Blue Tiles

Helical Tomotherapy
Sterzing, Schubert, Sroka-Perez, Kalz, Debus, Herfarth. Strahlentherapie und Onkologie. 2008 Jan 11;184(1):8-14.

Presenter: Dr Santam ChakrabortyDate: 21.02.08

Introduction

Tomotherapy is a method of radiation therapy using a rotating x-ray beam source.

Amalgam of the words: (Mackie,1993)

Tomography

Radiation Therapy

Tomotherapy in it's serial format preceded MLC based IMRT (1994)

One of basic driving rationale for Tomotherapy is it's coupling with a superior IGRT technology.

Tomography: A sectional image is taken through a body by moving an X-ray source and the film in opposite directions during the exposure. Consequently, structures in the focal plane appear sharper, while structures in other planes appear blurred. By modifying the direction and extent of the movement, operators can select different focal planes which contain the structures of interest.

Need for Image Guidance

IMRT and other conformal techniques prone to geographic errors

Significant changes in patient anatomy during treatment have been demonstrated

Intrafraction organ motion and deformation errors create further uncertainties

Advanced volumetric imaging techniques allow quantification of these errors basis of IGRT

Overview of IGRT Techniques

IGRT Panacea or Problem ?

Need for IGRT spurred the development for more advanced image guidance systems

However flip side is:

Cost of technology

Cost of time

Cost of personnel training

Cost of additional QA and maintainence

Present study attempts to answer some of these questions.

Patients and Methods

Study Design

Retrospective chart review

Study period: July 2006 and May 2007

Number of patients: 150

Reasons for the use of tomotherapy:

Complex tumor geometry and proximity of organs at risk

Need for image guidance when immobilization was problematic or interfraction variations were to be minimized

Site Distribution

Tumor Types Treated

Nine patients were treated with single-fraction radiosurgery (liver n = 6, lung n = 3).

12 patients were treated for multiple lesions in one procedure.

Radiotherapy Planning

Done on dedicated workstation by dosimetrist and oncologist software (Siemens)

Inverse Planning: Tomotherapy planning station v 2.2.1.55 (TomoTherapy Inc. Madison, US)

4D CT scans obtained for thoracic and abdominal tumors where respiratory motion was felt to be a problem

Plan verification:

Film and ionization chamber based.

Integrated verification software provided in TPS

Important Clarifications missing here are:

Use of other imaging modalities and availability of fusion software.

Image registration accuracy of software for CT scans with other imaging modalities.

Why a separate contouring software was needed?

DICOM connectivity with other Treatment planning and image acquisition software.

Immobilization

Individualized devices were used

Cranial Leisons:

Scotch-cast masks = 18

Soft thermoplastic masks = 11

Other sites:

Combined mask and vacuum bag = 60

Vacuum bag only = 31

Pillow for legs and feet = 30

3 point isocenter localization technique used.

MV CT Imaging

Pretreatment MV-CT in 98.2% in 3,026 fractions

Exception: Patients immobilized by scotch cast masks deviations < 1mm Imaged weekly (N = 18)

Length of CT scan: Variable

Image registration with KV-CT :

Algorithm based fusion:

Bony landmark based

Soft tissue based

Manual Correction:

50% of fractions: Range of 1mm ( < 5% for 3mm)

Details regarding scan procedure is missing a vital part of IGRT technique?Performed by therapists who needed training supervision by oncologist / physicist not clarified?Not given information regarding the criteria for selecting the technique of image registration and the point at which manual correction was used?Who did the dose distribution analysis prior to and after the correction if the therapists analyzed the dose distributions what criteria were used to deem the correction acceptable?

Setup Correction

Dose distribution to OAR and target analyzed on MV-CT scans

Translations corrected by couch movement directly

Roll correction : Adjusting the gantry start position.

Vector for correction:

Dose distribution analysis possible after re-contouring in the MV CT scan.

Using the ICRU quadratic formula as given in ICRU report 64

Results

Time Effort

Fast introduction: 20 patients per day after 4 weeks

Average on table time 24.8 min

Average treatment time 10.7 min

Time required for image guidance remained constant

Mean Treatment time: 11.3 min vs 10.6 months after 30 and 100 patients respectively

Mean On table time: 25 min and 24.6 min after 30 and 100 patients respectively.

Essentially same time for image guidance means realistic gains in time should not be expected beyond the initial learning curve,However image guidance takes 9 -10 minutes of additional time.

Time Effort

Mean values given and these tend to be influenced by the extremes also no standard deviations mentioned. - implication in not knowing the range of values encountered.

Despite the higher correction vector for prostate the treatment time was actually shorter points to treatment of a smaller ROI and with less complex intensity modulated profiles.

Average number of fractions delivered to prostate and head and neck cancer patients was 24 -25 what was the dose schedule used?

In breast was respiratory gating used?

Time Effort

Mean on table time and treatment time did not vary significantly for the patients with multiple targets and single targets.Patients undergoing radiosurgery had a treatment time that was almost 4 times more than the conventional treatment. But the time of 48 min is comparable to stereotactic radiosurgery done with cyberknife for extracranial targets.

Imaging and Position Accuracy

Prostate cancer patients are detailed.

In 7.7% fractions patients had detected rectal filling that impaired treatment delivery significantly.

Rotational setup variations that could have lead to increased dose to organs at risk required correction in 6 patients.

Mean detected correction vector 6.9 mm

< 5 mm in 833 fractions (27.5% of fractions)

5 mm in 2193 fractions (72.5% of fractions)

Discussion

Major Advantages highlighted

Applicable where highly conformal dose distributions are required.

Also considered useful for long segment and multiple target involvement or in targets in close proximity to critical organs

Image guidance for precise treatment of difficult targets in difficult patients.

MV-CT advantage in reducing bony targets

Possibility of dose guided radiotherapy

Critiques

A retrospective study with a highly heterogeneous population.

No worthwhile statistical considerations can be derived

In essence a descriptive study

However the importance of the study is that it helps the reader to go through one of the largest published working experience with helical tomotherapy.

Allows us to gain a quick insight into the pitfalls and benefits of this technology

Design Characteristics

Tomotherapy

Tomotherapy essentially means slice therapy radiation is delivered in a thin fan beam configuration

Tomotherapy was the earliest type of functional IMRT system available.

Two types:

Serial (NOMOS corp): The delivery of multiple fan beams with discrete table increments between each axial gantry arc.

Helical (Tomotherapy Inc.): Continuous synchronized gantry and table motion. From the patients point of view, the source describes a helical trajectory.

Design Principles

Designed around a ring gantry similar to a helical CT scanner.

Non coplanar treatments ruled out

Use of IMRT obviates need for non coplanar treatment.

Ring gantry maintains its isocenter to tens of microns as compared to a millimeter diameter on the best C-arm Linac gantries1.

Nominal SAD = 85 cm

1 6 rotations per minutes

Jeswani S, Mackie TR, Aoyama H. Overview of the HI-ART TM Helical Tomotherapy System.

1 . Jeraj R, Mackie TR, Balog J, Olivera G, Pearson D, Kapatoes J, et al. Radiation characteristics of helical tomotherapy. Med. Phys. 2004 Feb 0;31(2):396-404.

Machine Interior

Fenwick JD, Tome WA, Soisson ET, Mehta MP, Rock Mackie T. Tomotherapy and Other Innovative IMRT Delivery Systems. Seminars in Radiation Oncology. 2006 Oct ;16(4):199-208.

LINAC Design

Standing wave S band LINAC 2 operating energies1

6 MV photon beam output for treatment

3.5 MV photon for imaging

No flattening filter

Output increased to 8 Gy/min1 at center of bore 2 times that of periphery

The beam energy spectrum is more constant (< 5%)

There is less scatter contamination.

However a beam hardener and electron stopper is provided

1 . Jeraj R, Mackie TR, Balog J, Olivera G, Pearson D, Kapatoes J, et al. Radiation characteristics of helical tomotherapy. Med. Phys. 2004 Feb 0;31(2):396-404.

Jaw Characteristics

23 cm of 95% tungsten shielding is used in the linac support fixture and combination of primary collimator and jaws.

The average leakage from head is 0.01%.

Independent Y jaws have been provided field width 1 5 cm at the isocenter.

Output in the fan beam drops dramatically below 1 cm due to loss of lateral electron equilibrium and partial source occlusion

A primary beam stop precludes need for a primary barrier

Main scatter is from the beam itself and minimal scatter from the head.

Machine Head configuration

Beam Characteristics: Projection

For treatment delivery the full rotation is divided into 51 projections.

Each projection is characterized by it's own leaf opening and closing pattern

Each projection covers an arc segment of 7.

Between each projections all leaves are closed for a short period of time highly segmented step and shoot approach.

Constant dose rate for the LINAC assumed but monitor chambers have inbuilt safety interlocks

Beam Characteristics: Pitch

The rotational fan beams overlap with each point seeing from 2 to 5 rotations or about 100 to 250 possible beamlets.

Three fan beam widths used 1, 2.5 and 5 cm.

Pitch of the delivery: Defined as the fraction of the beam width that the couch translates (moves in or out) during each rotation of the gantry

The pitch can be defined and helps in offsetting the threading effect.

Typical values for the pitch are 0.25 - 0.5

Binary MLC Characteristics

Binary MLCs are provided 2 positions open or closed

Pneumatically driven 64 leaves

Open-close time of 20 ms

Width 6.25 mm at isocenter

10 cm thick

Interleaf transmission 0.5% in field and 0.25% out field

Maximum FOV = 40 cm

LINAC

Cone Beam

Y jaw

Y jaw

Fan Beam

Binary MLC

Couch Characteristics

Flat Couch provided allows automatic translations during treatment

Target Length long as 160 cm can be treated

Cobra action of the couch limits the length treatable

Manual lateral couch translations possible

Automatic longitudinal and vertical motions possible

Possible to treat anywhere within a cylindrical volume 40 cm in diameter by 160 cm long

Workstation

Includes:

An operator station

Planning station

32 CPU computer cluster attached to database server

Treatment machine

Parallel processor architecture for optimization of thousands of beamlets involved

System offers no contouring tools rather contours have to imported from other TPS

Shielding Requirements

A tomotherapy primary beam shield is:

Reduced in width by a factor of almost 10

Increased in thickness by more than a tenth value layer in comparison to a conventional accelerator.

Furthermore, the secondary shielding requirements are enhanced by more than two tenth value layers with respect to conventional shielding demands.

However primary beam stopper included significantly reduces room shielding requirements.

Balog J, Lucas D, DeSouza C, Crilly R. Helical tomotherapy radiation leakage and shielding considerations. Med. Phys. 2005 Mar 0;32(3):710-719.

Planning Process

Planning process

3 D contouring done on treatment planning system any DICOM-RT compatible TPS is capable for contouring

Contoured CT images pushed to TPS

Three factors are predefined before starting the calculations:

Pitch

Modulation Factor: Is the ratio of maximum to the mean leaf opening time for all non-zero leaf opening values per projection.

Field width: Describes the fixed width of field 1, 2.5 or 5 cm

At a modulation factor of 1, the only modulation that the leaves can make is open at the same amount of time (or arc length) as all the other open leaves or the leaves are closed. At the highest level of 5, this factor is set to allow for the most flexibility in the optimizer possible, but the optimizer must work harder to find the best solution

Planning Process

The number of projections are fixed at 51 and all projections are utilized.

Using a small field size, smaller pitch and larger modulation factor lead to more conformal plans

Two sets of calculations are done:

Precalculation: More time consuming but an automatic overnight process.

Basic calculations describing the beamlets and their interaction with the patient are computed.

Optimization: Similar to the process of conventional IMRT where constraints and doses are prescribed using data from the precalculation stage. Less time consuming.

Planning Process

One important aspect of the TPS (v2.0) is that the system doesn't allow voxel overlap between two organs of the same type eg. two types of tumor volumes or two types of sensitive organs.

Where two organs of same type do overlap the dose volume information of the organ with higher priority will be displayed.

PTV 1

PTV 2

If priority of PTV 1 is less than PTV 2 then its DVH wont be computed at all !!!

If PTV 2 has a lesser priority but remains an overlapping organ for DVH calculation, volume inside PTV1 wont be taken into DVH calculation

Saibishkumar EP, Jha N, Scrimger RA, MacKenzie MA, Daly H, Field C, et al. Sparing the parotid glands and surgically transferred submandibular gland with helical tomotherapy in post-operative radiation of head and neck cancer: A planning study. Radiotherapy and Oncology. 2007 Oct ;85(1):98-104.

Generated H&N Plan

Generated Plans

Cranio-caudal Penumbra

As radiation starts during patient motion when the target reaches the lower end of the fan beam there is increased dose penumbra in the cranio-caudal region

This can be detrimental in situations where critical organs lie in close proximity in this direction e.g. nasophayrnx

Efforts on way to reduce this effect by primary jaw shielding for the start of the treatment.

Bauman G, Yartsev S, Rodrigues G, Lewis C, Venkatesan VM, Yu E, et al. A Prospective Evaluation of Helical Tomotherapy. International Journal of Radiation Oncology*Biology*Physics. 2007 Jun 1;68(2):632-641.

MV-CT Imaging and Adaptive RT

Imager Characteristics

Arc-shaped xenon detector has 738 channels, each with two ionization cavities filled with xenon gas and divided by 0.32 mm tungsten septa.

The detector array has a 110 cm radius of curvature

540 out of 738 channels are used for the MVCT image reconstruction.

The source to detector distance is 145 cm.

Majority of photon beam interacts with tungsten septae which also prevent detector cross talk greater effeciency

FOV = 40 cm

Maximum number of slices = 80

Image Characteristics

Allows higher image resolution than cone beam MV CT (3 cm diameter with 3% contrast difference)

Tissue heterogeneity calculations can be done reliably on the CT images as scatter is less (HU more reliable per pixel)

Not affected by High Z materials (implant)

Dose 0.3 3 Gy depending on slice thickness

3 imaging modes: Coarse (12 mm), Normal (8 mm) and fine (4 mm) thickness.

Image Characteristics

FOV of 40 cm available in the tomotherapy MVCT system may lead to a degradation of image quality because the tissue outside the FOV is not properly accounted for in the reconstruction process

Typical result is bowl artifacts - the reconstructed CT values are increased in the peripheral regions of the images

However sufficient information for checking setup even if the MV-CT image width is half that of the patient thickness.

Matching of images

Two types of matching algorithms:

Automatic mutual information algorithm

Manual translations and rotations

Calculated couch shifts adjusted automatically except for lateral translations

Limit of manual lateral adjustment = 2.5 cm

Roll correction possible adjust gantry start position

MV-CT Images

Cone beam CT vs Tomo MV CT

Cone beam CT on C-arm gantry designs restricted to 1 rotation per second due to collisional considerations

In Tomotherapy CT length of 1.2 cm can be imaged in 10 sec or 1 rotation

Acquisition occurs automatically

In KV cone beam CT takes 1.7 min to acquire 285 projections and few minutes to reconstruct

However, this effort would yield 256 slices, almost twice as many slices per minute as the tomotherapy unit.

Adaptive Planning process

3 categories of dose data are required for adaptive treatment

Dose Prediction: Display of changed dose due to changes in setup and anatomy from fraction to fraction it is a pretreatment verification process and an online process.

Dose Verification: Uses the MV CT to verify the dose distribution it is an offline process so can incorporate data acquired during the treatment.

Dose Reconstruction: This process includes determination of the actual dose received at the time of treatment done from sinogram data acquired by the Xe detectors that work at the time of treatment it is again an offline process.

ART: Concept

Conventional Rx

Sample Population based margins

Accommodates variations of setup for the populations

No or infrequent imaging

Largest margin

Offline ART

Individual patient based margins

Frequent imaging of patients

Estimated systemic error corrected based on repeated measurements

A small margin kept for random error

Plans adapted to average changes

Online ART

Individual patient based margins

Daily imaging of patients

Daily error corrected prior to the treatment

Smallest margin required

Plans adapted to the changing anatomy daily!

1.

2.

3.

ART: Why ?

Due to a change in the contours (e.g. Weight Loss) the actual dose received by the organ can vary significantly from the planned dose despite accurate setup and lack of motion.

Importance: Adaptive RT

Without replanning in Head and neck cancer:

Dose to 95% target volume reduces in 92% patients

Magnitude of dose reduction: 0.2 7.4 Gy

Spinal cord dmax increases in all patients

Magnitude of increase: 0.2-15.4 Gy

Brain stem dmax increased in 82% patients.

Magnitude of increased by 0.6-8.1 Gy

Hansen EK, Bucci MK, Quivey JM, Weinberg V, Xia P. Repeat CT imaging and replanning during the course of IMRT for head-and-neck cancer. Int J Radiat Oncol Biol Phys. 2006 Feb 1;64(2):355-62.

ART: Problem

Real time adaptive RT is not possible today

The treatment to an imaged tumor position can only be delivered after a certain period of time in which the image is processed.The time for image processing and the signal processing for MLC / machine movement is unavoidable.Thus actual real time Adaptive radiotherapy is not possible.

ART: Steps..

ART: Steps

Clinical Planning and Treatment Experience

Atypical Treatment Planning

Some of the atypical radiation techniques that have been planned using HT are:

Craniospinal irradiation

Total marrow irradiation

Total Lymphoid irradiation

Total Marrow Irradiation

TMI to 10 Gy was delivered as part of a tandem transplant regimen using HT to the 53-year-old patient with multiple myeloma

After TMI, the patient experienced the expected blood count nadir, followed by successful engraftment

Grade 2 nausea and grade 1 emesis occurred only briefly on day 2 of TMI

Skin erythema, oral mucositis, esophagitis, and enteritis were not observed

Organ doses were substantially lower than those associated with standard TBI

TMI and TMLI (Total Marrow & Lymphoid irradiation) potential alternative to TBI and targeted radio-immunotherapy

Wong JYC, Liu A, Schultheiss T, Popplewell L, Stein A, Rosenthal J, et al. Targeted total marrow irradiation using three-dimensional image-guided tomographic intensity-modulated radiation therapy: an alternative to standard total body irradiation. Biol Blood Marrow Transplant. 2006 Mar ;12(3):306-15.

Craniospinal Radiation

Penagaricano JA, Papanikolaou N, Yan Y, Youssef E, Ratanatharathorn V. Feasibility of cranio-spinal axis radiation with the Hi-Art tomotherapy system. Radiotherapy and Oncology. 2005 Jul ;76(1):72-78.

Results: When considering D50% and D10%, CSA-TOMO has a dosimetric advantage over CSA-RT for most organs at risk. The body integral dose was higher for the CSA-TOMO plan by approximately 6.5%.

Dosimetric Comparison

Fiorino C, Dell'Oca I, Pierelli A, Broggi S, Martin ED, Muzio ND, et al. Significant improvement in normal tissue sparing and target coverage for head and neck cancer by means of helical tomotherapy. Radiotherapy and Oncology. 2006 Mar ;78(3):276-282.

Dosimetric Comparison

Fiorino C, Dell'Oca I, Pierelli A, Broggi S, Martin ED, Muzio ND, et al. Significant improvement in normal tissue sparing and target coverage for head and neck cancer by means of helical tomotherapy. Radiotherapy and Oncology. 2006 Mar ;78(3):276-282.

Clinical Results: Bauman et al

Bauman G, Yartsev S, Rodrigues G, Lewis C, Venkatesan VM, Yu E, et al. A Prospective Evaluation of Helical Tomotherapy. International Journal of Radiation Oncology*Biology*Physics. 2007 Jun 1;68(2):632-641.

Clinical results: Bauman et al

In 61 patients treated with HT the plan was deemed acceptable in 80%

11 patients could not complete 80% of their treatment with HT due to machine downtime

Approximately 90% of the planned fractions could be delivered with HT

Median times:

Positioning: 20 minutes (11 82 min)

Treatment: 6 minutes (2 68 min)

Median overall treatment time similar

78% patients were satisfied or very satisfied - Patients were least satisfied with treatment duration.

Clinical Results: Bauman et al

OAR DVH metrics:

A total of 566 clinically relevant OAR DVH points compared

In 59% of these comparisons, HT OAR metrics were as good as or better than the 3D-CRT metric

In 41% 3DCRT was better however improvement minimal to the tune of 1 2 Gy in most cases

HT better than 3DCRT for:

53% of head/neck OAR comparisons

55% of thoracic OAR comparisons

72% of abdomen/pelvis OAR comparisons.

Non coplanar treatment

10 patients with skull base tumors stereotactic radiotherapy planned 50-50.4 Gy

HT compared with noncoplanar IMRT

PITV ratio reduced with LINAC based plans 2.22 for HT vs 1.44 for LINAC

Inhomogeneity Index was higher as compared to LINAC based plans (0.1 vs 0.08)

13% - 540% increase in low dose isodose volumes with HT

Where the disease spread inferiorly HT was considered superior.

Soisson ET, Tome WA, Richards GM, Mehta MP. Comparison of linac based fractionated stereotactic radiotherapy and tomotherapy treatment plans for skull-base tumors. Radiotherapy and Oncology. 2006 Mar ;78(3):313-321.

Concerns with HT

4 major concerns identified by Bauman and Sterzing et al in their series were:

Increase in the integral dose to normal tissues due to the rotational arc nature of the treatment

Increased penumbra in the cranio-caudal direction due to inherent problem with machine design

In certain situations noncoplanar beam arrangements may give potentially superior results.

Extra dose of the order of 0.62 cGy per MV-CT depending on the chosen pitch can lead to increased dose to normal tissues with unknown consequences.

Conclusions

Helical tomotherapy is a new paradigm at delivering megavoltage radiation therapy

Machine designed with express purpose of delivering Image guided IMRT

Cost benefit remains to be determined

Several issues remain of concern

Novel treatment techniques now possible

New advances may render present technology obsolete:

Volumetric Modulated Arc Therapy - Varian

IMPT (Helical / C-arm based)

Cobalt Tomotherapy with integrated MRI (developmental)

Muokkaa otsikon tekstimuotoa napsauttamalla

Muokkaa jsennyksen tekstimuotoa napsauttamalla

Toinen jsennystaso

Kolmas jsennystaso

Neljs jsennystaso

Viides jsennystaso

Kuudes jsennystaso

Seitsems jsennystaso

Kahdeksas jsennystaso

Yhdekss jsennystaso

Journal Club: Helical Tomotherapy Sterzing et al. Strahlentherapie und Onkologie. Presenter Dr Santam Chakraborty

Site

CNS7

Head and Neck28

Thorax37

Abdomen58

Skeletal20

SiteTumor TypeNumberCNSChordoma2Meningioma2Ependymoma1Medulloblastoma1Head and NeckPharyngeal Cancers14AbdominalProstate28Esophageal Cancer2Gastric Cancer3Pancreatic Cancer5Anal Cancer2Cholangiocarcinoma4Gastric Lymphoma3Ovarian Cancer4Total71

???Page ??? (???)17/02/2008, 16:06:28Page / SiteTumor TypeNumberThoracicBreast17Lung Cancer3Pleural Mesothelioma5Other SitesLymphoma13Sarcomas8Skin Tumors3Metastatic SitesBone14Liver6Lung4Lymph Node2Other4Total79

???Page ??? (???)17/02/2008, 16:06:29Page / Column 1

Scotch casts18

Thermoplastic11

Cast + Bag60

Bag only31

Pillows30