Studies Of CTV To PTV Margins And Dose Reconstruction In Prostate Cancer Treated By Helical Tomotherapy System

Part OneStudies of CTV to PTV margins in prostate cancer treated by helical TomotherapyObjective:To evaluate the CTV to PTV margins in prostate cancer treated by Tomotherapy system using MVCT.Methods:From Aug.2008to Oct.2010,10Tic-2c prostate cancer patients were treated by helical tomotherapy. CTV included prostate and seminal vesicles with a prescription dose of76Gy/34F. Patients received daily pre-and post-treatment online MVCT imaging and alignment. These data allowed the analyses of inter-fractional and intra-fractional errors, and determination of the optimal CTV to PTV margins.Results:A total of680online MVCT scans were performed for these10patients. For inter-fractional errors, the smallest was noted in LR direction where95%of them were≤5mm, and the largest was noted in SI direction where38.8%of them were≥8mm. In AP direction,81.8%of the errors were≤5mm. The intra-fractional errors of <3mm in LR, SI and AP direction accounted for99.1%,90.6%,95.6%, respectively. Without IGRT correction, CTV to PTV margin is5.3mm,14.2mm and8.5mm in LR, SI, and AP direction, respectively. While daily pre-treatment online IGRT correction can reduce CTV to PTV margin to1.8mm,2.1mm, and2.4mm in LR, SI and AP direction, respectively.Conclusions:Daily online pre-treatment IGRT correction can reduce inter-fractional errors, and CTV to PTV margin. However, because of the existence of intra-fractional errors and other uncertainties,3-5mm is proposed as an optimal CTV to PTV margin in prostate cancer radiation therapy. Daily dose reconstruction in prostate cancer treated by helical TomotherapyObjective:To evaluate the daily changes in delivered doses to the CTV, rectum, and bladder in prostate cancer treated by Tomotherapy system.Methods:From Aug.2008to Oct.2010,10Tic-2c prostate cancer patients were treated by helical tomotherapy. CTV included prostate and seminal vesicles with a prescription dose of76Gy/34F. Patients received daily pre-treatment online MVCT imaging and alignment. Prostate, seminal vesicles, rectum, and bladder were re-contoured on each MVCT image. Daily delivered doses were recalculated and compared with the primary plan.Results:Based on10patients with340daily MVCT images, change in D95of CTV was not significant compared to primary plan, the average change of fractionated D95(cD95)(±SD) was2.9%±0.8%(0～8.4%). Unlike CTV, there were significant changes in rectum and bladder volumes and doses. The average rectal volume was2.9%±0.8%(0-8.4%) and average rectal V2was11.76±5.05cm3(4.49～37.42cm3). The average bladder volume was127.13±33.34cm3(54.09～242.98cm3) and average bladder V2was13.09±7.81cm3(5.16～35.36cm3).Conclusions:Small changes in CTV volume were seen in prostate cancer treated by Tomotherapy system. With daily guidance for targeting of the prostate, the deformation of the prostate did not significantly affect coverage of the CTV using5mm CTV-PTV margin.