

transendothelial channels.14 The relative impermeability

of the endoneurial blood vessels is an extension of the bar-

rier function of the perineurium surrounding this com-

partment and is a key feature of the blood-nerve barrier.

Descriptions of PNI have included tumor cells

within every layer of the peripheral nerve sheath, from

abutment of tumor cells within the perineurum to well

formed cancerous glands within the perineurium or endo-

neurium, to small clusters of tumor cells within a nerve

that is surrounded by normal tissue well away from tu-

mor. Which of these tumor nerve configurations are true

examples of PNI and which features are necessary for

defining PNI? In his 1985 article on neurotropic carcino-

mas, Batsakis offered a broad definition of PNI, character-

izing it as tumor cell invasion in, around, and through the

nerves.8 The article is cited widely in the literature on PNI,

and the definition has become the generally accepted 1 for

PNI. It is sufficiently broad to cover most of the histopa-

thologic varieties of this entity previously described in the

Review Article

3380 Cancer August 1, 2009

literature and has helped to focus later research efforts by

eliminating any confusion about perineural lymphatics.

However, in, around, and through has left tremendous

room for further clarification. Some authors have sug-

gested that tumor cells must be observed inside the peri-

neurial layer, specifically, to cite PNI.16 This definition

seems overly stringent and would exclude several examples

of clear nerve invasion, such as gland formation within the

collagen layers of the epineurium. We advocate that the

finding of tumor cells within any of the 3 layers of the

nerve sheath represents PNI. A much more frequent find-

ing in PNI is tumor-nerve contact within the perineurum

without the finding of tumor cells inside of the sheath,

and there is wide variability among authors regarding the

degree to which this tumor-nerve contact is necessary to

call it PNI. Various growth patterns have been described,

including complete and incomplete encirclement, con-

centric lamination, and tangential contact.17 For instances

in which this occurs outside of the main body of the tu-

mor, the finding is recognized more easily as malignant

carcinomatous invasion of a neural structure. However, in

instances in which this occurs within the main body of the

tumor, it is less clear cut. For these circumstances, many

authors have proposed that at least 33% of the circumfer-

ence of the nerve should be surrounded by tumor cells to

call it PNI; anything less than 33% represents focal abut-

ment and not invasion.17-19 In summary, we advocate a

somewhat broad definition of PNI, in keeping with the

original definition by Batsakis of in, around, and through

the nerves, while incorporating many features that were

cited previously in the literature: tumor in close proximity

to nerve and involving at least 33% of its circumference or

FIGURE 1. Perineural invasion: Illustrated is a tumor enveloping a peripheral nerve in cross section revealing tumor cell spread.

Molecules expressed by tumors interact with surrounding stroma as well as receptors associated with peripheral nerves.

Perineural Invasion in Cancer/Liebig et al


tumor cells within any of the 3 layers of the nerve sheath

(Fig. 2).

Pathogenesis

It has become evident that PNI is not an extension of lym-

phatic metastasis, as once was suggested. Definitive stud-

ies have demonstrated that lymphatic channels do not

penetrate the inner sanctum of the nerve sheath.8-10,20 For

the last 40 years, the predominant theory behind the

pathogenesis of PNI has been that tumor cells spreading

along neural sheaths are privileged to a low-resistance

plane, which serves as a conduit for their migration. Once

inside of the nerve sheath, tumor cells may be in a privi-

leged growth environment that facilitates metastasis, but

the multiple layers of collagen and basement membrane

that compose the nerve sheath make access to this path

anything but low-resistance. The reason certain carcino-

mas exhibit a predilection for PNI and others do not

remains unknown.

More recently, studies have demonstrated that PNI

may involve reciprocal signaling interactions between

tumor cells and nerves and that these invading tumor cells

may have acquired the ability to respond to proinvasive

signals within the peripheral nerve milieu. In a PNI model

using mouse dorsal root ganglia (DRG) cocultured in a

Matrigel matrix with prostate cancer cells, Ayala et al.

demonstrated tumor cell migration along neurites toward

FIGURE 2. These photomicrographs depict perineural invasion (PNI) in human colorectal cancer specimens. Sections of human

colorectal cancers were stained with hematoxylin and eosin and were reviewed by a pathologist for PNI. Tumor cells located

within the peripheral nerve sheath either (a) in clusters or (b) forming glandular elements are clear examples of PNI. When tumor

cells are not located inside of the nerve sheath but are in close proximity to the nerve in the perineural environment, at least 33%

of the circumference of the nerve must be involved by tumor to diagnose PNI. This is true weather the nerve is located (c) within

the main body of the tumor or (d) at a site outside of the primary tumor focus.

Review Article


the ganglia of origin as well as focused, directional out-

growth of neurites toward cancer cell colonies.21 The

addition of stromal cells to the aforementioned in vitro

model resulted in increased neurite outgrowth and cell

colony formation.22 This suggests that the signaling

mechanisms behind PNI likely involve at least 3 different

cellular elements, including tumor cells, nerve cells, and

stromal cells, and may include autocrine and paracrine

mechanisms.

The increased neurite formation demonstrated in

the previous in vitro studies suggests that axonal migration

may be a key element of PNI. Axonal growth is a complex

process that requires neurotrophic growth factors and axo-

nal guidance molecules.23,24 The neurotrophins are the

best characterized family of neurotrophic factors and

comprise nerve growth factor (NGF), brain-derived neu-

rotrophic factor (BDNF), neurotrophin 3 (NT-3) and

neurotrophin 4/5 (NT-4/5).25 It is their potent effects on

neuronal growth that have made the neurotrophins prime

candidates for study in the PNI invasion pathway. There

is a growing body of literature implicating these molecules

as well as other neurotrophic factors in cancer (Table 1).

Recent evidence in prostate cancer suggests that there is an

up-regulation in neurotrophin expression by tumor cells

as an escape mechanism from dependence on paracrine

expression by stromal elements.26 Ketterer et al., by using

polymerase chain reaction analysis of microdissection

specimens, discovered that there is an up-regulation in

neurotrophin expression by tumor cells as well as intratu-

moral nerves in pancreas cancer.27 Tumor cells cannot

migrate through extracellular matrix or nerve sheath with-

out expressing proteinases. Matrix metalloproteinases

(MMPs), and, in particular, the gelatinases (MMP-2 and

MMP-9), seem to play a pivotal role in PNI. Okada et al.

reported that exogenous NGF led to a dose-dependent

increase in MMP-2 expression and tumor cell invasion in

pancreatic cancer cells.28 This effect was mediated by

binding NGF to its tropomyosin receptor kinase A (trkA)

receptor, which is expressed on the tumor cell surface,

with subsequent activation of the p44/42 mitogen-associ-

ated protein kinase signaling pathway.28 Pancreatic cancer

overexpresses glial cell line-derived neurotrophic factor

(GDNF).29 Migration of pancreatic cancer cells is

increased by GDNF-secreting glioma cells in a dose-de-

pendent fashion, suggesting both a chemotactic effect and

a chemokinetic effect of GDNF on tumor cells.29 Fur-

thermore, this pancreatic cancer cell migration depends

on GDNF-induced up-regulation of MMP-9 expression

and activity.30

Experimental Models

Our understanding of the pathogenesis of PNI has been

limited by a lack of effective models for this complex

Table 1. Neurotrophic Factors and Their Possible Functions and Expression Patterns in Cancer

Factor Possible Function in Cancer Expression in Cancer References

NGF May stimulate epithelial cancer cell growth and

mediate nerve invasion through its interaction

with trkA, an NGF-specific receptor; binding

of NGF to trkA leads to activation of the

p44/42 MAPK signaling pathway and

up-regulation of MMP-2, a proinvasive mediator

Overexpressed in pancreas cancer and

prostate cancer cell lines; trkA is

strongly expressed on the perineurium

of peripheral nerves

Ketterer 2003,27 Okada 2004,28

Zhu 1999,31 Zhang 2005,32

Zhu 2002,33 Geldov 1997,34

Kowalski 200235

BDNF May be overexpressed by tumor cells to

promote neurite growth; stimulates tumor cell

invasion at low-to-moderate concentrations

Overexpressed in pancreas cancer and

adenoid cystic carcinoma; expression

does not correlate with the presence of

PNI, suggesting that the BDNF-expressing

phenotype may appear before nerves

Ketterer 2003,27 Zhu 2002,33

Kowalski 2002,35

Miknyoczki 199936

GDNF Exhibits a chemotactic and chemokinetic effect

on tumor cells and mediates increased

MMP-9 expression and activity

Overexpressed in specimens of human

neural plexi; multiple pancreatic cancer

cell lines express the RET protein

tyrosine kinase receptor for GDNF

Okada 1999,29 Okada 200330

NT-3 Stimulates tumor cell invasion at

low-to-moderate concentrations

Overexpressed in pancreas cancer

specimens

Ketterer 2003,27 Miknyoczki 199936

NGF indicates nerve growth factor; trkA, tropomyosin receptor kinase A (a high-affinity catalytic receptor for nerve growth factor); MAPK, mitogen-associated

protein kinase; MMP-2, matrix metalloproteinase 2; BDNF, brain-derived neurotrophic factor; PNI, perineural invasion; GDNF, glial cell line-derived neurotrophic

factor; NT-3, neurotrophin 3.



interaction between nerve, tumor cell, and stroma. Few in

vitro models have proven capable of capturing even a sin-

gle aspect of the disease process, and much of their limita-

tions stemmed from the difficulty in culturing peripheral

nerve preparations and in replicating the neural microen-

vironment. In vivo models have proven more promising,

but these only recently have been developed. Below, we

outline the available, effective in vitro and in vivo models

of PNI and highlight some of the current research using

these experimental designs.

A critical element in the metastatic cascade is the

ability of tumor cells to invade through basement mem-

branes. These thin, specialized sheets of extracellular ma-

trix act as barriers against cellular and macromolecular

movement, particularly across epithelial layers and endo-

thelial-lined spaces of the vasculature. Basement mem-

brane also lines several cell layers of the neural sheath,

making this metastatic process elemental to PNI as well.

By using an in vitro assay that mimics tumor cell invasion

of basement membrane and correlates with in vivo meta-

static potential of cells, Miknyoczki et al. and Albini

observed that BDNF and NT-3 stimulated pancreatic tu-

mor cell invasion.36,37 In similar experiments, human

prostate cancer cell invasion was up-regulated markedly in

response to exogenous NGF and to NGF-like proteins

secreted by human prostate stromal cells.34,38 These

experiments suggest that there are complex signaling

interactions between the peripheral nerve milieu, tumor

stromal elements, and tumor cells.

In 2001, Ayala et al. described a novel in vitro PNI

model in which mouse DRG were cocultured with pros-

tate cancer cells in a Matrigel matrix.21 The 3-dimen-

sional Matrigel matrix suspends the ganglia and allows

for multidirectional neurite outgrowth (Fig. 3). Some

studies of neurite outgrowth have used cell suspensions of

isolated neurons rather than explanted mammalian DRG;

FIGURE 3. This in vitro model of perineural invasion in prostate cancer was developed by Ayala et al.21 Mouse dorsal root ganglia

(DRG) are cocultured in Matrigel with circumferentially placed prostate cancer cells. DRG exhibit directional outgrowth of neu-

rites in response to tumor cells and tumor cell colonies reveal increased growth in (a) a brightfield image and (b) a darkfield

image (original magnification, �40).

Review Article


however, the influence that supportive glial cells have in

promoting neurite growth and PNI certainly is lost.39

Three phenomena, as outlined above in the discussion of

pathogenesis, were observed in the Matrigel/DRGmodel:

1) focused outgrowths of neurites projected into cancer

cell colonies within 24 hours of culture; 2) cancer cell col-

ony formation increased; and, 3) cancer cells migrated

along contacted neurites toward the ganglion of origin.

The results were reproducible across 3 different prostate

cancer cell lines. This experiment effectively establishes

the presence of a reciprocal growth interaction between

cancer cells and neurites in vitro and also suggests that

actively growing nervous tissue somehow may promote tu-

mor cell invasion. To date, there are no data supporting

this phenomenon in vivo.

Several animal models recently have been developed

for tumors that are known to metastasize through the peri-

neural route. Pour et al. developed a Syrian hamster

model of both carcinogenesis-induced and orthotopically

transplanted pancreatic cancer.40 Both tumors demon-

strated PNI in about 90% of cases. Furthermore, in this

model, those authors demonstrated not only that tumor

cells invade extrapancreatic neural plexi by this route but

that they also reach distant metastatic sites, such as lymph

nodes, through the perineural space. Eibl and Reber

developed a model for pancreatic cancer recurrence in

nude mice that underwent resection of pancreatic tumors

at 4 weeks, 6 weeks, and 8 weeks after orthotopic implan-

tation.41 Eighty percent of pancreatic tumors that were

resected 6 weeks after orthotopic implantation recurred

with extensive invasion of retroperitoneal nerves, but

tumors that were resected at 4 weeks did not show signs of

recurrence. These findings suggest that metastasis of retro-

peritoneal nerves may occur later in the disease process.

This model may serve as an effective tool for studying

early recurrences and may help with elucidating the role

of nervous structures as sources of recurrence.

Other animal models used for studying PNI include

a chemically induced prostate carcinoma in Wistar rats

that demonstrates frequent PNI and a nude mouse ortho-

topic injection model of human head and neck squamous

cell carcinoma that leads to PNI in 100% of tumors.42,43

Both of these models result in tumors that closely mimic

their human counterparts histologically and phenotypi-

cally and, thus, serve as useful tools for studying the mo-

lecular pathogenesis of PNI.

Clinical Significance

Some of the earliest observations of neural involvement by

tumor were in cancers of the head and neck in which large

nerves seemed to serve as ready conduits for intracranial

extension. The incidence of PNI in head and neck cancers

varies considerably by histology but is reported most com-

monly in squamous cell carcinoma in which the incidence

is as high as 80%.44-46 PNI is a significant pathologic fea-

ture in head and neck cancers, heralding decreased sur-

vival, increased locoregional recurrence rates, and a

shorter time to recurrence.17,45,47,48 In 1 series of 239

patients with mucosal squamous cell carcinomas, PNI was

associated with a 23% 3-year survival rate versus 49% in

patients with stage-matched, PNI-negative tumors in uni-

variate analysis.45 Although treatment strategies for head

and neck malignancies were beyond the scope of this

review, it is noteworthy that PNI status often significantly

affects surgical strategies and adjuvant treatments in head

and neck cancers.49-53 The rationale for changing therapy

based on PNI status is largely anecdotal without signifi-

cant evidence.53 This is 1 of the only malignancies for

which assessment of PNI status is a required component

of the pathologic analysis according to the reporting pro-

tocols published by the College of American

Pathologists.54

PNI is observed in as many as 75% of resected pros-

tate cancer specimens and in 25% of biopsies from

patients without lymph node metastases, and many

authors agree that it is the most significant route of extrac-

apsular spread in this malignancy.55,56 In 1 series of 78

prostatectomy specimens with extracapsular invasion,

50% of specimens had tumor penetrating the capsule

solely along nerves; and, in the remaining 50% of speci-

mens, capsule penetration through nerves was the pre-

dominant route.57 Several studies have demonstrated that

the presence of PNI on prostate needle biopsy specimens

can reliably predict capsular penetration at prostatectomy,

although this claim remains disputed.58-60 In a series of

381 patients who received for low-risk prostate cancer,

Beard et al. reported that the 5-year prostate-specific anti-

gen (PSA) failure-free survival rate was 50% versus 80%

in patients with and without PNI in their needle biopsy

specimens, respectively.5 This association was observed

only on univariate analysis, and PNI was not an independ-

ent predictor of PSA failure on multivariate analysis, as



were pretreatment PSA level and Gleason score. However,

there was a significant correlation between the presence of

PNI and higher Gleason scores, suggesting that PNI may

predict occult, high-grade disease in otherwise low-risk

patients. Perineural involvement of benign prostatic

glands is well recognized; PNI alone, without other signs

of neoplasia, is not always diagnostic of prostate cancer.61

In a recent systematic review of investigations of PNI in

prostate needle biopsy specimens, wide variability in study

design, PNI frequency, and statistical analyses were

observed; although the results suggested that PNI in pros-

tate needle biopsy specimens may predict an adverse out-

come after surgery or radiation, no conclusion could be

reached based on data in that review, and there is no solid

evidence that a PNI-positive biopsy should alter ther-

apy.62 With regard to prostatectomy specimens, a study

of 1550 patients from the University of Michigan indi-

cated that PNI was a significant predictor of higher patho-

logic stage and positive margins on both univariate and

multivariate analysis.63 However, investigations of PNI in

prostatectomy specimens, like in studies of needle biop-

sies, lack consensus regarding biochemical failure rates

and survival differences after surgery. Although there were

2 studies that revealed significantly worse outcomes asso-

ciated with PNI, in the remaining studies, no statistical

difference was oberved.55,64-70

In 1 study of 90 pancreatic adenocarcinoma speci-

mens, PNI was observed in 88 tumors (98%).71 In that

study, there was no association between PNI and tumor

size, differentiation, or lymphovascular invasion; but there

was a significant correlation between PNI and tumors

with lymph node metastases. Reports on the incidence of

PNI in pancreatic cancer cite a minimum of 70% of these

tumors, and some authors even claim that 100% of these

tumors. will exhibit PNI if enough sections are eval-

uated.3,72-75 Positive PNI status in pancreatic cancer pre-

dicts decreased survival, often independent of stage, but

treatment remains unchanged by PNI status.3,71,73 In a

subgroup analysis of 72 patients with lymph node-nega-

tive disease (stage I and stage II), the 5-year survival rate of

patients without PNI was 75% versus 29% for those with

PNI in their tumor specimens (P< .02).3 The pancreas is

richly innervated by the autonomic nervous system, pri-

marily through plexi from the celiac and superior mesen-

teric artery ganglia. Studies of extratumoral PNI in

pancreatic cancer have focused on these plexi as likely sites

of micrometastatic spread and sources for retroperitoneal

recurrence, although these possibilities remain to be con-

firmed. Up to 72% of PNI-positive pancreatic adenocar-

cinomas demonstrate extrapancreatic nerve plexus

involvement at the time of resection.71,76,77 The associa-

tion between intratumoral PNI and extrapancreatic plexus

involvement seems to correlate with the degree of invasion

observed in the primary tumor and with the presence of

PNI within the pancreas but outside of the main body of

the tumor.71,76,77

PNI has been recognized in many series as a preva-

lent pathologic feature of colorectal cancer and is reported

in up to 33% of these tumors at the time of resection.78-81

Several studies have demonstrated a significant correlation

between the presence of PNI in colorectal tumors and

increased locoregional recurrence rates, a 5-year survival

rates, and an increased likelihood of finding metastatic

disease at the time of resection.78,80,82-84 In 1 study of 563

rectal and rectosigmoid cancers, the presence of PNI was

associated with a 27% cancer-specific 5-year survival rate

versus a 78% 5-year survival rate in PNI-negative tumors

(P< .001).4 Similar results were reported by Krasna et al.

in their review of 77 patients with colorectal carcinoma.

In that series, not only was survival lower in PNI-positive

patients, but those patients were almost 3 times more

likely to have metastatic disease at the time of diagnosis

(27% vs 73%; P < .01).84 These results suggest a correla-

tion between PNI status and advanced tumor stage, a rela-

tion that also has been demonstrated in other studies. For

instance, in a review of 373 patients with rectal cancer

who underwent curative resection, the incidence of PNI

in patients with stage III disease was 20%, which was

twice the incidence reported in the overall study popula-

tion (10%).85 Perhaps more noteworthy is the prognostic

significance of PNI in lymph node-negative disease, par-

ticularly stage II, in which it has been demonstrated that

chemotherapy has no survival benefit. In an analysis of

124 patients with lymph node-negative colorectal cancer

who underwent curative resection, the 5-year survival rate

was 87% for patients with PNI-negative tumors versus

57% for patients with PNI-positive tumors (P < .006).86

Other studies similarly have indicated that positive PNI

status portends a worse outcome in patients with lymph

node-negative colorectal tumors.87-89 This suggests that

PNI indicates a more aggressive tumor phenotype and

may be useful in selecting a subgroup of lymph node-

Review Article


negative patients who could benefit from adjuvant ther-

apy. Some patients with stage II disease currently are

offered adjuvant therapy after resection of PNI-positive

tumors, but outcomes data are not yet available.90

There is scant information on PNI in malignancies

other than those discussed above. The reported incidence

of PNI in cholangiocarcinoma is approximately 75% to

85%, and at least 2 studies have noted a significant corre-

lation between PNI and decreased survival in patients

with this malignancy.6,91,92 One series of 354 gastric can-

cer specimens cited a PNI-positive rate of 60% and a sig-

nificant correlation with disease progression and overall

survival.7 Reported rates of PNI in breast cancer range

from 3% to 38%.93-95 PNI is reported often in conjunc-

tion with lymphovascular invasion as 1 pathologic feature

of breast cancer, confounding its significance as an inde-

pendent prognostic variable. Some studies have demon-

strated a correlation between lymphovascular invasion/

PNI and decreased survival in breast cancer, whereas other

studies have failed to demonstrate a correlation between

PNI and outcome.96-99

Underreporting of PNI remains an obstacle to gain-

ing an adequate understanding of its true prognostic sig-

nificance. First, several factors make nerve invasion very

difficult to recognize. Inflammatory cells or large, muci-

nous pools may obscure the presence of tumor cells

around nerves. Microscopic foci of nerve invasion are

common and also may escape detection.72 It has been

demonstrated that evaluating specimens after staining for

nerve-specific antigens markedly increases PNI detection

rates compared with standard hematoxylin and eosin

staining techniques.46,79 On a re-review of slides from 40

patients with head and neck squamous cell carcinoma, the

rate of detection of PNI almost tripled from 30% to 82%

when specimens were stained for protein S100.46 It is con-

ceivable that nerve-specific staining may become a routine

part of the pathologic evaluation of malignancies in which

PNI status affects treatment.

In addition to problems with detection, to date,

there are no concrete guidelines on the reporting of PNI

in malignancies other than head and neck cancers. This

contributes further to the underreporting of PNI. Accord-

ing to the latest protocols published by the College of

American Pathologists, evaluation of PNI is not a required

element in pancreatic, colorectal, or prostate cancer pa-

thology reports.100

Conclusion

Since PNI first was described over a century ago, there has

been a paradigm shift in our understanding of its pathoge-

nesis. Initially, it was believed that PNI represented lym-

phatic spread of tumor into nerves; however, studies

performed in the mid-1900s called this theory into ques-

tion. By the late 1900s, the predominant theory became

that of the nerve sheath as a low-resistance path for tumor

spread. Although the ‘‘path of low resistance’’ theory still

persists in the literature today, evidence is emerging indi-

cating that the PNI phenomenon is more like invasion

than simple diffusion. New models have been developed

that provide strong evidence for signaling between the

nerves and invading tumor cells. Stromal elements, includ-

ing fibroblasts, also seem to play a key role in the complex

signaling interactions driving PNI. Studies involving neu-

rotrophins and axonal guidance molecules have implicated

a few of the molecular mediators involved in the process,

although we have only just begun to scratch the surface.

We have highlighted the clinical significance of PNI

in those malignancies in which it has been studied best.

These include cancers of the head and neck, pancreas, co-

lon and rectum, and prostate. Overall, PNI portends a sig-

nificantly lower 5-year survival rate and signifies more

advanced disease. There are many malignancies, however,

for which we have only begun to recognize the signifi-

cance of PNI. Breast cancer and hepatobiliary cancers are

among these malignancies; however, preliminary evidence

suggests that, 1 day, PNI also will be recognized as a sig-

nificant pathologic feature in these and other tumors.

With a better understanding of the pathogenesis and

through work using animal models, we can begin to target

PNI in our treatment strategies against those malignances

for which PNI is significant. Gene profiling already has

been used to identify an expression profile predictive

of PNI in biliary tract cancers.101 These expression

profiles are being evaluated for use in risk stratification

and in guiding the extent of surgical resection; however,

1 day, the candidate genes or their downstream molec-

ular pathways could serve as therapeutic targets. Sev-

eral neurotrophins, including NGF, BDNF, and NT-3,

have been implicated in promoting tumor cell invasion

and may be key mediators in the pathogenesis of

PNI.34,36,38 Researchers have begun searching for viable

therapeutic targets among these neurotrophins and their



receptors.36,102-104 Both anti-NGF antibodies and small

interfering RNA against NGF have demonstrated efficacy

against cell proliferation and angiogenesis in a breast can-

cer murine model.104 Intratumoral and peritumoral injec-

tion with antibodies against receptors for NGF and NT-3

have produced significant growth inhibition in both

human pancreatic cancer and prostate cancer xenografts

in a nude mouse model.105

Although much of our research has focused on tu-

mor cell invasion of nerves, the PNI story is beginning to

include axonal growth and possibly nerve ‘‘invasion’’ of tu-

mor. A study performed by Maru et al. indicated that PNI

diameter, measured with an ocular micrometer and

defined as the largest focus of PNI in a tumor, was a better

predictor of outcome in prostate cancer than PNI status

alone.55 There is no clear explanation why this is the case.

It is possible that tumors with larger foci of PNI are simply

more advanced cancers, which we would expect to have

worse outcomes; or perhaps there is nerve enlargement

within these foci of PNI as a result of a cancer cell-pro-

moted nerve growth phenomenon during PNI. Clearly,

this aspect of PNI needs to be explored further; however,

in vitro evidence certainly suggests that there is a reciprocal

growth interaction occurring between nerves and tumors.

It seems that progress in understanding PNI has

been stymied by the lack of a concise definition for this

pathologic process. We conclude that the definition

offered by Batsakis in 1985—tumor cell invasion in,

around, and through the nerves—leaves too much room for

interpretation and is not precise enough to promote uni-

formity among researchers. After a thorough review of the

literature, we have compiled criteria for the identification

of PNI in a histologic specimen. Finding tumor cells

within any of the 3 layers of the nerve sheath or tumor foci

outside of the nerve with involvement of �33% of the

nerve’s circumference are sufficient features for calling

PNI. It is likely that, as our understanding of the pathoge-

nesis of PNI evolves, so too will this definition.

Conflict of Interest Disclosures

The authors made no disclosures.

References

1. Cruveilheir J. Maladies Des Nerfs Anatomie PathologiqueDu Corps Humain. 2nd ed. Paris, France: JB Bailliere;1835.

2. Neumann E. Secondare cancroid infiltration des nervusmentalis bei einem. Arch Pathol Anat. 1862;24:201-201.

3. Ozaki H, Hiraoka T, Mizumoto R, et al. The prognosticsignificance of lymph node metastasis and intrapancreaticperineural invasion in pancreatic cancer after curative resec-tion. Surg Today. 1999;29:16-22.

4. Law WL, Chu KW. Anterior resection for rectal cancerwith mesorectal excision: a prospective evaluation of 622patients. Ann Surg. 2004;240:260-268.

5. Beard CJ, Chen MH, Cote K, et al. Perineural invasion isassociated with increased relapse after external beam radio-therapy for men with low-risk prostate cancer and may be amarker for occult, high-grade cancer. Int J Radiat OncolBiol Phys. 2004;58:19-24.

6. Su CH, Tsay SH, Wu CC, et al. Factors influencing post-operative morbidity, mortality, and survival after resectionfor hilar cholangiocarcinoma. Ann Surg. 1996;223:384-394.

7. Duraker N, Sisman S, Can G. The significance of perineu-ral invasion as a prognostic factor in patients with gastriccarcinoma. Surg Today. 2003;33:95-100.

8. Batsakis JG. Nerves and neurotropic carcinomas. Ann OtolRhinol Laryngol. 1985;94(4 pt 1):426-427.

9. Hassan MO, Maksem J. The prostatic perineural space andits relation to tumor spread: an ultrastructural study. Am JSurg Pathol. 1980;4:143-148.

10. Rodin AE, Larson DL, Roberts DK. Nature of the perineu-ral space invaded by prostatic carcinoma. Cancer. 1967;20:1772-1779.

11. Akert K, Sandri C, Weibel ER, et al. The fine structure ofthe perineural endothelium. Cell Tissue Res. 1976;165:281-295.

12. Peters A. Connective tissue sheaths of peripheral nerves.In: Peters A, Palay SL, Webster H, eds. The Fine Structureof the Nervous System: Neurons and Their SupportingCells, 3rd ed. New York, NY: Oxford University Press;1991:494.

13. Stolinski C. Structure and composition of the outer con-nective tissue sheaths of peripheral nerve. J Anat. 1995;186(pt 1):123-130.

14. Olsson Y. Microenvironment of the peripheral nervous sys-tem under normal and pathological conditions. Crit RevNeurobiol. 1990;5:265-311.

15. Larson DL, Rodin AE, Roberts DK, et al. Perineural lym-phatics: myth or fact? Am J Surg. 1966;112:488-492.

16. Veness MJ. Perineural spread in head and neck skin cancer.Australas J Dermatol. 2000;41:117-119.

17. Fagan JJ, Collins B, Barnes L, et al. Perineural invasion insquamous cell carcinoma of the head and neck. Arch Oto-laryngol Head Neck Surg. 1998;124:637-640.

18. Bockman DE, Buchler M, Beger HG. Interaction of pan-creatic ductal carcinoma with nerves leads to nerve damage.Gastroenterology. 1994;107:219-230.

Review Article


19. Nagakawa T, Kayahara M, Ohta T, et al. Patterns of neuraland plexus invasion of human pancreatic cancer and experi-mental cancer. Int J Pancreatol. 1991;10:113-119.

20. Reina MA, Lopez A, Villanueva MC, et al. Morphology ofperipheral nerves, their sheaths, and their vascularization [inSpanish]. Rev Esp Anestesiol Reanim. 2000;47:464-475.

21. Ayala GE, Wheeler TM, Shine HD, et al. In vitro dorsalroot ganglia and human prostate cell line interaction: rede-fining perineural invasion in prostate cancer. Prostate. 2001;49:213-223.

22. Cornell RJ, Rowley D, Wheeler T, et al. Neuroepithelialinteractions in prostate cancer are enhanced in the presenceof prostatic stroma. Urology. 2003;61:870-875.

23. Chilton JK. Molecular mechanisms of axon guidance. DevBiol. 2006;292:13-24.

24. Chedotal A, Kerjan G, Moreau-Fauvarque C. The brainwithin the tumor: new roles for axon guidance molecules incancers. Cell Death Differ. 2005;12:1044-1056.

25. Boyd JG, Gordon T. Neurotrophic factors and their recep-tors in axonal regeneration and functional recovery after pe-ripheral nerve injury. Mol Neurobiol. 2003;27:277-324.

26. Dalal R, Djakiew D. Molecular characterization of neuro-trophin expression and the corresponding tropomyosin re-ceptor kinases (trks) in epithelial and stromal cells of thehuman prostate. Mol Cell Endocrinol. 1997;134:15-22.

27. Ketterer K, Rao S, Friess H, et al. Reverse transcription-PCR analysis of laser-captured cells points to potentialparacrine and autocrine actions of neurotrophins in pancre-atic cancer. Clin Cancer Res. 2003;9:5127-5136.

28. Okada Y, Eibl G, Guha S, Duffy JP, Reber HA, Hines OJ.Nerve growth factor stimulates MMP-2 expression and ac-tivity and increases invasion by human pancreatic cancercells. Clin Exp Metastasis. 2004;21:285-292.

29. Okada Y, Takeyama H, Sato M, et al. Experimental impli-cation of celiac ganglionotropic invasion of pancreatic-can-cer cells bearing c-ret proto-oncogene with reference toglial-cell-line-derived neurotrophic factor (GDNF). Int JCancer. 1999;81:67-73.

30. Okada Y, Eibl G, Duffy JP, et al. Glial cell-derived neuro-trophic factor upregulates the expression and activation ofmatrix metalloproteinase-9 in human pancreatic cancer.Surgery. 2003;134:293-299.

31. Zhu Z, Friess H, diMola FF, et al. Nerve growth factorexpression correlates with perineural invasion and pain inhuman pancreatic cancer. J Clin Oncol. 1999;17:2419-2428.

32. Zhang Y, Dang C, Ma Q, et al. Expression of nerve growthfactor receptors and their prognostic value in human pan-creatic cancer. Oncol Rep. 2005;14:161-171.

33. Zhu Z, Kleeff J, Kayed H, et al. Nerve growth factor andenhancement of proliferation, invasion, and tumorigenicityof pancreatic cancer cells. Mol Carcinog. 2002;35:138-147.

34. Geldof AA, De Kleijn MA, Rao BR, et al. Nerve growthfactor stimulates in vitro invasive capacity of DU145

human prostatic cancer cells. J Cancer Res Clin Oncol.1997;123:107-112.

35. Kowalski PJ, Paulino AF. Perineural invasion in adenoidcystic carcinoma: its causation/promotion by brain-derivedneurotrophic factor. Hum Pathol. 2002;33:933-936.

36. Miknyoczki SJ, Lang D, Huang L, et al. Neurotrophinsand Trk receptors in human pancreatic ductal adenocarci-noma: expression patterns and effects on in vitro invasivebehavior. Int J Cancer. 1999;81:417-427.

37. Albini A. Tumor and endothelial cell invasion of basementmembranes. The Matrigel chemoinvasion assay as a tool fordissecting molecular mechanisms. Pathol Oncol Res. 1998;4:230-241.

38. Djakiew D, Pflug BR, Delsite R, et al. Chemotaxis andchemokinesis of human prostate tumor cell lines inresponse to human prostate stromal cell secretory proteinscontaining a nerve growth factor-like protein. Cancer Res.1993;53:1416-1420.

39. Tonge DA, Golding JP, Edbladh M, et al. Effects of extrac-ellular matrix components on axonal outgrowth from pe-ripheral nerves of adult animals in vitro. Exp Neurol. 1997;146:81-90.

40. Pour PM, Egami H, Takiyama Y. Patterns of growth andmetastases of induced pancreatic cancer in relation to theprognosis and its clinical implications. Gastroenterology.1991;100:529-536.

41. Eibl G, Reber HA. A xenograft nude mouse model forperineural invasion and recurrence in pancreatic cancer.Pancreas. 2005;31:258-262.

42. Cabanillas R, Secades P, Rodrigo JP, et al. Orthotopic mu-rine model of head and neck squamous cell carcinoma [inSpanish]. Acta Otorrinolaringol Esp. 2005;56:89-95.

43. Bosland MC, Prinsen MK, Dirksen TJ, et al. Characteriza-tion of adenocarcinomas of the dorsolateral prostateinduced in Wistar rats by N-methyl-N-nitrosourea, 7,12-dimethylbenz(a)anthracene, and 3,20-dimethyl-4-aminobi-phenyl, following sequential treatment with cyproterone ac-etate and testosterone propionate. Cancer Res. 1990;50:700-709.

44. Carter RL, Foster CS, Dinsdale EA, et al. Perineural spreadby squamous carcinomas of the head and neck: a morpho-logical study using antiaxonal and antimyelin monoclonalantibodies. J Clin Pathol. 1983;36:269-275.

45. Soo KC, Carter RL, O’Brien CJ, et al. Prognostic implica-tions of perineural spread in squamous carcinomas of thehead and neck. Laryngoscope. 1986;96:1145-1148.

46. Kurtz KA, Hoffman HT, Zimmerman MB, et al. Perineu-ral and vascular invasion in oral cavity squamous carci-noma: increased incidence on re-review of slides and byusing immunohistochemical enhancement. Arch Pathol LabMed. 2005;129:354-359.

47. Goepfert H, Dichtel WJ, Medina JE, et al. Perineural inva-sion in squamous cell skin carcinoma of the head and neck.Am J Surg. 1984;148:542-547.



48. O’Brien CJ, Lahr CJ, Soong SJ, et al. Surgical treatment ofearly stage carcinoma of the oral tongue—would adjuvanttreatment be beneficial? Head Neck Surg. 1986;8:401-408.

49. Shah N, Saunders MI, Dische S. A pilot study of postoper-ative CHART and CHARTWEL in head and neck cancer.Clin Oncol (R Coll Radiol). 2000;12:392-396.

50. Brandwein-Gensler M, Teixeira MS, Lewis CM, et al. Oralsquamous cell carcinoma: histologic risk assessment, butnot margin status, is strongly predictive of local disease-freeand overall survival. Am J Surg Pathol. 2005;29:167-178.

51. Garcia-Serra A, Hinerman RW, Mendenhall WM, et al.Carcinoma of the skin with perineural invasion. Head Neck.2003;25:1027-1033.

52. Mendenhall WM, Amdur RJ, Williams LS, et al. Carci-noma of the skin of the head and neck with perineuralinvasion. Head Neck. 2002;24:78-83.

53. Mendenhall WM, Amdur RJ, Hinerman RW, et al. Skincancer of the head and neck with perineural invasion. Am JClin Oncol. 2007;30:93-96.

54. Pilch BZ, Gillies E, Houck JR, et al. Upper AerodigesticTract Cancer Protocols and Checklists. Northfield, Ill: Col-lege of American Pathologists, 2005.

55. Maru N, Ohori M, Kattan MW, et al. Prognostic signifi-cance of the diameter of perineural invasion in radical pros-tatectomy specimens. Hum Pathol. 2001;32:828-833.

56. Ayala GE, Dai H, Ittmann M, et al. Growth and survivalmechanisms associated with perineural invasion in prostatecancer. Cancer Res. 2004;64:6082-6090.

57. Villers A, McNeal JE, Redwine EA, et al. The role of peri-neural space invasion in the local spread of prostatic adeno-carcinoma. J Urol. 1989;142:763-768.

58. Bastacky SI, Walsh PC, Epstein JI. Relationship betweenperineural tumor invasion on needle biopsy and radical pros-tatectomy capsular penetration in clinical stage B adenocarci-noma of the prostate. Am J Surg Pathol. 1993;17:336-341.

59. de la Taille A, Katz A, Bagiella E, et al. Perineural invasionon prostate needle biopsy: an independent predictor of finalpathologic stage. Urology. 1999;54:1039-1043.

60. Rubin MA, Mucci NR, Manley S, et al. Predictors ofGleason pattern 4/5 prostate cancer on prostatectomy speci-mens: can high grade tumor be predicted preoperatively?J Urol. 2001;165:114-118.

61. Ali TZ, Epstein JI. Perineural involvement by benign pros-tatic glands on needle biopsy. Am J Surg Pathol. 2005;29:1159-1163.

62. Harnden P, Shelley MD, Clements H, et al. The prognos-tic significance of perineural invasion in prostatic cancerbiopsies: a systematic review. Cancer. 2007;109:13-24.

63. Lee IH, Roberts R, Shah RB, et al. Perineural invasion is amarker for pathologically advanced disease in localized pros-tate cancer. Int J Radiat Oncol Biol Phys. 2007;68:1059-1064.

64. Ozcan F. Correlation of perineural invasion on radicalprostatectomy specimens with other pathologic prognosticfactors and PSA failure. Eur Urol. 2001;40:308-312.

65. Endrizzi J, Seay T. The relationship between early bio-chemical failure and perineural invasion in pathological T2prostate cancer. BJU Int. 2000;85:696-698.

66. Merrilees AD, Bethwaite PB, Russell GL, et al. Parametersof perineural invasion in radical prostatectomy specimenslack prognostic significance. Mod Pathol. 2008;21:1095-1100.

67. Ng JC, Koch MO, Daggy JK, et al. Perineural invasion inradical prostatectomy specimens: lack of prognostic signifi-cance. J Urol. 2004;172(6 pt 1):2249-2251.

68. Miyake H, Sakai I, Harada K, et al. Limited value of peri-neural invasion in radical prostatectomy specimens as a pre-dictor of biochemical recurrence in Japanese men withclinically localized prostate cancer. Hinyokika Kiyo. 2005;51:241-246.

69. Ito K, Nakashima J, Mukai M, et al. Prognostic implicationof microvascular invasion in biochemical failure in patientstreated with radical prostatectomy. Urol Int. 2003;70:297-302.

70. van den Ouden D, Hop WC, Kranse R, et al. Tumourcontrol according to pathological variables in patientstreated by radical prostatectomy for clinically localized car-cinoma of the prostate. Br J Urol. 1997;79:203-211.

71. Takahashi T, Ishikura H, Motohara T, et al. Perineuralinvasion by ductal adenocarcinoma of the pancreas. J SurgOncol. 1997;65:164-170.

72. Pour PM, Bell RH, Batra SK. Neural invasion in the stag-ing of pancreatic cancer. Pancreas. 2003;26:322-325.

73. Hirai I, Kimura W, Ozawa K, et al. Perineural invasion inpancreatic cancer. Pancreas. 2002;24:15-25.

74. Noto M, Miwa K, Kitagawa H, et al. Pancreas head carci-noma: frequency of invasion to soft tissue adherent to thesuperior mesenteric artery. Am J Surg Pathol. 2005;29:1056-1061.

75. Kayahara M, Nagakawa T, Futagami F, et al. Lymphaticflow and neural plexus invasion associated with carcinomaof the body and tail of the pancreas. Cancer. 1996;78:2485-2491.

76. Nakao A, Harada A, Nonami T, et al. Clinical significanceof carcinoma invasion of the extrapancreatic nerve plexus inpancreatic cancer. Pancreas. 1996;12:357-361.

77. Nagakawa T, Kayahara M, Ueno K, et al. A clinicopatho-logic study on neural invasion in cancer of the pancreatichead. Cancer. 1992;69:930-935.

78. Horn A, Dahl O, Morild I. Venous and neural invasion aspredictors of recurrence in rectal adenocarcinoma. Dis Co-lon Rectum. 1991;34:798-804.

79. Bellis D, Marci V, Monga G. Light microscopic and im-munohistochemical evaluation of vascular and neural inva-sion in colorectal cancer. Pathol Res Pract. 1993;189:443-447.

80. Matsushima T, Mori M, Kido A, et al. Preoperative estima-tion of neural invasion in rectal carcinoma. Oncol Rep.1998;5:73-76.

Review Article


81. Moore PA, Dilawari RA, Fidler WJ. Adenocarcinoma ofthe colon and rectum in patients less than 40 years of age.Am Surg. 1984;50:10-14.

82. Bentzen SM, Balslev I, Pedersen M, et al. Time to locore-gional recurrence after resection of Dukes’ B and C colo-rectal cancer with or without adjuvant postoperativeradiotherapy. A multivariate regression analysis. Br J Can-cer. 1992;65:102-107.

83. Ross A, Rusnak C, Weinerman B, et al. Recurrence andsurvival after surgical management of rectal cancer. Am JSurg. 1999;177:392-395.

84. Krasna MJ, Flancbaum L, Cody RP, et al. Vascular andneural invasion in colorectal carcinoma. Incidence andprognostic significance. Cancer. 1988;61:1018-1023.

85. Shirouzu K, Isomoto H, Kakegawa T. Prognostic evalua-tion of perineural invasion in rectal cancer. Am J Surg.1993;165:233-237.

86. Onate-Ocana LF, Montesdeoca R, Lopez-Graniel CM,et al. Identification of patients with high-risk lymph node-negative colorectal cancer and potential benefit from adju-vant chemotherapy. Jpn J Clin Oncol. 2004;34:323-328.

87. Burdy G, Panis Y, Alves A, et al. Identifying patients withT3-T4 node-negative colon cancer at high risk of recur-rence. Dis Colon Rectum. 2001;44:1682-1688.

88. Horn A, Dahl O, Morild I. The role of venous and neuralinvasion on survival in rectal adenocarcinoma. Dis ColonRectum. 1990;33:598-601.

89. Di Fabio F, Nascimbeni R, Villanacci V, et al. Prognosticvariables for cancer-related survival in node-negative colo-rectal carcinomas. Dig Surg. 2004;21:128-133.

90. De Dosso S, Sessa C, Saletti P. Adjuvant therapy for coloncancer: present and perspectives. Cancer Treat Rev. 2009;35:160-166.

91. He P, Shi JS, Chen WK, et al. Multivariate statistical anal-ysis of clinicopathologic factors influencing survival ofpatients with bile duct carcinoma. World J Gastroenterol.2002;8:943-946.

92. Nagakawa T, Mori K, Nakano T, et al. Perineural invasionof carcinoma of the pancreas and biliary tract. Br J Surg.1993;80:619-621.

93. Ho CM, Mak CK, Lau Y, et al. Skin involvement in inva-sive breast carcinoma: safety of skin-sparing mastectomy.Ann Surg Oncol. 2003;10:102-107.

94. Elmore JG, Moceri VM, Carter D, et al. Breast carcinomatumor characteristics in black and white women. Cancer.1998;83:2509-2515.

95. Cowan WK, Kelly P, Sawan A, et al. The pathological andbiological nature of screen-detected breast carcinomas: amorphological and immunohistochemical study. J Pathol.1997;182:29-35.

96. McCready DR, Chapman JA, Hanna WM, et al. Factorsaffecting distant disease-free survival for primary invasivebreast cancer: use of a log-normal survival model. Ann SurgOncol. 2000;7:416-426.

97. McCready DR, Chapman JA, Hanna WM, et al. Factorsassociated with local breast cancer recurrence after lumpec-tomy alone: postmenopausal patients. Ann Surg Oncol.2000;7:562-567.

98. Mate TP, Carter D, Fischer DB, et al. A clinical and histo-pathologic analysis of the results of conservation surgeryand radiation therapy in stage I and II breast carcinoma.Cancer. 1986;58:1995-2002.

99. Roses DF, Bell DA, Flotte TJ, et al. Pathologic predictorsof recurrence in stage 1 (T1N0M0) breast cancer. Am JClin Pathol. 1982;78:817-820.

100. College of American Pathologists. Cancer Protocols andChecklists. Northfield, Ill: College of American Patholo-gists; 2007.

101. Murakawa K, Tada M, Takada M, et al. Prediction oflymph node metastasis and perineural invasion of biliarytract cancer by selected features from cDNA array data.J Surg Res. 2004;122:184-194.

102. Papatsoris AG, Liolitsa D, Deliveliotis C. Manipulation ofthe nerve growth factor network in prostate cancer. ExpertOpin Investig Drugs. 2007;16:303-309.

103. Desmet CJ, Peeper DS. The neurotrophic receptor TrkB: adrug target in anticancer therapy? Cell Mol Life Sci.2996l63(7-8):755-759.

104. Adriaenssens E, Vanhecke E, Saule P, et al. Nerve growthfactor is a potential therapeutic target in breast cancer. Can-cer Res. 2008;68:346-351.

105. Miknyoczki SJ, Wan W, Chang H, et al. The neuro-trophin-trk receptor axes are critical for the growth andprogression of human prostatic carcinoma and pancreaticductal adenocarcinoma xenografts in nude mice. Clin Can-cer Res. 2002;8:1924-1931.



perineural invasion in cancer

Documents