comparison of the digital rectal examination, endorectal ultrasound, and body coil magnetic...

Urol Radiol 13:110-118 (1991)

Urologic Radiology

© Spfinger-Vedag New York Inc. 1991

Comparison of the Digital Rectal Examination, Endorectal Ultrasound, and Body Coil Magnetic Resonance Imaging in the Staging of Adenocarcinoma of the Prostate

M.L. Schiebler, ~ S. McSherry, 2 B. Keefe, ~ C.A. Mittelstaedt, ~ J.L. Mohler, 2 G.A. Dent, 3 and W.H. McCartney ~ Departments of ~Radiology, 2Surgery, and 3pathology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA

Abstract. A series of 25 patients with biopsy proven adenocarcinoma of the prostate underwent preoperative staging evaluation with a digital rectal examination, endorectal ultrasound, and body coil magnetic resonance imaging (MRI) before their radical retropubic prostatectomy. The sensitivity and specificity of the digital rectal examination for the detection ofextracapsular disease were 17 and 100%, respectively. The sensitivity and specificity of endorectal ultrasound for the detection of extracapsular disease were 35 and 89%, respectively. The sensitivity and specificity of body coil MRI for the detection of extracapsular disease by adenocarcinoma of the prostate were 47 and 63%, respectively. Microscopic disease of the capsule and seminal vesicles was the principle reason for understaging by both imaging modalities. This small series suggests that both imaging modalities are marginally more sensitive, albeit less specific, for extracapsular disease of the prostate than the digital rectal examination, with ultrasound having a slight edge in specificity and MRI having a slight edge in sensitivity.

Key words: Adenocarcinoma of the prostate -- Preoperative staging -- Endorectal ultrasound --

Magnetic resonance imaging -- Radical retropubic prostatectomy.

The preoperative staging accuracy of endorectal ultrasound and body coil magnetic resonance imaging (MRI) for adenocarcinoma of the prostate is under investigation [1-12]. If the tumor nodule can be shown accurately outside the capsule by an imaging study, most urologists recommend conservative management with radiation therapy in lieu of radical retropubic prostatectomy. However, if an imaging modality can accurately show organ-confined disease, radical retropubic prostatectomy is a cu- rative surgical procedure [13]. The clinical aspects of this staging study are the subject of a separate publication [ 14].

The purpose of this study was to compare the staging accuracies of the digital rectal examination, endorectal ultrasound, and body coil MRI in patients with known adenocarcinoma of the prostate prior to their radical retropubic prostatectomy. Postoperatively, results of the digital rectal examination and imaging modalities were compared with the histopathologic findings.

Address offprint requests to: Mark L. Schiebler, M.D., Department of Radiology, University of North Carolina, CB 7510, Chapel Hill, NC 27599-7510, USA

Materials and Methods

From June 1988-November 1989, 25 patients with biopsy proven adenocarcinoma of the prostate and with a life expectancy of

M.L. Schiebler et al.: DRE, US, and MRI: Staging Prostate Adenocarcinoma 111

greater than I0 years had pelvic MRIs and endorectal ultrasound performed to help stage their adenocarcinoma of the prostate prior to a planned radical retropubic prostatectomy. The patients' ages ranged from 52-80 years with a mean age of 64.2 years. Both studies were done within 6 weeks of the biopsy. Each radical retropubic prostatectomy was performed within 2 months of the imaging studies. Routine clinical staging workup consisted of a digital rectal examination by a urologic oncologist (JM), serum prostatic acid phosphatase, and prostate-specific antigen.

The MRIs were all performed on a Philips 1.5-T Gyroscan with the following parameters: (1) transaxial SE TR 600 msec/ TE 20 msec (10 ram, 2 mm skip, 2 AVE, 205 x 256 matrix) from the mid-aspect of the kidneys through the pubic symphysis, and (2) transaxial SE TR 2500 msec/TE 20 and 80 msec (0.5 mm, 1 mm skip, 2 AVE, 205 x 256 matrix) through the seminal vesicles and prostate. No dedicated Tl-weighted 0.5-mm runs were performed through the prostate because of time constraints.

The endorectal ultrasounds for this study were performed within 1 month of the MRIs on either an Acuson machine with two dedicated 7-MHz endoreetal probes (one sagittal and one transverse) or a General Electric machine with a single endorectal probe with two 6.2-MHz transducers within its tip. The inter- pretations of all of the ultrasounds (BK, CM) were done in a prospective manner and were blinded to the pathology.

For endorectal ultrasound, the tumor was considered extracapsular if any one of the following findings was present: (1) irregularity of, or asymmetry to, the pericapsular fat; (2) if the seminal vesicles appeared enlarged or asymmetric; (3) if the angle between the posterior superior aspect of the gland and the seminal vesicles appeared unsharp; (4) if the hypoechoic nodule deformed the outer margin of the gland and was also accompanied by hypoechoic changes in the usually hyperhoic fat; and (5) if there was no fat plane between the obturator internus muscle and the nodule.

For MRI, the adenocarcinoma of the prostate was considered extracapsular if any one of the following findings were present: (1) if either of the seminal vesicles were of low signal intensity (obstructed and/or asymmetric seminal vesicles were called normal in this analysis), (2) if the capsule appeared deformed, (3) if the periprostatic venous plexus was asymmetric or unilaterally absent, (4) if the periprostatic fat appeared irregular, or (5) if there were lymph nodes and/or bone metastases. The capsule as a distinct structure per se was infrequently seen, its position, however, was inferred by the site of the surrounding periprostatic venous plexus. For all 25 cases, the MRI (MS) reader was blinded to the clinical examination, ultrasound results, and pathology. The sensitivities and specificities for accurately determining extracapsular disease were calculated.

All pathology specimens (prostates and pelvic lymph nodes) were analyzed and graded according to the Gleason classification by one pathologist (GD).

Pathological evaluation of the prostate specimens was performed in the following manner: (1) the gland was fixed in 10% buffered formalin immediately postresection; (2) the fixed gland was cut into four quarters and sliced into approximately equal 8-16 cubes with a knife depending on its size; (3) each quarter of the gland was labeled with a letter (A, anterior right; B, anterior left; C, posterior right; D, posterior left) and the individual slices of these cubes were labeled with a number-- 1 for the most superior slice through 6 for the most inferior slice; (4) these tissue cubes were then embedded in wax within plastic cassettes for routine microtome cutting of the tissue, stained with hematoxylin and eosin, and then mounted; (5) each tissue block's slides were then rigorously evaluated for the presence of transcapsular adenocarcinoma of the prostate, or invasion of the lumen of the

seminal vesicles; (6) the status of the gland and its margins were recorded on a coding sheet detailing the following items: (A) Gleason grade, (B) lymph node involvement with adenocarcinoma, (C) adenocarcinoma in the lumen or around the seminal vesicles, (D) involvement of the gland margins with adenocarcinoma, (E) invasion of the prostate capsule by adenocarcinoma, (F) penetration of the prostatic capsule by adenocarcinoma, (G) maximal tumor diameter in centimeters, (H) tumor volume in grams, (I) percent of gland involved with adenocarcinoma. For the purposes of this study, stage C disease was determined only if there was capsular penetration by adenocarcinoma.

The prostatic volume was calculated by using the fact that the prostate has a specific gravity of 1.0. Therefore, the volume was considered equal to the fresh prostatic weight in grams [ 15].

In three difficult cases films were also reviewed together with the pathology slides retrospectively to determine where the staging errors were. In all the other cases, the pathology data sheets were used in combination with the MRI data sheets and films.

Three patients with advanced disease in this population received surgery because they demanded aggressive management. Statistical analysis using Fischer's exact test for differences was performed on the ultrasound and MRI staging data.

Results

Sta t i s t i ca l ana lys i s o f t h i s d a t a set was p e r f o r m e d

even t h o u g h the s a m p l e was ve ry smal l . Th i s l imi t s

a n y s t a t i s t i ca l ly s ign i f ican t s t a t e m e n t s t h a t can be

m a d e f r o m the da ta . T h e N for s e n s i t i v i t y ca l cu la -

t i ons was 17 ( n u m b e r o f t r u e - p o s i t i v e s p lu s n u m b e r

o f fa l se -nega t ives ) . T h e s e n s i t i v i t y (_+95% conf i -

d e n c e l imi t s ) for u l t r a s o u n d in the ser ies was 0 .35 ( + 0 . 2 3 ) , a n d for M R I was 0 .47 (+_0.24). F i s c h e r ' s

exac t tes t for d i f fe rence was n o t s ign i f ican t (P =

0.36). T h e N for spec i f ic i ty was 8 ( f a l s e -pos i t i ve p lu s

fa l se -nega t ives ) . T h e spec i f ic i ty (+_95% c o n f i d e n c e l imi t s ) for u l t r a s o u n d was 0 .88 (+_0.24), w h e r e a s for

M R I i t was 0.63 (+_0.34). F i s c h e r ' s exac t tes t for d i f fe rence was n o t s ign i f ican t (P = 0.29). T h e p o s -

i t i v e p r e d i c t i v e v a l u e o f u l t r a s o u n d (_+95% conf i - dence l imi t s ) was 0 .86 (_+0.25) a n d for M R I was 0.73 ( + 0 . 3 4 ) . T h e n e g a t i v e p r e d i c t i v e v a l u e for ul - t r a s o u n d (___ 95 % conf idence l imi t s ) was 0.39 (___ 0.2 3 ), a n d for M R I was 0 .36 (+_0.25; see T a b l e 1).

O n e f ac to r l e a d i n g to u l t r a s o u n d ' s u n d e r s t a g i n g o f th i s d i s ease w i th r e spec t to M R I was d u e to the p re sence o f p e l v i c l y m p h a d e n o p a t h y ( T a b l e 2). A

s e c o n d fac to r was m i c r o s c o p i c i n v a s i o n o f t he cap - sule.

T h e r e were m a n y fac to rs r e s p o n s i b l e for M R I ' s

u n d e r s t a g i n g o f a d e n o c a r c i n o m a o f the p r o s t a t e in

th i s ser ies (Tab l e 2), t he p r i m a r y one b e i n g m i c r o - s cop i c d i sease .

T a b l e s 3 a n d 4 c o m p a r e o t h e r p u b l i s h e d s tag ing d a t a w i t h th i s ser ies , fo r e n d o r e c t a l u l t r a s o u n d a n d b o d y coi l M R I , r e spec t ive ly .

112

Table 1.

M.L. Schiebler et al.: DRE, US, and MRI: Staging Prostate Adenocarcinoma

Maximal stage Digital rectal using both US examination Endorectal US Body coil MRI and/or MRI

True (+) 3 6 8 10 True ( - ) 8 7 5 7 False (+) 0 1 3 3 False ( - ) 14 11 9 5 Sensitivity TP/(TP + FN) 18% 35% 47% 59% Specificity TN/(TN + FP) 100% 89% 63% 62% Test confined 14 + 8 7 + 11 9 + 5 5 + 7

Pathology confined 8 7 5 5 Pathology unconfined 14 11 9 7 Negative predictive value 36% 38% 35% 42%

Test unconfined 3 + 0 6 + 1 8 + 3 10 + 3 Pathology confined 0 1 3 3 Pathology unconfined 3 6 8 10 Positive predictive value 100% 88% 72% 77%

Table 2. Analysis of understaging errors

Body coil MRI Endorectal US

Major pathologic findings related to understaging by

Microscopic invasion of the capsule Macroscopic invasion of the capsule Microscopic invasion of the neurovascular bundle Macroscopic invasion of the neurovascular bundle Microscopic invasion of the seminal vesicle Macroscopic invasion of the seminal vesicle Microscopic involvement of lymph nodes Macrosco )lC involvement of lymph nodes

Total number of patients

Major imaging findings related to understaging by

Anterior location of mass Confusion with postbiopsy changes Subtle involvement of the periprostatic venous plexus Asymmetric seminal vesicles Obstructed seminal vesicles (symmetric) Poor quality examination Significant central zone BPH with small periperipheral zone Pelvic lymphadenopathy No imaging findings suggestive of advanced disease

Total number of patients

2 6 2 0 1 1 1 1 2 1 1 0 0 0 0 2

9 11

0 1 2 1 1 2 1 1 1 1 1 0 2 1 0 2 1 2

9 11

Discussion

Previous reports on the sensitivity and specificity for the staging of adenocarcinoma of the prostate usually relied on retrospective analysis of one modality or the other, and these were infrequently blinded. Rarely have both endorectal ultrasound and body coil MRI at 1.5 T been compared prospectively with one another for the staging of adenocarcinoma of the prostate, until the recent publication of the Radiologic Diagnostic Oncology Group study (RDOG) [ 12].

Our results are much poorer than those pre- sented in the recent radiologic literature. We have purposefully included those cases of microscopic transcapsular penetration by adenocarcinoma of the prostate as stage C disease, which is somewhat con- troversial. For this small series, clear definition of the capsule by either MRI or endorectal ultrasound was at best only 50% correct for determining whether or not disease was confined to the gland. Sensi- tivities reported in the literature for the staging of adenocarcinoma of the prostate by endorectal ultrasound varied from a minimum of 0.57 [2] to a

M.L Schiebler et al.: DRE, US, and MRI: Staging Prostate Adenocarcinoma

Table 3. Reported endorectal ultrasound staging versus pathology from radical retropubic prostatectomy (RRP)"

113

Andri- Harde- Perra- ole man pato Pontes Resnick Rifkin Salo [2] [10] [11] [1] [81 [121 [18]

This series

All series c o m -

b i n e d

No. of patients RRP 20 43 25 41 23 219 31 (TP) True (+) Test+ 4 18 5 23 17 84 12

Path+ (TN) True ( - ) Tes t - 13 15 17 7 6 42 16

Path - (FP) False (+) Test+ 0 3 0 2 0 50 1

Path - (FN) False ( - ) Tes t - 3 7 3 9 0 43 2

Path+ Sensitivity TP/(TP + FN) 57% 72% 62% 59% 100% 66% 85% Specificity TN/(TN + FP) 100% 77% 100% 77% 100% 56% 94% Predictive value for TN/(TN + FN) 81% 68% 85% 43% 100% 49% 88%

confinement disease Predictive value for TP/(TP + FP) 100% 85% 63% 59% 100% 62% 92%

unconfined disease Series prevalence of 35% 58% 32% 78% 74% 58% 45%

extracapsular disease

25 6

8

1

11

35% 89% 42%

85%

68%

427 159

124

57

77

67% 68% 61%

73%

550

"Adapted from McSherry et al. [14].

Table 4. Reported body coil MRI staging vs. pathology from radical retropubic prostatectomy (RRP) specimens"

All series Bezzi Biondetti Hricak Rifkin This com- [9] [4] [6] [ 12] series bined

Field strength MRI 1.5T 1.5T 0.5T 1.5T 1.5T No. RRP 37 15 46 194 25 b (TP) True (+) Test+ 13 4 23 86 8

Path+ (TN) True ( - ) Test+ 16 9 5 47 5

Path+ (FP) False (+) Test+ 3 1 5 35 3

Path+ (FN) False ( - ) Test+ 5 l 1 26 9

Path+ Sensitivity TP/(TP + FN) 72% 80% 96% 76% 47% Specificity TN/(TN + FP) 84% 90% 50% 57% 63% Predictive value for TN/(TN + FN) 81% 90% 83% 64% 35%

confined disease Predictive value for TP/(TP + FP) 76% 80% 82% 71% 72%

unconfined disease

317 134

82

47

42

76% 63% 66%

74%

"Adapted from McSherry et al. [ 14]. b Twenty-four RRP and one attempted with biopsy-positive lymph nodes.

maximum of 1.0 [8] and specificities ranging from 0.77 [10] to 1.0 [2, 8, 11] (Table 3). For body coil MRI, the reported sensitivities have ranged from 0.72 [9] through 0.96 [6] and specificities have ranged from 0.50 [6] through 0.90 [4] (Table 4). When all published series are combined, the sensitivities and specificities for the two modalities are remarkably similar with endorectal ultrasound 67% sensitive

and 68% specific (Table 3) and body coil MRI 76% sensitive and 63% specific (Table 4).

The understaging of adenocarcinoma of the prostate in this series by MRI was due to many factors (Table 2). On re-review of all nine cases where MRI failed to show advanced disease (adenocarcinoma of the prostate spread beyond the capsule) on the initial reading, the following errors in interpre-

114 M.L. Schiebler et al.: DRE, US, and MRI: Staging Prostate Adenocarcinoma

A

/

Fig. 1. Ultrasound postbiopsy true-negative/MRI postbiopsy false-positive. A Transrectal ultrasound angled to the right lateral margin of the gland showing preservation of the pericapsular fat (thin arrows). Subtle hypoechoic nodule in right peripheral zone (open arrow). B Body coil MRI (SE 2500/80, 5-mm thick, 205 x 256, 2 AVE) showing an apparent deformity and loss of the normal capsule at 7-8 o'clock against the rectum (arrow). This area showed only subtle evidence for methemoglobin on the short TR/TE images in this same region.

tation were discovered (see Table 2): subtle disrup- tion of the neurovascular bundle at the insertion of the periprostatic venous plexus, asymmetric or symmetric obstruction of the seminal vesicles without low signal intensity on the long TR/TE images, microscopic transcapsular invasion seen only at his- topathology, and confusion with the postbiopsy changes (Fig. 1). Thus, our errors were not related to one type of misinterpretation.

This series had five cases that were correctly upstaged to advanced disease (stage C or D) by the MRI study prospectively when compared to the endorectal ultrasound interpretation. The reasons for the body coil MRI's success in these patients were investigated retrospectively. In three cases, the major MRI finding of advanced disease was seminal vesical invasion (Fig. 2), in one case the periprostatic venous plexus was gone at the neurovascular bundle (Fig. 3) and in the last case the anterior edge of the periprostatic fat was invaded by tumor (best seen on the SE 600/20 images). In two other cases, more accurate staging by body coil MRI was possible because pelvic lymphadenopathy was found. These two cases had both been called stage C by endorectal ultrasound, so for the purposes of determining whether or not disease was confined vs. unconfined, endorectal ultrasound was still accurate. In this se-

ries the comparative strengths of MRI were in determining tumoral involvement of the seminal vesicles and whether or not spread to the pelvic lymph nodes had occurred. Evaluation of capsular penetration by body coil MRI was unreliable.

In this series endorectal ultrasound did a fair job evaluating for nodular disease of the capsule from approximately 2 o'clock through 10 o'clock in the transverse plane of the prostate and it also was able to evaluate the seminal vesicle/prostatic angle on the sagittal images. Disease anterior to the in- terior fibrous band was more difficult to see by endorectal ultrasound.

There were three cases in which the endorectal ultrasound correctly showed advanced disease while the body coil MRI was read as disease confined to the gland. In all three cases the ultrasound showed capsular and periprostatic fat deformity indicative of locally advanced disease. MRI in these cases failed to show signs of these changes, possibly due to problems with the slice thickness and volume averaging.

Problems in Staging Adenocarcinoma of the Prostate

The R D O G study suggests that the future of adenocarcinoma of the prostate staging will be in the hands of MRI [12]. The following discussion will highlight staging difficulties of adenocarcinoma of the prostate that both body coil MRI and endorectal ultrasound had in this series, with the primary em- phasis on body coil MRI.

Gland Heterogeneity

There are many difficulties in accurately staging adenocarcinoma of the prostate. The first problem is


A

Fig. 2. False-negative ultrasound for seminal vesicle involvement/true-positive MRI for seminal vesicle involvement. A Sag- ittal ultrasound of the right seminal vesicle (arrow) which in retrospect shows invasion of the base of the seminal vesicle (open arrow) not noticed on primary reading. B Transaxial MRI (SE 2500/80, 5-mm thick, 205 x 256 matrix, 2 AVE) showing low signal intensity of both of the seminal vesicles indicative of tumoral involvement (white arrow).

the gland itself and its heterogeneity in the central zone, where most benign prostate hypertrophy (BPH) occurs. The MRI appearances of BPH are extremely variable. From in vitro studies we know that fibro- muscular BPH can have the same signal character- istics as tumor [16]. We suspect that in vivo, tumor can simulate benign disease on MR imaging and benign disease can mimic tumor; this is especially true for the central zone. Ultrasound experiences similar difficulties distinguishing adenocarcinoma of the prostate nodules from benign hyperplasia in the central zone. In addition, iatrogenic changes in- duced by biopsy can interfere with the determina- tion of capsular penetration. Then one has to deal with the unpredictable placement of microscopic foci of carcinoma, which can be multifocal and/or nodular, and in either the central zone or peripheral zone. These are never going to be seen by a macroscopic imaging modality.

A second major factor in the difficulty of staging adenocarcinoma of the prostate is the expected in- terobserver variability that is always in existence, even in radiologists of similar levels of experience. For both endorectal ultrasound and body coil MRI of the prostate there will be differences in the per- ceptual thresholds for each observer as to what indeed constitutes an abnormal area. Most published reports of the staging specificity and sensitivity for adenocarcinoma of the prostate are from academic centers specializing in prostatic imaging and resec- tions for adenocarcinoma of the prostate. The added

variability introduced by having radiologists not as practiced in the interpretation of these images has not even begun to be addressed. Thus, the high sensitivity and specificity for staging adenocarcinoma of the prostate by endorectal ultrasound and body coil MRI may indeed be limited to a few very practiced observers and may not be applicable to other, less experienced observers.

Technical Problems

There were six main technical issues that may have contributed to MRI's poor performance in this study: (1) field of view, (2) no endorectal coil, (3) slice thickness, (4) time, (5) imaging planes, and (6) no fat saturation techniques.

The field of view was large in this series (300- 350 mm) depending upon patient size. The larger the field of view, the less in-plane resolution one has, and thus the ability to see subtle lesion decreas- es.

In order to increase in-plane resolution with smaller fields of view without losing signal-to-noise one needs to use an endorectal coil, which was not used in this series. With the increase in signal-to- noise obtained from this probe, one should routinely be able to use 3-mm slices with (200-160 mm) fields of view. This increase in the in-plane resolution should improve upon the detection of subtle lesions.

A major problem in staging adenocarcinoma of the prostate by body coil MRI is the large slice thickness resulting in some volume averaging. The in- plane resolution of body coil MRI with a 256-mm field-of-view and 256 × 256 matrix is slightly less than 1 mm/pixel. Can we really expect ourselves with a 5-mm thick imaging voxel, skipping 1 mm, to be able to assess 1-2 mm transcapsular of disease? This amount of penetration, however, is quite ab-


Fig. 3. Negative ultrasound/positive body coil MRI for the periprostatic venous plexus. A Transverse endorectal ultrasound of the left peripheral zone nodule (fat arrow) in the peripheral zone without evidence for spread into the surrounding fat (long arrow). B Body coil MRI (SE 2500/80, 5-mm thick, 205 x 256, 2 AVE) showing a large low signal intensity nodule in the left peripheral zone (long white arrow) with loss of the left periprostatic venous plexus at the neurovascular bundle (curved white arrow).

normal for the pathologist. To help illustrate the problem of volume averaging on body coil MRI, consider the following example. An adenocarcinoma of the prostate nodule with 2 mm of capsular penetration could be half in and half out of the imaging voxel. This is because the 2-ram nodule could be straddling the imaging voxel and the skip area of 1 mm so that only 1 mm of abnormal capsule (penetrated by 2 mm) would be in the voxel. This abnormal area of capsule would be volume averaged away and not appreciated on the final image. With the large slice thicknesses in this series there was a significant problem with volume averaging. The use of thinner slices may have improved upon our ability to see smaller lesions.

Imaging time is a major factor in MRI of the prostate. Ideally, T2-weighted images in three planes (axial, coronal, and sagittal) would be useful along with T 1-weighted 3-mm transaxial slices of the gland. These sequences are only performed after imaging the retroperitoneum for lymph nodes with T1- weighted spin-echo sequences. It is not unusual for our patients to spend 1 t/2 h in the magnet. This severely tests any cooperative patient's ability to stay still. Many of these men have some form of bladder outlet obstruction anyway, increasing their postvoid residual. So unless they have a Foley cath- eter, they often need to void about halfway through the examination. A patient with a full bladder can- not hold as still as a patient without a full bladder.

New rapid spin-echo techniques may allow for

a three-dimensional T2-weighted volumetric ac- quisition of the prostate. This data set could then be analyzed on a computer work station for a dy- namic real-time multiplanar examination of this complex organ with so much internal architectural heterogeneity.

The last main technical problem with the way the MRIs were performed in this study relates to pulse sequences. Some centers advocate using fat suppression techniques to help in the evaluation of the periprostatic fat and true prostatic capsule. More work is needed to definitively establish the role, if any, that these types of pulse sequences should play in prostate imaging.

Pathology

Another major difficulty in these imaging studies dealing with staging adenocarcinoma of the prostate is microscopic disease and trying to accurately sort out the stage B-2 lesions from the early C-1 lesions. Body coil MRI is being held to a very high standard in this analysis (e.g. surgical pathology at 40--400 × magnification) and it fails. Microscopic disease will never be seen by a macroscopic imaging modality. This fact was clearly supported by the R D O G study as well [12]. Transcapsular penetration anywhere along the gland surface of more than 0.5 mm, however, must be called stage C-1 disease by the pathologist. Although our pathologists did not define the depth of transcapsular invasion in millimeters by adenocarcinoma of the prostate, we suggest that body coil MRI with 0.5-cm thick slices should be held accountable for adenoeareinoma of the prostate in the seminal vesicle lumen, lymphadenopathy of 0.5 cm or greater, and nodules with capsular invasion of more than 0.4 cm in size in the peripheral zone. We have found that detection of more subtle disease is unreliable. Some centers use sagittal and


coronal T2-weighted images to supplement the axial images. This adds time to the study, but may also help in determining subtle transcapsular disease.

Role of Radical Retropubic Prostatectomy in Minimal Disease

The role of radical retropubic prostatectomy in the setting of minimal tumoral spread beyond the capsule (without positive lymph nodes) remains to be proven. Clearly, an important percentage of patients have already had radical retropubic prostatectomy with positive surgical margins and negative lymph nodes (minimal disease). In our series, 9 of 25 patients (36%) fell into this group (e.g. false-negative on their imaging tests). Perhaps these radical retropubic prostatectomy patients with minimal disease will ultimately have similar life expectancies as those patients with adenocarcinoma of the prostate confined to the gland (negative surgical margins). How- ever, a useful outcome analysis proving this true will take many years, and would probably best be accomplished with a cooperative study.

Difficulties in Comparing Staging Studies

There are tremendous problems encountered when trying to compare one prostatic staging paper with another one even when the same imaging modality was used. From Table 3, it can be seen that the disease prevalence varied from 35-78%, and thus the patient populations studied were different. Also, the number of patients in most studies has been small making them less statistically valid. Aside from disease prevalence issues, variability in the imaging criteria used for determining extracapsular disease probably also varied between readers.

The equipment varied for each one of these studies as well. Higher resolution transducers have only been available for general use from some man- ufacturers since 1988. One of the MRI staging pa- pers only used a 0.5 T magnet. Prostate images obtained at 0.5 T are inferior to those that are obtained at 1.5 T.

It is very difficult to assess accuracy in these studies, because each used a different method to correlate the imaging study to the pathology. The recent R D O G study has come closest to standard- izing all of these problems.

Prevalence of Extracapsular Disease

Patient selection prior to radical retropubic prostatectomy is a key variable in these types of staging

studies. Some centers are better than others at weed- ing out those patients with capsular penetration by adenocarcinoma of the prostate prior to their radical retropubic prostatectomy; as shown in Table 3, disease prevalence varies from a low of 32% [11] to a high of 78% [1]. Any reader, given the same accuracy, is going to necessarily have a higher overall number of false-negative readings as the disease prevalence in a series increases. Conversely, his/her number of false-negative readings will decrease as the overall possible number of positive cases is smaller. This is just one of the many reasons that the staging series are not directly comparable.

Conclusions

The importance of this small series is that it shows that: (1) published radical retropubic prostatectomy series are not alike in their prevalence for extracapsular adenocarcinoma of the prostate, possibly due to differences in patient selection; (2) different centers are not alike in their ability to prospectively stage adenocarcinoma of the prostate by body coil MRI or endorectal ultrasound; (3) different centers are not alike in the methods they used to determine extracapsular disease by imaging the modalities of ultrasound or MRI; (4) for our center, staging adenocarcinoma of the prostate prior to radical retropubic prostatectomy remains quite difficult and is almost a chance guess; (5) a primary reason for this difficulty in staging is microscopic disease, which has no imaging correlates; and (6) the degree of sensitivity and specificity obtained in the R D O G trial was not duplicated in this series, and may not necessarily be directly applicable to other centers.

With the introduction of the endorectal MRI coil [17] and as our team becomes more familiar with the strengths and weaknesses of each part of the staging workup, better care will ultimately be given to those patients under consideration for a radical retropubic prostatectomy. Due to the mul- tidisciplinary nature of the preoperative assessment of adenocarcinoma of the prostate and evaluation of the final histopathologic specimen, future work at this institution will involve closer cooperation between urologists, radiologists, and pathologists.

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comparison of the digital rectal examination, endorectal ultrasound, and body coil magnetic...

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