Radiation Oncology, Physics, Image Guided Radiotherapy (IGRT)

ID: 000491 Approved:30 Aug 2011 Last Modified: 29 Oct 2012 Review Due:30 Aug 2012

Target Audience: This protocol is aimed at providing information on Image Guided Radiotherapy (IGRT) principles and practices for the following radiation oncology health professionals:

Radiation Oncologists Radiation Oncology Registrars Radiation Oncology Medical Physics Registrars Radiation Therapists

Overview This protocol is designed to be used as an educational resource and provide an overview to Image Guided Radiotherapy (IGRT) principles and practices. IGRT involves daily imaging and positioning intervention to accurately target the tumour.

IGRT delivery has developed quickly for some treatment sites. The inherent complexity of IGRT for each treatment site is variable and dependent on factors such as target dose, organs at risk (OAR) doses, stability of position and internal organ motion.

Advances in available technology, such as remote couch motion, and the capability for kV imaging on modern linear accelerators have facilitated IGRT development.

Naturally, as technology evolves so do the modalities to perform IGRT. In 2011, IGRT might be performed based on a range of imaging modalities including electronic portal imaging (EPID), megavoltage cone beam CT (MV­CBCT), kV images, kV­CBCT, ultrasound and respiratory motion sensors.

Key References: 1. TG­104 The Role of In­room kV X­Ray Imaging for patient setup and target localisation Dec 2009

2. TG­144 Klien E, Hanley J, Bayouth J, Yin F­F, Simon W, Dresser S, Serago C, Aguirre F, Ma L, Arjomandy B, Liu C. AAPM Task Group Report 144 Quality assurance of medical accelerators 2009

3. TG­101 Benedict S, Yenice K, Followill D, Galvin J, Hinson W, Kavanagh B, Keall P, Lovelock M, Meeks S, Papiez, Purdie T, Sadagopan R, Schell M, Salter B, Schiesinger D, Shiu A, Solberg T, Song D, Stieber V, Timerman R, Tome W, Verellen D, Wang L, Yin F­F. Stereotactic body radiation therapy: The report of AAPM Task group 101. MedPhys 37(8) 2010

4. TG­75 Murphy M, Balter J, Balter S, BenComo J, Das I, Jiang S, M C­M, Oliviera G, Rodebaugh R, Ruchala K, Shirato H, Yin F­F. The management of imaging dose during image­guided radiotherapy: Report of the AAPM Task Group 75 MedPhys 34(10) 2007

5. TG­111 AAPM Task Group 111 Report Comprehensive Methodology for the evaluation of Radiation Dose in X­Ray Computed Tomography: The Future of CT dosimetry Feb 2010

6. Seminars in Radiation Oncology IGRT special 2004. Vol 14, (1):1­100 7. IMRT, IGRT, SBRT ­ Advances in the Treatment planning and Delivery of

Radiotherapy. Ed John L Meyer. Karger publishing.

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IGRT Modalities

Imaging Modality Example of images1. Electronic Portal Imaging (EPI) ­ Most linear accelerators have an electronic imaging panel attached to capture treatment verification images using the treatment beam.

this device can be used to acquire radiographic of fluoroscopic images for patient alignment

the EPI will typically require 1­4 MU and will produce an image of quality high enough to visualise bones clearly

an image of the treatment portal is usually accompanied by an expanded field size image to allow visualisation of surrounding anatomy and the megavoltage (MV) imaging doses can be easily included in the planned dose in most treatment planning software

2. Megavoltage Cone Beam CT (MV­CBCT) ­ MV­CBCT has been implemented on Siemens linear accelerators utilising the treatment beam to acquire projections for reconstruction as a CBCT image.

the use of MV reduces streaking artefact for high Z materials that would be present with kV imaging, but lowers contrast for near­tissue materials and dose has been reported to be higher than kV­CBCT

it should be noted, that some linear accelerator (linac) manufacturers now offer a low MV (1­3MV) imaging beam that has a higher contrast than the 6MV beam

potentially, such a low MV beam might be synchronised between the 6MV treatment pulses for "live imaging" during a treatment and this would have various applications

typical 6MV dose range for MV­CBCT is 5­10cGy 1

3. Kilovoltage (kV) ­ Elekta and Varian linear accelerators can be purchased with additional X­ray tubes mounted on the linac gantry to produce a kV beam that projects orthogonally to the treatment beam to an imaging receptor mounted on the opposite side.

this device can be used to acquire radiographic or fluoroscopic images for patient alignment

the high image quality and minimal dose (of the order of 1/100 cGy) have made orthogonal kV imaging the current standard for IGRT

stand alone (room mounted) systems like Brainlab Exactrac® have floor and ceiling mounted x­ray systems that provide a true rooms­eye­view independent of the treatment system

one disadvantage of kV imaging is that it is difficult to incorporate the imaging related dose into planned treatment dose, due to the difference in biological effects between kV and MV beams

4. Kilovoltage Cone beam CT (KV­CBCT) ­ Varian and Elekta can provide a kV­CBCT option that acquires a CBCT from the extra x­ray generators.

these provide high quality image for positioning, however

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Residual accuracy of the IGRT system

While it is important to setup a thorough QA regimen to test all components of the IGRT process, there are residual uncertainties within the system that should also be carefully considered by your IGRT team. Residual uncertainties are covered by the PTV margin (encompassing the internal target volume (ITV) and will include:

mechanical reproducibility – flex in imaging panel and x­ray tube with gantry rotation, couch movement stepping and couch slop

contouring inaccuracy – different imaging modalities, window/level (W/L), SUVs, mean CT versus 4DCT intrafraction motion accuracy/reproducibility of surrogate

These may be difficult to address but must be considered by the team.

Image Registration

Increasingly, the planning CT image is being fused with other modalities (CBCT, MRI and PET) to better identify targets, and reduce contouring uncertainty. In some cases, the images acquired outside the radiotherapy department will not be acquired with the patient in the treatment position and this complicates the registration process. Most radiotherapy planning systems have rigid registration and many will have automated processes. However, deformable registration may be more useful in overcoming misregistration due to patient position and is particularly useful in adaptive radiotherapy (see below).

image quality is currently lower than helical CT and not good enough for diagnosis

the images are adequate for dose calculation in many situations, however this should only be attempted after careful investigation

significant artefact (mainly cupping artefact) and off axis hardening may cause problems for use of electron density calibration curves. Artefact from internal gas and bones may also obscure anatomy for alignment

images are approximately 15cm long with couch centred, due to separation and dimension of the imaging panel. This may result in partially missing large target volumes and organs at risk (OARs), which is important for studies where you need complete organs

the range of central doses from CBCT is variously quoted as 0.2 to10 cGy 2

5. Ultrasound ­ Ultrasound images provide a radiation free image.

however, the accuracy of positioning based on these images can be heavily user dependent 21 22

ultrasound is useful in delineation of soft tissue detail, and has limited utility in close proximity to bone

remote ultrasound devices to remove user induced imaging variations real time application are an area of active investigation

6. Respiratory/external motion sensors A number of devices are available to image or detect markers on the patient’s skin, thereby using external body contour as a surrogate for internal motion.

these devices can use infra­red to detect the position of markers placed on the patient relative to isocentre (Brainlab, Varian Optical Guidance Platform) or, could read the deformation of a grid projected onto the patient (Align­RT)

other systems, like Varian® Real­time Position Management™ (RPM), Elekta Active Breathing Coordinator™ and ANSAI belts are specifically designed to gate the treatment delivery or the image acquisition to account for breathing motion

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Autosegmentation tools will usually use gradient techniques to fit from a vendor or user library of structure contours. These should be checked to confirm that they match your department contour protocol or Clinical Trial protocol. Other segmentation tools can match by weighting a certain part of the image so that more emphasis is placed on accuracy in a defined region of the image.

CBCT Imaging Dose

The dose to the patient from a kV CBCT acquisition may be highly variable, and differ significantly from the vendors published values. It is important to have a good understanding of the metrics used by vendors to describe patient dose from CBCT and to understand the limits of that metric. Dose related quantities that might be used include:

central dose (CDTC) ­ (measured dose in Gy for a particular phantom geometry)

dose length product (DLP) ­ (dose in Gy m ie per length of field) computed tomography dose index (CTDI) and it’s varieties

measured in water CTDIw measured along a 100mm length CTDI100 along the phantom centrec peripheralp

Most of the vendors will publish a dose in cGy and a CTDIw; the former is a central dose point measured in a unit density cylindrical phantom and the second is a weighted average of measured dose centrally and peripherally. Both of these metrics were originally designed for helical CT and do demonstrate some shortcomings from CBCT as the off­axis beam quality hardens. Further detail on these quantities can be obtained from AAPM TG 111: the future of CT Dosimetry­ Feb 2010 3. See also Point/Counterpoint by Brenner 2006. 4

It is good practise to measure the dose in a range of scenarios with various phantom geometries locally. For new IGRT techniques it may be useful to calculate a CBCT dose factor as the ratio of dose measured in the particular geometry and technique to a standard geometry/technique. CBCT dose should be measured with an appropriate detector that will exhibit low energy dependence, high reproducibility, low stem effect (reading from detector stem in field), high signal/noise ratio, low dose rate effects. Be aware of varying dose rate, chamber volume and the atomic number of the material used for the ion chambers central electrode. Several studies have shown computer simulations (Monte Carlo modelling) of kV CBCT dose distributions 23,24. For some sites with small high or low density volumes (like pelvis), the kV dose distribution in the patient from CBCT has been reported to be fairly uniform. It might be reasonable in such cases to attribute a single dose value per CBCT for these patients (based on measurement in the centre of a cylindrical phantom of suitable dimension). The patient imaging dose for a CBCT acquisition will vary due to filters in the beam, aperture dimension, beam settings, geometry of subject, and density of internal structures. Some of these parameters can be exploited to reduce the dose, together with post processing.

Site specific IGRT

Prostate

Prostate tumours were an early proponent for IGRT because:

1. The prostate is a mobile organ (requiring daily targeting) 2. Fiducial markers are visible in EPID acquisitions (allowing targeting) 3. Improved targeting aids in dose escalation 4. Queries around the prostates sensitivity to fractionation (a/ß ratio) leading to the attraction of hypo­fractionation

The prostate will deform and move and so will the surrounding normal tissue. There have been many studies showing potential geographic misses from positioning based on bony matches. The current standard is daily orthogonal imaging, which is generally kV imaging. CBCT may provide further information on compliance with bladder/rectum protocols, patient contour variations and patient tilt. Fiducial markers are implanted using trans rectal ultrasound (TRUS) 1 week prior to simulation, allowing any swelling to reduce prior to CT simulation. Usually 3­4 markers (each ~1x3mm) are implanted for easy visualisation in pre­treatment images. Stability of these markers (non­migration) through a course of treatment has been established 19 20. Fiducial markers are now widely thought of as a standard of care for intact prostate RT. Markers may be accidentally implanted in the rectal wall or seminal vesicles (SV). These may be used for positioning if they remain in situ, but extreme care should be taken to identify the correct marker and some soft tissue reference might be useful. If a marker is placed in the SV's or rectal wall it should be clearly noted, so it can be ignored during online registration. Dose calculations may be performed on CBCT images in some situations and this will depend on the dose accuracy required. It

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is important to involve a Physicist in your IGRT team to investigate such potential situations before they occur. A dose calculation without homogeneity correction will consider any shape changes, but not density changes. A full dose calculation with a superposition algorithm will compensate for all contoured structures. As mentioned previously this will not account for any sudden intra­fraction anatomy changes. There are several methods to reduce prostate motion including:

rectal balloons – some evidence suggests improved stability of the prostate with rectal balloons. Studies have also shown reduced rectal dose due to expansion of rectal wall, variations in prostate tilt due to variable insertion, increased superior­inferior motion. However, patient comfort and tolerability needs to be considered 5

dietary – the patient diet can be modified, however there are mixed results in the literature about this improving prostate stability

bladder filling protocol – 'comfortably full' ask patient to drink fixed volume of water prior to simulation and treatment. It is recommended to assess compliance with pre­treatment imaging if possible ie: CBCT, ultrasound

rectal vacuum /enema – this can be used at simulation where a full rectum may cause a significant systematic error if not replicated through course of treatment

Auto registration algorithms work well for prostate fiducial markers and the pelvis generally. However, manual confirmation of the auto match is recommended. Autosegmentation of pelvic anatomy is now available through a number of vendors. This technology will present the newly acquired CBCT with contours when it first appears on screen. It is suggested that the accuracy will need to be thoroughly investigated prior to use. This technology holds great promise for the development of adaptive planning and IGRT.

Head and Neck

IGRT has been rapidly developed for head and neck radiotherapy because rapid tumour proliferation/shrinkage during treatment, and patient weight­loss due to poor diet induced by treatment/tumour side effects results in large changes in patient anatomy. These patients are generally well fixated and there is little intra­fraction variation (except for swallowing action). The IGRT imaging options that are best suited include;

matching to bone from orthogonal images matching to bone from CBCT matching to bone from CBCT with offline CBCT review to assess magnitude of shape changes, possibly leading to replanning

matching to bone with autosegmentation, auto registration and recalculation

If the patient shape change will cause a significant dose error on delivery then the plan may be replanned. Currently, changes noted on visual check of the patient by the radiation therapists during daily patient positioning or FSD check would indicate the need for a repeat planning CT. However, a number of other indices including: body contour variation, treatment DVH variation, point dose variation and normal tissue shape changes may also indicate the need for replanning. The replanning process usually takes about one to three days, incorporating independent checking/QA processes, so it can be incorporated quickly into the treatment.

Lung and Breast IGRT

Both lung and breast IGRT require management of respiratory motion. The options available include reducing the motion, gating delivery, accounting for motion in PTV expansion and tracking. Organ motion may be reduced by abdominal compression, breath hold and coaching of the patient, active breathing control and biofeedback technologies. Both of these sites are challenging for dose calculation (due to tangential beams, lung/bone interfaces, small fields) and generally have poor CBCT image quality. These issues remain an active area of investigation and will need to be resolved prior to effective implementation of high precision radiotherapy techniques for lung and breast radiotherapy.

Stereotactic Body Radiotherapy (SBRT) SBRT involves extra­cranial hypo­fractionated treatments (<5 fractions) that require the precision accuracy of IGRT. Like cranial stereotactic, the IGRT methods are trending away from the origins of using a stereotactic frame bolted to the patient towards image guidance. The need for image guidance accuracy is heightened for these treatments, as demonstrated in AAPM Report 14415 by a separate list of requirements for this modality compared against conventional radiotherapy. (See also AAPM TG101 SBRT16) The literature shows several sites currently being treated with SBRT including liver, lung, spine and prostate. Note that many of these publications report treatment with an Accuray® Cyberknife device, none of which are currently present in Australia. IGRT prerequisites for these include pre, during and post treatment imaging to verify the mean tumour position (and isocentre), and qualify any correlations for monitoring a tumour motion surrogate. The residual error in the IGRT system should be quantified as < 2mm. The radiobiology of hypo­ fractionation is poorly understood, hypo­fractionation offers a reduced number of fractions to ‘correct’ for any errors and often the targets are within mm of OARs. It is recommended to follow the guide of current clinical trials for SBRT. A limited list is provided below, noting that technical and clinical issues with different patient groups are highly distinct and individualized.

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Radiation Oncology, Physics, Image Guided Radiotherapy (IGRT)

Liver – RTOG 04­38 is a dose escalation study for liver metastases with 10 fractions of 3.5­5.0 Gy. Some of the IGRT issues specific to liver SBRT include:

liver is a parallel organ so does exhibit a volume effect. the liver is mobile and deforms during treatment liver lesions are difficult to see in treatment images suggesting that fiducial markers could be used.

Motion management techniques should be employed from planning CT onwards to accurately contour the liver and internal target volume (ITV). The treatment can be gated to respiratory motion with setup using fluoro or CBCT. Poulsen et al 2011 have implemented a fluoroscopic tracking technique to monitor the position of fiducials during treatment.

Lung ­ RTOG 0813 is a dose escalation study starting with 5x10Gy. RTOG 0915 compares 1x34Gy vs 4x12Gy. There is also a TROG Trial ­ TROG 0902 CHISEL comparing 3x20Gy against conventional 30x2Gy. Some of the IGRT issues specific to lung SBRT include:

requirement for a 4DCT dataset for planning; with an ITV created from the maximum intensity projection (MIP) and if treating free breathing, then the dose is calculated on the mean CT dataset.

option for gating a free­breathing treatment or treating during breath hold to reduce ITV and planned lung dose. there are a number of gating signals that could be used including a RPM marker block, Ansai belt, surface contour AlignRT or respiratory output. Exhale breathe hold is generally considered more stable than inhale, but may be difficult for the patient given that they have lung disease. The breathe hold can be guided by audio and/or visual assistance. The Automated Breathing Control (ABC) device will restrict breathing remotely.

treatment planning usually requires a 7­14 non­coplanar beam arrangement. However, there is literature investigating the role of Volumetric Modulated Arc therapy (VMAT). An early concern arose regarding interplay for dMLC delivery with breathing motion but the literature is not conclusive either way on this point. During treatment, the simulation setup will be reproduced and for free breathing, verified with multiple CBCT images during each fraction as there is evidence of base line shifting after about 6 minutes of treatment. Treatment time can be extensive due to the complexity of the delivery (many non coplanar beams, large MU, and careful setup).

Spine – An ablative dose is delivered to spine metastases avoiding the spinal cord. RTOG 0631 is comparing a single 16Gy fraction to the standard 8Gy fraction. Some of the IGRT issues specific to spine SBRT include:

highly accurate patient setup with the PTV often only 1­2mm from the spinal cord or its planning risk volume (PRV).

the use of an ablative dose in a single fraction should be differentiated from the existing spinal cord tolerance data on 2Gy fractions

Prostate ­ There is emerging evidence for prostate treatments of 35Gy ­45Gy in 5 fractions. The most mature data is from MSKCC and Stanford. Some IGRT issues specific to prostate SBRT include:

potential for rapid and erratic prostate movement intrafraction (evidences from Calypso data) requirement for intrafraction motion detection and action. Actions might include gating the treatment or tracking the couch/MLC to 'follow' the motion

most of the published data is from centres using the Accuray® Cyberknife device that has regular intrafraction imaging and tracking capability

QA of an IGRT system

There is guidance in three main references; Yoo et al 6, Verellen et al 7 and AAPM TG­134 (2009). The QA program will depend on the use and the system. The QA program will include mechanical system tests, image quality tests, daily, monthly, and treatment type tests.

Clinical Trials Increasingly, clinical trials involve high doses that require precise targeting. Uniformity in the application of these protocols across participating sites is critical to avoid misadministration and consistency for accurate reporting. In Australia and New Zealand, IGRT is considered explicitly through the TROG­IGRT working party. A standard questionnaire has been implemented in the PROFIT, trial and developed further for RAVES and CHISEL to require stringent credentialing activities. The credentialing activities have included submission of IGRT protocols, dummy runs, dose measurements, setup accuracy measurements and viewing of IGRT systems in play. In the future, credentialing activities might be performed by the newly setup Australian Clinical Dosimetry Service (ACDS), similar to RTOG who use the RPC for credentialing.

Advanced research topics

Adaptive Radiotherapy ­ ART

Adaptive radiotherapy is the concept of adapting the treatment plan based on the geometry on the day. The basic steps for online ART include

acquiring a patient CT image deformably register the image and contours recalculate dose with original plan assess suitability of plan if acceptable then treat otherwise, reoptimise, recalculate and then treat

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There are several ways to perform each step and some of the process could be performed offline. The Offline ART process might include taking the decision of assessing suitability of the original plan to the treated patient geometry offline. In the current setting ie standard technology, this is a more feasible option. The assessment will be made on the previous days' images, or possibly a number of previous days images to review daily and systematic changes. Current research shows that the online ART process can be performed within a research environment in as low as 40s using graphical processing units (GPU) 8, 9 , however with commercial products the time required for this process is closer to 10­20min. Research is also continuing into replanning during VMAT delivery.

4D planning This research area involves planning a treatment delivery that has a time component and might involve tracking the tumour in realtime ie: moving couch/MLC to 'follow' the tumour using a variety of imaging modalities. The planning process will require either a surrogate for motion (external – marker block, skin surface infra­red; internal ­ fiducial) or a way of visualising the soft tissue, generally through imaging. Verification may use fluoroscopic imaging, and/or 4DCBCT.

References

1. Vanantwerp, A. E., S. M. Raymond, M. C. Addington, et al. 2010. "Dosimetric evaluation between megavoltage cone­beam computed tomography and body mass index for intracranial, thoracic, and pelvic localization." Med Dosim.

2. Islam, M. K., T. G. Purdie, B. D. Norrlinger, et al. 2006. "Patient dose from kilovoltage cone beam computed tomography imaging in radiation therapy." Med Phys 33(6):1573­1582.

3. AAPM TG 111 Report ­ The Future of CT Dosimetry 2010 4. Brenner, D. J. 2006. "It is time to retire the computed tomography dose index (CTDI) for CT quality assurance and

dose optimization. For the proposition." Med Phys 33(5):1189­1190. 5. Smeenk, R. J., B. S. Teh, E. B. Butler, et al. 2010. "Is there a role for endorectal balloons in prostate radiotherapy?

A systematic review." Radiother Oncol 95(3):277­282. ­ Link to external article 6. Yoo, S., G. Y. Kim, R. Hammoud, et al. 2006. "A quality assurance program for the on­board imagers." Med Phys

33(11):4431­4447. 7. Verellen, D., M. De Ridder, K. Tournel, et al. 2008. "An overview of volumetric imaging technologies and their

quality assurance for IGRT." Acta Oncol 47(7):1271­1278. 8. McNutt, T., Jacques. S. Radiation 2010 Therapy Dose Calculation using Graphics Processing Units (Abstract).

MedPhys 37(6):3452 9. Jiang, S. Gu, X. Men, C. et al 2010. a Real time re­planning for online adaptive radiotherapy AAPM 2010 Abstract

MedPhys 37 (6): 3542. 10. Rong, Y., J. Smilowitz, D. Tewatia, et al. 2010. "Dose calculation on kV cone beam CT images: an investigation of

the Hu­density conversion stability and dose accuracy using the site­specific calibration." Med Dosim 35(3):195­207.

11. Alaei, P., G. Ding and H. Guan. 2010. "Inclusion of the dose from kilovoltage cone beam CT in the radiation therapy treatment plans." Med Phys 37(1):244­248.

12. Sharpe, M. B., D. J. Moseley, T. G. Purdie, et al. 2006. "The stability of mechanical calibration for a kV cone beam computed tomography system integrated with linear accelerator." Med Phys 33(1):136­144.

13. Walter, C., J. Boda­Heggemann, H. Wertz, et al. 2007. "Phantom and in­vivo measurements of dose exposure by image­guided radiotherapy (IGRT): MV portal images vs. kV portal images vs. cone­beam CT." Radiother Oncol 85(3):418­423. ­ Link to external article

14. AAPM ­ TG­104 ­ The Role of In­room kV X­Ray Imaging for patient setup and target localisation ­ Dec 2009 ­ Link to external article

15. AAPM ­ TG­144 ­ Klien E, Hanley J, Bayouth J et al 2009. AAPM Task Group Report 144 Quality assurance of medical accelerators.

16. AAPM ­ TG­101 ­ Benedict S, Yenice K, Followill D et al 2010. Stereotactic body radiation therapy: The report of AAPM Task group 101. MedPhys 37(8) 2010

17. AAPM ­ TG­75 ­ Murphy M, Balter J, Balter S et al 2007. The management of imaging dose during image­guided radiotherapy: Report of the AAPM Task Group 75 MedPhys 34(10) 2007

18. AAPM ­ TG­111 ­ AAPM Task Group 111 ­ Report Comprehensive Methodology for the evaluation of Radiation Dose in X­Ray Computed Tomography: The Future of CT dosimetry Feb 2010

19. Wu, J., T. Haycocks, H. Alasti, et al. 2001. "Positioning errors and prostate motion during conformal prostate radiotherapy using on­line isocentre set­up verification and implanted prostate markers." Radiother Oncol 61(2):127­133.

20. Schallenkamp, J. M., M. G. Herman, J. J. Kruse, et al. 2005. "Prostate position relative to pelvic bony anatomy based on intraprostatic gold markers and electronic portal imaging." Int J Radiat Oncol Biol Phys 63(3):800­811.

21. Kuban, D. A., L. Dong, R. Cheung, et al. 2005. "Ultrasound­based localization." Semin Radiat Oncol 15(3):180­191.

22. Artignan, X., M. H. Smitsmans, J. V. Lebesque, et al. 2004. "Online ultrasound image guidance for radiotherapy of prostate cancer: impact of image acquisition on prostate displacement." Int J Radiat Oncol Biol Phys 59(2):595­601.

23. Ding, G. X., P. Munro, J. Pawlowski, et al. 2010. "Reducing radiation exposure to patients from kV­CBCT imaging." Radiother Oncol 97(3):585­592.

24. Downes, P., R. Jarvis, E. Radu, et al. 2009. "Monte Carlo simulation and patient dosimetry for a kilovoltage cone­beam CT unit." Med Phys 36(9):4156­4167.

The currency of this information is guaranteed only up until the date of printing, for any updates please check www.eviq.org.au

­ 02 Apr 2013

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