ESTRO 37 Abstract book

fiducial markers implanted in the prostate are used as a surrogate of the daily prostate position. Although the acquisition of kV/MV CBCT images is more time- consuming, this type of image is often used in our department, in detriment of portal images, due to logistics issues related to the implantation of the fiducial markers. The purpose of this study was to compare the impact on the dose distribuition when both this daily images are incorporated in the treatment plan. Material and Methods Twenty patients with radical treatment for high risk prostate cancer were selected, with a prescription of 48Gy (2Gy per day) to a PTV1 (pelvic lymph nodes + prostate) followed by a boost of 28 Gy (2Gy per day) to a PTV2 (prostate). For all selected patients, two plans were prepared: one with MV CBCT and the other with MV EPI, the imaging modalities avaliable in our department. For both cases, 38 imaging fractions were considered: two daily orthogonal EPIs, whose daily contribution is of the order of 1 cGy and was not included in the prescribed 2Gy/fraction; or one MV CBCT, whose daily contribution is approximately 10 cGy and was considered as part of the 2 Gy/fraction. The PTV was analyzed according to ICRU 83 (D50, D98, V100 and D2) and organs at risk: rectum (V60), bladder (V60), femoral heads (V50), bowel bag (V45) and bulb (D90), according to Quantec/RTOG guidelines. Statistical analysis was performed using SPSS v. 24. The comparative analysis between the two plans (MV CBCT images and EPI images) was enhanced using the Wilcoxon Signed-Rank Test. Results The comparison of the plans showed that there are significant differences in the PTV coverage (D50 and V100), with lower values for MV CBCT vs portal image (D50: Z= -2.236, p= 0.025; V100: Z= -2.023, p=0.043) and in the bowel bag, with higher values for MV CBCT vs portal image (Z= -2.041, p= 0.041). Conclusion Radical treatment of prostate cancer should not be performed without daily imaging due to the prescription of high doses of radiation and the complexity of the techniques used. The daily MV CBCT image is undoubtedly more time-consuming and showed to determine a lower PTV coverage and higher doses in the bowel bag when compared with the portal image. EP-2383 Triggered imaging for hypofractionated prostate treatment on Varian TrueBeam linear accelerator J. Ter-Minasjan 1 , M. Marjamäe 1 1 North-Estonian Regional Hospital Cancer Center, Radiation Therapy, Tallinn, Estonia Purpose or Objective Dose delivery accuracy for hypofractionated radiation therapy of prostate cancer is very important in achieving the high dose in prostate whilst not exceeding the dose to organs at risk. There is a need for comprehensive imaging protocols in order to get the planned dose distribution. The aim of the poster is to present the introduction of the treatment and image guidance method for the patients who underwent hypo- fractionated external beam irradiation of prostate cancer. Material and Methods From September 2016 to September 2017 25 patients were treated on Varian TrueBeam linear accelerator with hypofractionated treatment using VMAT delivery technique (1-2 arcs). Patient with normal anatomy, localized prostate cancer with medium and high progression risk were selected for hypofractionated treatment. 3 fiducial gold markers (1x3 mm) were implanted in each patient for the imaging system to localize the target. The prescribed dose was 3 Gy per fraction to PTV for 20 consecutive fractions. Firstly, kV

images from AP and LAT position were taken and 2D/2D match based on gold seeds and couch shift were performed. Then, Auto Beam Hold window is activated and appropriate settings are chosen: image every 60 degrees with adjusted field size to reduce the radiation dose to the patient outside the target. This results in typically 12 intrafractional images during 2 arc treatment. This is a compromise between imaging dose and accuracy achievable. The allowed movement diameter for each marker was set to 6 mm. To detect markers automatically ROI/VOI as small as possible around markers is selected. If automatic registration is not successful then the markers can be selected manually. Treatment starts and continues until markers are in allowable range. Otherwise, the treatment is paused and additional imaging is performed: marker match or 2D/2D or 2D/3D match are in option. After shifting of the couch the treatment can be resumed. Results Using described method it was possible to implement the hypofractionated prostate external beam irradiation. As a result the number of fractions (from 35–38 to 20) decreased, while the accuracy of dose delivery increased. Due to successful implementation of triggered imaging it was possible to reduce the PTV margins from 7 to 5 mm. The interruptions of the treatment due to internal movement were seen for almost all patients indicating the importance of the intrafractional imaging procedure. In most cases the fraction took on average 7 min more than the standard fraction (without intrafractional imaging). In some cases however, the treatment took as long as 40 min due to large internal prostate movements. The treatment for most patients was accomplished in the standard timeslot of 15 min. Conclusion Successful implementation of intrafractional imaging for hypofractionated treatment of prostate cancer results in decreased number of fractions and reduced PTV margins. Despite slight increase in time per fraction, the treatment accuracy and dose delivery are improved. EP-2384 interfractional seminal vesicle motion for prostate cancer with/without androgen deprivation therapy T. Waki 1 , K. Katsui 2 , N. Katayama 2 , M. Takemoto 3 , S. Kanazawa 2 1 Tsuyama Chuo Hospital, Radiology, Tsuyama-shi, Japan 2 Okayama University Hospital, Radiology, Okayama, Japan 3 Japanese Red Cross Society Himeji Hospital, Radiology, Himeji, Japan Purpose or Objective We investigated differences in seminal vesicle (SV) length and interfractional SV motion relative to the prostate gland in prostate cancer patients. Material and Methods We compared 32 patients who received androgen deprivation therapy (ADT) before radiotherapy with 12 patients receiving radiotherapy alone at Okayama University Hospital in August 2008-July 2011. We examined the right and left SVs’ length and motion by computed tomography (CT) to determine the ADT’s effects and analyzed 347 CT scans in a multiple linear regression model. Results The ADT patients’ SV length was significantly shorter than the non-ADT patients’. The differences in right and left SV lengths between the ADT and non-ADT patients were 6.8 mm (95% CI 2.0-11.7 mm) and 7.2 mm (95% CI 3.1-11.3 mm) respectively in an adjusted regression model. SV motion did not differ between the ADT and non-ADT patients in terms of interfractional motion of