S376 I. J. Radiation Oncology d Biology d Physics Volume 69, Number 3, Supplement, 2007

Conclusions: Treating early stage prostate cancer with a SIB technique using helical tomotherapy provides excellent target cov-erage with improved correlation between the prescribed and delivered doses, as well as decreased rectal dose when compared withSEQ delivery. The SIB technique allows biologically equivalent treatment to be given in fewer fractions (25 vs 36) with significantrectal sparing, which will likely translate into decreased rectal symptoms in patients treated with this technique.

Author Disclosure: H.L. Lee, None; J.M. Baisden, None; H.D. Skinner, None; B.F. Schneider, None.

2309 Relative Motion of Implanted Prostate Fiducials During Stereotactic Body Radiation Therapy

M. Edwards, J. Anderson, T. Boike, T. Li, R. Timmerman

UT Southwestern Medical Center, Dallas, TX

Purpose/Objective(s): To evaluate inter-fraction relative motion of implanted fiducials in prostate cancer patients during treat-ment with Stereotactic Body Radiation Therapy (SBRT).

Materials/Methods: Inter-fraction fiducial motion of the first 5 patients treated on an IRB approved protocol involving SBRT forprostate cancer is assessed for consistency and reliability of fiducial position. SBRT was completed between October 2006 andMarch 2007. SBRT was given in 5 fractions of 900 cGy each to the prostate. A stereotactic body frame, pretreatment enema,60 cc rectal balloon and full bladder are used for immobilization and reproducibility in the daily treatment setup. All patientshave 3 implanted gold fiducials: two at the left and right base and one in the apex of the prostate. The rectal balloon is filled toa consistent volume of 60 cc. CBCT images are registered to the initial planning scan and necessary couch shifts made to alignthe fiducials. The rectal balloon is repositioned relative to the fiducials by a soft tissue CBCT match. A second CBCT is doneto verify fiducial shifts and rectal balloon match. An algorithm was developed to automatically determine the absolute positionsof the gold fiducials from projection data obtained from the CBCT images. The sum of the 3 interfiducial distances is calculatedfor each fraction and patient. The interfiducial sums for the 1st, 3rd, and 5th fractions for each patient are compared. The time intervalbetween fiducial placement and start of therapy as well as the interval between start of therapy and SBRT completion are correlatedwith change in relative fiducial position.

Results: From the 1st to 3rd fraction there is a mean decrease in total interfiducial distance of 2.5 mm ± 1.2 mm (p = 0.015). Betweenthe 1st and 5th fraction there is a mean decrease in total interfiducial distance of 2.3 mm ± 1 mm (p = 0.016). There was no significantchange in interfiducial distance between the 3rd and 5th fraction (p = 0.65). For all patients the median treatment time from fiducialplacement to start of therapy was 22 days. The median time from delivery of the 1st radiation fraction to the completion of the 5th

was 14 days. Time from fiducial placement to start of treatment does not correlate with the observed decrease in total interfiducialdistance (r2 = 0.63, p = 0.11). Time from start of therapy to completion of therapy does correlate with the total interfiducial dif-ference observed between the 1st and 5th fraction (r2 = 0.85, p = 0.03).

Conclusions: A small but significant decrease in interfiducial distance for SBRT patients was noted through the course of treat-ment. Due to the small magnitude of this distance, fiducial markers remain a consistent and reliable tool for positioning of prostatecancer patients even when treating with the high doses per fraction used in SBRT.

Author Disclosure: M. Edwards, None; J. Anderson, None; T. Boike, None; T. Li, None; R. Timmerman, PC050628 from the USDepartment of Defense, B. Research Grant.

2310 Megavoltage Cone Beam Digital Tomosynthesis in Target Volume Localization for Patients Undergoing

Prostate External Beam Radiotherapy: A Novel Method of IGRT

T. S. Lim, G. Morton, G. Pang, A. Loblaw, P. Cheung, P. O’Brien, J. Rowlands, A. Wighton, P. Au

Toronto Sunnybrook Regional Cancer Center, Toronto, ON, Canada

Purpose/Objective(s): Interfraction and intrafraction prostate movement during prostate radiotherapy (RT) is a well-known phe-nomenon. Accurate target localization and verification is therefore an important step toward enhancing treatment accuracy. Thepurpose of the study is to evaluate the clinical feasibility of a novel x-ray tomographic imaging technique called MegavoltageCone Beam Digital Tomosynthesis (MV-CBDT). In contrast to megavoltage cone beam computed tomography (MV-CBCT)that uses a full gantry rotation in the image acquisition, MV-CBDT achieves this via a partial gantry rotation (typically 20–40�). The advantages of MV-CBDT over MV-CBCT are that it is quicker in image acquisition and reconstruction, and deliversless radiation doses to patients. To our knowledge, this is the first ever study to investigate this in a clinical setting.

Materials/Methods: A clinical investigational MV-CBDT system was installed on an existing LINAC (Primus, Siemens). Thesystem is also capable of performing MV-CBCT. All patients undergoing radical RT for clinically localized prostate cancer(PC) were eligible. Patients underwent standard RT planning and treatment as prescribed by their physicians. For any 2 fractionsof RT, once electronic portal images (EPI) were taken, MV-CBDT and MV-CBCT imaging at 6MV were performed. The imagequality obtained by MV-CBDT and its suitability for use in image guidance was assessed qualitatively by comparing image qualityof EPI, MV-CBDT and MV-CBCT.

Results: Between December 2006 and April 2007, 10 patients were accrued; four patients with fiducial markers within the prostateand the remainder without. One also had a prosthetic hip replacement. Both MV-CBDT and MV-CBCT could acquire images ofthe clinical target volume (CTV) and surrounding organs at risk (OAR). The image quality of MV-CBDT is comparable to that ofMV-CBCT. Both modalities provided better image quality than that of the standard EPI. However, at present the quality of rawimages from both MV-CBDT and MV-CBCT obtained using #12 monitor units (MUs) at 6MV are still suboptimal for contouringthe CTV and OAR. No image distortion was noted in patients with fiducial markers or hip prosthesis. Overall duration for imageacquisition was shorter with MV-CBDT (1 minute vs. 2 minutes) as well as lower radiation doses to patients (6 MUs vs.12 MUs).

Conclusions: Our preliminary findings suggest that MV-CBDT is feasible for image-guided prostate RT. However, further im-provement of image quality is needed particularly for imaging soft tissues before it can replace standard imaging modalities.

Author Disclosure: T.S. Lim, None; G. Morton, None; G. Pang, The project was partially supported by Siemens., C. OtherResearch Support; A. Loblaw, None; P. Cheung, None; P. O’brien, None; J. Rowlands, None; A. Wighton, None; P. Au, None.