ESTRO 37 Abstract book

2 Comprehensive Cancer Center in Bialystok- Poland, Department of Medical Physic, Bialystok, Poland 3 Comprehensive Cancer Center in Bialystok- Poland, Department of Radiotherapy-, Bialystok, Poland Purpose or Objective Sparing hippocampus during brain irradiation could avoid damage of neural progenitor cells located in subgranular zone of the hippocampus and subsequently avoid neurocognitive decline. The aim of the study was to asses the possibility of the reduction of radiation dose to contralateral or bilateral hippocampi in patients with primary malignant brain tumors using planning RT system and volumetric modulated arc therapy (VMAT). Material and Methods The study assigned 15 patients with high grade glial tumors (HGGT) III or IV WHO who received radiotherapy (VMAT) or 3D conformal RT (60 Gy in 30 fractions). Depending on tumor location contralateral or both hippocampi were delineated. RT plans were optimized to reduce the doses to hippocampus with maintaining the plan's acceptability parameters. Hippocampal dose and treatment parameters of VMAT plans were compared to the 3D conformal RT plans. Results All RT plans achieved PTV coverage of at least 95%. Average volume of delineated hippocampi was 5,2 cm 3 . Using VMAT technique median dose D 100% to the contraleral hippocampus was less than 16 Gy (10,8 Gy), whereas the maximal dose to the contralateral hippocampus was 15,8 Gy and ipsilateral - 23 Gy. VMAT technique allowed for median dose reduction to contralateral hippocampus by 28% comparing to 3D conformal RT plans. The total treatment RT fraction time was reduced by 4 min in VMAT technique vs 3D conformal. In 85 % cases there was no technical possibility for spare ipsilateral hippocampus, maintainly in centrally, large tumor location. Conclusion VMAT RT is feasible and might provide substantial reduction of the dose to contralateral hippocampus in HGGT. Large, centrally located tumors makes impossible excluded to spare both hippocampi and in some cases even contralateral. Sparing hippocampus in patients with glioblastoma in relation to neurocognitive toxicity needs to be investigated in the further studies. EP-2365 Comparing the dosimetric impact of fiducial marker according to density override method D.Y. Sung 1 , S.J. PARK 1 , J.H. PAKR 1 , Y.C. PARK 1 , H.C. Park 1 , B.K. Choi 1 1 Samsung Medical Center, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective The application of density override is very important to minimize dose calculation errors by fiducial markers of metal material in proton treatment plan. However, density override with actual material of the fiducial marker could make problem such as inaccurate target contouring and compensator fabrication. Therefore, we perform density override with surrounding material instead of actual material and we intend to evaluate the usefulness of density override with surrounding material of the fiducial marker by analyzing the dose distribution according to the position, material of the fiducial marker We supposed that the fiducial marker of gold, steel, titanium is located in 1.5, 2.5, 4.0, 6.0 cm from the proton beam’s end of range using water phantom. Treatment plans were created by applying density override with the surrounding material and actual material of the fiducial marker. Also, a liver cancer patient who received proton therapy was selected. We and number of beams. Material and Methods

EP-2363 Comparison of VMAT and IMRT for post- operative RT in prostate cancer patients with a rectal balloon H. Jung 1 1 Seoul National University Bundang Hospital, radiation oncology, Seongnam-si-, Korea Republic of Purpose or Objective In radiotherapy for prostate cancer, different treatment techniques result in different dose distributions in the major organs and healthy tissues. This study established treatment plans for VMAT and IMRT, which have different numbers of rotations, in post-operative RT in prostate cancer patients with a rectal balloon. The dose distribution characteristics of these treatment plans were compared, and their treatment efficiency was evaluated. Material and Methods The subjects consisted of 10 patients who underwent post-operative RT for prostate cancer. CT images were obtained from patients with a rectal balloon using a slice thickness of 3 mm, and Eclipse (Ver 11.0, Varian, Palo Alto, USA) was used to apply 10 MV from a Truebeam STx (Varian, Palo Alto, USA) fitted with an HD120MLC. 1-arc VMAT, 2-arc VMAT, and 7-field IMRT plans (1A-V, 2A-V, 7F-I) were generated for each patient, using the same dose volume constraint and plan normalization values in all plans. In order to evaluate the 3 plans, we compared the coverage of the PTV, the conformity index (CI), and the homogeneity index (HI); in order to examine the dose delivered to adjacent healthy tissue, we calculated the ratio of 50% isodose volume to the PTV (R 50% ). Among the organs at risk (OAR), D 25% in the rectum and D mean in the bladder were compared. In order to evaluate treatment efficiency, MU and irradiation time were measured; for each evaluated variable, the mean was calculated across the 10 patients and this value was used in the comparative analysis. Portal dosimetry was also performed using an EPID to test the accuracy of dose delivery. Results Although there was no significant difference between treatment plans in PTV coverage or HI, CI was 1.036, 1.035, and 1.230 in 1A-V, 2A-V, and 7F-I, respectively, meaning that it was significantly higher in 7F-I than in VMAT (p=0.00). R 50% was 3.083, 3.054, and 3.991, respectively, meaning that it was lowest in 2A-V and highest in 7F-I (p=0.00). D 25% in the rectum was similar between VMAT strategies, but was approximately 7% higher for 7F-I (p=0.02), while D mean in the bladder did not show any significant differences (p>0.05). Total MU was highest for 7F-I at 757.9 (p=0.00), compared to 494.7 and 479.7 for 1A-V and 2A-V, respectively. Irradiation times were 65.2s, 133.1s, and 145.5s, meaning that 1A-V had by far the shortest irradiation time (p=0.00). In portal dosimetry tests, all treatment plans showed a gamma pass rate (2 mm, 2%) of at least 99.5% (p=0.00). Conclusion This study showed that when post-operative RT was performed in prostate cancer patients with a rectal balloon, although there was little difference in PTV coverage according to treatment technique, 1A-V and 2A- V were effective at reducing the dose delivered to the rectum and to healthy tissue compared to 7F-I. Comparing the two VMAT techniques, R 50% and MU were slightly lower in 2A-V, but irradiation time was shorter in 1A-V, meaning that this treatment was more efficient, and could minimize patient movements during RT. EP-2364 Hippocampal sparing radiotherapy in primary brain tumors using volumetric modulated arc therapy M. Konopka-Filippow 1 , E. Sierko 1 , D. Hempel 1 , E. Szewczuk 2 , T. Filipowski 3 , M.Z. Wojtukiewicz 1 1 Medical University of Bialystok- Poland, Department of Clinical Oncology, Bialystok, Poland