ESTRO 36 Abstract Book

S853 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective To evaluate conformity, homogeneity and dose distribution to PTV and organ at risk (OAR) of three different types of radiotherapy techniques in pancreatic cancer. Material and Methods Three radiotherapy treatment plans, including 3DCRT, forward-planning IMRT and volumetric arc therapy (VMAT) were created for 18 consecutive patients with pancreatic cancer. Eight postoperative patients with a resectable pancreatic cancer and ten patients with unresectable locally advanced disease were selected for this dosimetric analysis. Dose Volume Histogram (DVH) comparative analysis was performed for PTV and OAR. Paired t-test was used for statistical analysis. Results All plans exhibited similar PTV coverage (V95%) and conformity (all p > 0.05). The Homogeneity Index (HI) was acceptable for all plans; in particular, it was higher in VMAT plans than in 3D-CRT and IMRT plans. The mean dose to the liver was 13.5 Gy for 3D, 12.1 Gy for IMRT, 10.9 Gy for VMAT (p<0.001) to the benefit of VMAT. Volumes of kidneys irradiated to doses of 20Gy (V20), 23Gy (V23), 28Gy (V28) by the VMAT plans were significantly less than those of the IMRT and 3D-CRT plans. The volume of kidneys irradiated to a dose of 12 Gy (V12) was not significantly different comparing the three techniques. Mean of the maximum point dose to spinal cord was better in VMAT plans (3D-CRT vs IMRT vs VMAT, 30.6 Gy, 34.1 Gy, 26.5 Gy, respectively; p<0.001). Conclusion VMAT can be a better option in treating pancreatic cancer as compared to IMRT and 3D-CRT. The VMAT plans resulted in equivalent or superior dose distribution with a reduction in the dose to organ at risk. EP-1582 Analysis of Risk of a Second Cancer from Scattered Radiation in Acoustic Neuroma Treatment Y. Oh 1 , D.O. Shin 2 , D.H. Shin 3 , W.K. Chung 4 , M. Chung 4 , D.W. Kim 4 1 Kyung Hee University Hospital at Gangdong, Research Institute of Clinical Medicine, Seoul, Korea Republic of 2 Kyung Hee University Hospital, Department of Radiation Oncology, Seoul, Korea Republic of 3 National Cancer Center, Proton Therapy Center, Goyang, Korea Republic of 4 Kyung Hee University Hospital at Gangdong, Department of Radiation Oncology, Seoul, Korea Republic of Purpose or Objective This study aimed to compare the risk of a secondary cancer from scattered and leakage doses in patients receiving intensity-modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), Proton therapy and Tomotheraphy. Material and Methods Acoustic neuroma patients were treated with IMRT, VMAT, SRS, Proton therapy or Tomotheraphy. Their excess relative risk (ERR), excess absolute risk (EAR), and lifetime attributable risk (LAR) of a secondary cancer were estimated using the corresponding secondary doses measured at various organs by using radio- photoluminescence glass dosimeters (RPLGD) placed inside a humanoid phantom. Results When a prescription dose was delivered in the planning target volume of the 4 patients, the highest organ equivalent doses (OED) was calculated in to Proton therapy, followed by SRS, VMAT, IMRT and Tomotherapy. The OED decreased as the distance from the primary beam increased. The thyroid received the highest OED compared to other organs. A lifetime attributable risk evaluation estimated that more than 0.03% of acoustic neuroma (AN)

patients would get radiation-induced cancer within 20 years of receiving radiation therapy. Conclusion The organ with the highest radiation-induced cancer risk after radiation treatment for AN was the thyroid. We found that the LAR could be increased by the transmitted dose from the primary beam. No modality-specific di ff erence in radiation-induced cancer risk was observed in our study. EP-1583 Quantifying Plan Quality Metrics using Conventional and Stereotactic dosimetric indices in Lung SBRT R. Yaparpalvi 1 , M. Garg 1 , W. Bodner 1 , J. Shen 1 , D. Mynampati 1 , H. Kuo 1 , N. Ohri 1 , J. Fox 1 , A. Basavatia 1 , S. Kalnicki 1 , W. Tome 1 1 Montefiore Medical Center, radiation Oncology, bronx, USA Purpose or Objective Several metrics have been proposed in literature for evaluating treatment plan quality in conventional and stereotactic planned dose distributions. In this study, we utilized these metrics for characterizing and quantifying Lung SBRT dose distributions. By applying various plan quality metrics available in the literature, we sought to not only characterize Lung SBRT target dose distributions but also evaluate merits of these metrics as tools for lung SBRT plan quality assessment. Material and Methods Treatment plans of 100 Lung SBRT patients treated in our institution were retrospectively reviewed. Dose calculations were performed using AAA algorithm with heterogeneity correction. A literature review on published plan quality metrics in the categories- coverage, homogeneity, conformity and gradient was performed. For each patient, using dose-volume histogram (DVH) data, plan quality metric values were quantified. Data were analyzed using descriptive statistics and two-tailed probability t-test (Wilcoxon signed-rank test). A p-value of < 0.05 was considered statistically significant. Results For the study, the mean (±SD) plan quality metrics in the four representative categories were: Coverage (96.4 ±2.4 %); Homogeneity (0.21 ±0.06); Conformity (0.90 ±0.06) and Gradient (1.27 ±0.30 cm). Geometric conformity strongly correlated with conformity index (p<0.0001). Gradient measures strongly correlated with target volume (p<0.0001). The RTOG lung SBRT protocol advocated conformity guidelines for prescribed dose in all categories were met in ≥94% of cases. Evaluating High Dose Spillage, the average cumulative volume of all tissue outside the PTV receiving a dose of > 105% of prescription dose was 0.94 (± 1.64) %. Considering Low Dose Spillage, the maximum % of prescription dose to any point at 2 cm distance in any direction from PTV was 56.0 (± 11.4) %. The proportion of lung volume (total lung volume – GTV) receiving doses of 20 Gy and 5 Gy (V 20 and V 5 ) were mean 4.9 % (± 3.1) and 16.9 % (± 9.0), respectively. Conclusion Our study metrics are valuable tools for establishing lung SBRT plan quality guidelines. Based on our data, we recommend using the following metrics as surrogates for establishing SBRT lung plan quality guidelines– Coverage % (ICRU 62), Homogeneity (HI ICRU83 ), Conformity (CN or CI Paddick ), and Gradient (R 50% ). EP-1584 Deformable image registration and dose accumulation for arc-Total Body Irradiation G. Guidi 1 , N. Maffei 1 , P. Ceroni 1 , G.M. Mistretta 1 , F. Lohr 2 , T. Costi 1 1 Az. Ospedaliero Universitaria di Modena, Medical Physics, Modena, Italy