REPORT ORGANIZATION ABSTRACT I. INTRODUCTION CALCULATION OF EFFECTIVE DOSE PER IMAGE ACQUISITION CALCULATION OF EFFECTIVE DOSE PER TREATMENT COURSE

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1 E S T I M A T I O N O F T H E I M A G I N G D O S E F O R T H E C Y B E R K N I F E R O B O T I C R A D I O S U R G E R Y S Y S T E M REPORT ORGANIZATION ABSTRACT I. INTRODUCTION II. III. IV. CALCULATION OF EFFECTIVE DOSE PER IMAGE ACQUISITION CALCULATION OF EFFECTIVE DOSE PER TREATMENT COURSE CALCULATION UNCERTAINTIES V. SUMMARY

2 A B S T R A C T The use of image guided radiation therapy (IGRT) has seen large scale acceptance as a method for accurate pre-treatment patient setup. Techniques such as cone-beam CT (CBCT) and imaging are used in several of the conventional gantry based radiation delivery systems. The Robotic Radiosurgery System uses stereoscopic X-ray imaging not only for patient set up, but also to continually track and correct for any target motion throughout the treatment delivery process. This paper presents an estimation of the dose delivered by the imaging system. This is compared with published data describing the imaging dose associated with gantry based IGRT devices. I. I N T R O D U C T I O N The System consists of a six-degrees-of-freedom 1 robotic manipulator which positions a compact 6 MV linac to accurately treat lesions anywhere in the body. 1,2,3 For target localization, the system uses a pair of amorphous silicon flat panel detectors arranged in a stereoscopic geometry with two ceiling-mounted kilovoltage X-ray sources at 45 degrees to the vertical, and the corresponding, floor-mounted flat panel detectors. The imaging system field of view, at the point where the central axes of these beams intersect (the imaging isocenter), is approximately 15 x 15 cm. In this paper we use the methodology described in the AAPM Task Group 75 Report (TG-75) 4 to obtain the effective dose 5 for kv X-ray imaging as utilized by the System to allow comparison with other image guidance modalities. It should be noted that the imaging system dose calculations presented here apply to all versions of the System, up to and including the recently released VSI System. Figure 1. VSI Robotic Radiosurgery System with stereoscopic in-floor flat panel detectors and ceiling mounted kv X-ray sources. 1 1 The equivalent of three Cartesian axes and three Euler angles are provided by the 6 rotational joints of the robot.

3 I I. C A L C U L A T I O N O F E F F E C T I V E D O S E P E R I M A G E A C Q U I S I T I O N The data in Table 1 show the surface dose per single image (averaged for the two imagers) as measured at the imaging isocenter using optically stimulated luminescent dosimeters (OSLDs), mounted on water equivalent slab phantoms to simulate backscatter. The ceiling mounted X-ray sources can achieve peak voltage of 150 kvp (depending upon imaging frequency) and have approximately 2.5 mm of aluminum inherent filtration. Table 1. Measured surface dose per single System image. kvp ma ms mas Dose (mgy) To compare the dose between different image guidance modalities we use the effective, dose, E (msv), which can be obtained from these absorbed dose measurements using the TG-75 method as follows: E = D*A*F where, D = absorbed dose (mgy) at the center of the field, A = X-ray beam area (cm 2 ) and F is a conversion factor of dosearea product to effective dose. Tables of F values are provided in TG-75 for a variety of anatomical target locations, beam orientations, and X-ray techniques. The X-ray technique used during treatment depends upon the target anatomy and patient size. A limited survey of sites was conducted to establish the range of kv, ma and exposure time settings used for a variety of treatment sites, and from these ranges the median values were used to calculate the typical effective dose per image pair according to the TG-75 method and the dose measurements reported in Table 1. Absorbed dose per mas varies approximately linearly with kvp within the kvp range presented in Table 1, and therefore linear interpolation was used to estimate the dose per mas for kvp settings not included in the measurements. The results of these calculations are shown in Table 2. It should be noted that the maximum available kv was increased to 150 kv only recently, and therefore there is little current user experience with kvp settings above about 125 kv, which might bias the kv settings in Table 2 for deep lesions and larger patients. Table 2. Calculated Effective Dose per image pair E, for typical System treatments. Anatomy kvp mas D (mgy) per image A (cm 2 ) F (msv/mgy cm 2 x10-5 ) E (msv) per image pair Head Chest Pelvis

4 In comparison, the effective doses for other image guidance modalities are listed in Table 3 (with the exception of the final row, these data points are taken from TG-75; the source references are listed for completeness): Table 3. Effective Dose for other image guidance modalities including imaging, diagnostic kv CT scanning and kv cone beam CT scanning. Modality Anatomy Effective Dose (msv) Reference Orthogonal MV Portal Image Pair (assuming 2MU per image) Orthogonal MV Portal Image Pair (assuming 2MU per image) Chest 8.6 [6] Male Pelvis 1.3 [6] Standard CT Head [7] [8][9][10] Standard CT Chest [7] [8][9][10] Standard CT Pelvis [7] [8][9][10] kv CBCT Head 3.0 [11] kv CBCT Chest 8.1 [11] kv CBCT Pelvis 3.0 (low dose mode) 23.0 (standard mode) [12] I I I. C A L C U L A T I O N O F E F F E C T I V E D O S E P E R T R E A T M E N T C O U R S E The calculations presented in Table 4 compare the total effective dose delivered by the X-ray image guidance system during a hypofractionated treatment course with the effective dose delivered by a diagnostic (or planning) CT scan of the same region, and by imaging or kv CBCT image guidance techniques. Unlike other image guidance techniques, the continual intra-fraction tracking and correction provided by the System requires X-ray image pairs to be acquired repeatedly throughout each fraction rather than just once for initial patient set-up. The range in typical numbers of image pairs acquired during treatment for various treatment sites was obtained from the limited survey of users described earlier, and the calculations presented in Table 4 are based on the median of those ranges (the total number of image acquisitions for each technique is stated in the table). For Synchrony treatments this number includes images to build the correlation model. All calculations are based on the data presented in Tables 2 and 3. Table 4. A comparison of effective doses delivered by image guidance methods during an entire hypofractionated treatment course. (Head) 1) Single-fraction intracranial SRS # images E (msv) # images E (msv) # images E (msv) # images E (msv) No data (Chest) 2) Hypofractionated (3 fractions) Lung SBRT # images E (msv) # images E (msv) # images E (msv) # images E (msv) (Pelvis) 3) Hypofractionated (4 fractions) Prostate SBRT # images E (msv) # images E (msv) # images E (msv) # images E (msv) (low dose) 92 (standard) 3

5 Table 5 shows a similar comparison for a conventionally fractionated prostate treatment, in which the image guidance dose resulting from a delivery of Robotic IMRT treatment is compared with that from set-up only IGRT technologies, assuming a 20 minute treatment fraction with 15 image pairs per fraction (with five minute patient set-up), which is approximately an imaging period of 60 s. Daily CBCT is included as an IGRT option in this table, but the relatively high effective dose from such a guidance strategy means that this is unlikely to be a viable option. Therefore, a hybrid IGRT strategy involving CBCT set-up at fractions 1-3 and weekly after that, with used every day 12 is also included for comparison. Table 5. A comparison of effective doses delivered by image guidance methods during an entire conventionally fractionated treatment course. 4) Conventionally Fractionated (N=40) Prostate IMRT CBCT set-up + # images E (msv) # images E (msv) # images E (msv) # images E (msv) # images E (msv) (low dose) 920 (standard) 10 (CBCT) + 40 (MV EPID) 82.8 (low dose) 283 (standard) I V. C A L C U L A T I O N U N C E R T A I N T I E S The uncertainties in these calculations are significant. Firstly, the X-ray technique and imaging frequency is dependent on the target anatomy, patient size and to some extent the patient behavior. While the calculations presented here represent a typical case, the data presented in Tables 1 and 2 enable these estimates to be varied as kv, mas, or imaging frequency are altered. Secondly, the F factors included in TG-75 do not correspond to the exact beam geometry or X-ray spectrum employed in the System. However, effective dose calculations made using these factors have been shown to be in reasonable agreement (within ~20%) of results calculated by Monte Carlo simulation of the actual imaging system. 13 Finally these F factors, taken from the Le Heron data, 14 were generated using Monte Carlo simulation to calculate the dose delivered to organs at risk (OARs) in an anthropomorphic mathematical phantom. The physical doses to each OAR were then multiplied by organ specific tissue weighting factors and summed to give the effective dose, using the data provided in ICRP This method has since been revised in ICRP with some significant changes made to tissue weighting factors, and therefore the TG-75 method does not provide an estimate of the effective dose that is consistent with the latest ICRP guidance. Unfortunately the raw dose to organ data is not provided by Le Heron, so it is not possible to convert. Recent calculations of the imaging system effective doses have shown that using ICRP 103 tissue weighting factors rather than ICRP 60 factors results in no significant change for head imaging, an increase of ~70% for thorax imaging, and a decrease of ~55% for pelvic imaging. 17,18 While this represents a significant uncertainty in the absolute values of effective dose given in sections II and III of this report, the relative comparisons between alternative imaging system doses for each treatment site presented here are likely to remain valid since all of these calculations are based on the same (ICRP 60) tissue weighting factors. V. S U M M A R Y In this paper we have provided data on the imaging dose delivered by the system for different treatment modalities and anatomical sites, by using in house measurements of surface dose to estimate the effective dose using the methodology described in TG-75. This will enable clinicians to make informed decisions regarding the dose associated with kv imaging during typical treatments and compare it with other commonly used modalities such as CBCT and imaging. 4

6 REFERENCES 1. C. Antypas and E. Pantelis, Performance evaluation of a G4 image-guided robotic stereotactic radiosurgery system, Phys. Med. Biol (2008). 2. W. Kilby, C. R. Maurer Jr, C. Amies et al., Platforms for image-guided and adaptive radiation therapy. In: R. D. Timmerman, L. Xing, eds. Image-Guided and Adaptive Radiation Therapy. Philadelphia: Lippincott Williams &Wilkins. pp (2010). 3. W. Kilby, J. R. Dooley, G. Kuduvalli, S. Sayeh, C. R. Maurer Jr, The Robotic Radiosurgery System in 2010, Technology in Cancer Research and Treatment (accepted). 4. M. J. Murphy et al., The management of imaging dose during image-guided radiotherapy: Report of the AAPM Task Group 75, Med. Phys. 34, (2007). 5. W. Jacobi, The concept of effective dose: A proposal for the combination of organ doses, J. Radiat. Environ. Biophys. 12, (1975). 6. S. P. Waddington and A. L. McKensie, Assessment of effective dose from concomitant exposures required in verification of the target volume in radiotherapy, Br. J. Radiol. 77, (2004). 7. M. Galanski, H. D. Nagel, and G. Stamm, Expositions-dosis bei CT-untersuchungen: Ergebnisse einer bundesweiten umgfrage, Fortschr. Rontgenstr. 172, M164 M168 (2000). 8. E. G. Fiberg, Norwegian Radiation Protection Authority, Department of Radiation Protection and Nuclear Safety, Østerås, Norway, (2000). 9. R. Smith-Bindman, J. Lipson, R. Marcus, K-P. Kim, M. Mahesh, R. Gould, A. Berrington de González, D. L. Miglioretti, Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer, Arch. Intern. Med (2009). 10. A. Berrington de González, M. Mahesh, K-P. Kim, M. Bhargavan, R. Lewis, F. Mettler, C. Land, Projected cancer risks from computed tomography scans performed in the United States in 2007, Arch. Intern. Med (2009). 11. M. K. Islam, T. G. Purdie, B. D. Norrlinger, H. Alasti, D. J. Moseley, M. B. Sharpe, J. H. Siewerdsen, and D. A. Jaffray, Patient dose from kilovoltage cone beam computed tomography imaging in radiation therapy, Med. Phys. 33, (2006). L R. Potts and M. Oatey, An investigation of the concomitant doses from cone beam CT and CT simulation in radiotherapy WC 2009, IFMBE Proc 25/I, (2009). 13. P. Francescon, Cyberknife: A users point of view, Rad Oncol 73 (sup1), S9-10 (2004). 14. J. C. Le Heron, Estimation of effective dose to the patient during medical x-ray examinations from measurements of the dose-area product, Phys Med Biol 37, (1992). 15. ICRP-60, The International Commission on Radiological Protection, The 1990 recommendations of the international commission on radiological protection, Annals of the ICRP 21 (1-3) (1990). 16. ICRP-103, The International Commission on Radiological Protection, Recommendations of the international commission on radiological protection, Annals of the ICRP 37 (2-4) (2007). 17. P. Francescon, Effective dose delivered by the Cyberknife image guidance system, Focus on Radiosurgery (Accuray Inc) 7, 1-3 (2009). 18. P. Francescon, private communication (2009). 5

7 The information provided in this report is intended for background and educational purposes only as it relates to the System. The information contained herein is not intended, nor should it be construed, as advocating the acquisition or purchase of a System. Accuray Incorporated reserves the right to update or change any specifications without notice. Please direct any questions or comments to the contact information below. Accuray Worldwide Headquarters 1310 Chesapeake Terrace Sunnyvale, CA USA Tel: Toll Free: , ext 4337 Fax: sales@accuray.com Accuray Europe Tour Atlantique 25 e 1 Place de la Pyramide Paris La Défense Cedex France Tel: Fax: Accuray Asia Ltd. Suites , Tower 6 The Gateway, Harbour City 9 Canton Road, T.S.T. Hong Kong Tel: Fax: Accuray Japan K.K. Daini Tekko Building 6F Marunouchi, Chiyoda-ku Tokyo Japan Tel: Fax: Accuray Incorporated. All Rights Reserved. Accuray, the stylized logo,, Synchrony, Xsight, Xchange and RoboCouch are among the trademarks and/or registered trademarks of Accuray Incorporated in the United States and other countries A