ESTRO 38 Abstract book

S939 ESTRO 38

filled endorectal balloon is used to spare part of the rectal wall. Material and Methods Six clinical hypo-fractionated prostate cancer treatments (5 x 7 Gy to the prostate with/without seminal vesicles, 5 x 10 Gy to the dominant intraprostatic lesion) were investigated. Treatment planning was done in Pinnacle (version 9.10, Philips, Fitchburg, WI, USA) using auto- planning with 2 VMAT arcs of 10 MV photons. Before treatment, the plans were verified with a pre-treatment measurement on a Delta4 phantom; all plans fulfilled our clinical gamma-criterion, i.e. 95% of the measured points within the 50% isodose surface were within 3%/3mm. After position verification with cone-beam CT and correction for translational errors, the patients were treated and EPID dose measurements were done in vivo (3 to 4 fractions per patient were measured). The measured dose distributions were compared “as such” to the clinical plan and also in aqua to the in aqua plan, i.e. with a density override equal to 1 on the whole CT dataset. Results In the figure the TPS dose, the EPID-reconstructed dose and the gamma analysis are shown for both the standard EPID-based dose reconstruction and for the in aqua method. With the standard reconstruction and comparison, the disagreement in the region of the endorectal balloon can clearly be seen in the gamma analysis (white dotted circle). This is due to the large density inhomogeneity caused by the endorectal balloon. The agreement improves considerably, when the in aqua method is used. On average (over fractions and patients) the percentage of points in agreement within the 50% isodose surface improves from 91%±2% (1SD) to 98%±2% for the clinical plan and the in aqua plan, respectively. The mean gamma improves from 0.46±0.05 to 0.36±0.05, respectively.

Imaging and Radiological Sciences, Central Taiwan University of Science and Technology, Taichung, Taiwan Purpose or Objective To evaluate the feasibility of N-isopropylacrylamide (NIPAM) polymer gel dosimeter in combination with MRI (magnetic resonance imaging) on dynamic dose distribution. Material and Methods The gel preparation was prepared with a composition of 5% NIPAM monomer, 5% gelatin and 3% BIS. 5 mM THPC was added to the solution to reduce the oxygen content and to improve sensitivity and reproducibility. The test treatment plan was a 4x5 cm 2 single-field treatment plan created using the Eclipse treatment planning system with an exposure angle of 180. A cylindrical acrylic model and a dynamic phantom were then selected to simulate the moving target with a motion period of 4 seconds and a range of 2 cm. The gel was irradiated with 0, 1, 2, 5, 8 and 10 Gy absorbed doses using a Varian iX linac with 6 MV X-rays. After the polymerization was completed, a slice image of the gel was extracted using a GE 1.5 Tesla MRI scanner. The slice images were analyzed using MATLAB and the results of the gamma test (γ-test) were performed on the treatment planning system and the NIPAM gel dosimeter. Since the estimated dynamic dose distribution is reduced, 4x4 and 4x3 cm 2 treatment plan are made to perform the gamma test together and the passing rate is evaluated. Results The gel results were compared to the 4x5, 4x4, and 4x3 cm 2 treatment plans. To verify the dose, the gamma-test was performed according to the gamma standards of 3% and 3 mm. The comparison results showed that the passing rates in the coronal section images were 60.76%, 95.07%, and 50.02%, respectively. Further analysis found that the dynamic dose distribution was the closest to the 4x4 cm 2 result and the highest passing rate. Shows that the dynamic dose distribution range is reduced, and the reduction range is 50% of the motion range. Conclusion The results demonstrate that it is feasible to validate the dynamic dose distribution using NIPAM gel and MRI scanners, and the dynamic dose range can be quantified by evaluating dynamic effects. In this study, the gel dosage and dynamic phantom simulate the breathing state of the human body, and the real situation is more complicated. It is recommended that future research can evaluate more types of treatment plans and design more impact parameters, such as: motion period and range, and get more careful assessment of dynamic effects, which can provide a more precise definition of treatment margin. EP-1742 In vivo EPID dosimetry for prostate cancer treatments with an endorectal balloon M. Wendling 1 , B. Sterckx 1 , I. Steinseifer 1 1 Radboud university medical center, Radiotherapy, Nijmegen, The Netherlands Purpose or Objective In vivo dose verification is an ideal QA method, because the dose distribution is verified during the actual treatment and no additional time for a pre-treatment measurement is needed. The commercial solution iViewDose (Elekta, Crawley, UK) offers EPID-based in vivo dose verification and has been proven for various treatment sites. Because large tissue inhomogeneities are not accurately handled by the algorithm, in vivo dose verification of lung cancer treatments is done “ in aqua ”, i.e. before dose reconstruction the images are first converted to a situation as if the patient consisted entirely of water [” In aqua vivo EPID dosimetry,” Med. Phys. 39, 367-377, 2012]. In this study we demonstrate that the in aqua method also leads to improvements in the verification of prostate cancer treatments, in case an air-

Conclusion Although originally developed for lung cancer treatments, the in aqua method for EPID dosimetry also works as a correction for other large density inhomogeneities. EPID- based dose verification with iViewDose can therefore be used for in vivo dose verification of prostate cancer treatments with an endorectal balloon. EP-1743 Dosimeter selection for small field percentage depth dose and tissue maximum ratio measurements S. Crowe 1,2 , E. Whittle 2 , C. Jones 3 , K. Tanya 1,2 1 Royal Brisbane and Women's Hospital, Cancer Care Services, Herston, Australia ; 2 Queensland University of Technology, Science and Engineering Faculty, Brisbane, Australia ; 3 Princess Alexandra Hospital, Radiation Oncology, Brisbane, Australia Purpose or Objective The measurement of small field tissue-maximum ratio (TMR) and percentage depth dose (PDD) data is necessary for the calculation of dose for stereotactic radiotherapy treatments. The measurement of beam configuration data, including TMRs and PDDs, is complicated by a loss of lateral charged particle equilibrium in small fields and