ESTRO 2020 Abstract book

S834 ESTRO 2020

Datasets of 52 patients (27 females and 25 males) with median age of 15 years (range 6 -17) were obtained. Thirty and twenty-two patients were treated with 3D-CRT and VMAT, respectively. Low dose (14.4-25.6 Gy) involved field radiotherapy following chemotherapy (COPP/ABV) was delivered to the mediastinum according to AIEOP 2004 HD study protocol. Heart, thyroid, breast, lungs and spinal cord were identified as organs at risk (OARs). Provided that the dose distribution in the target volume was similar between the two treatments, dose-volume histograms (DVHs) were evaluated and compared for OARs. Results Mean dose to the heart was 3.12 Gy (3D-CRT) and 2.72 Gy (VMAT) (p=0.33), respectively; the same feature for thyroid was 10.84 Gy (3D-CRT) and 8.75 Gy (VMAT) (p=0.07), respectively; among the female population, VMAT resulted in a better sparing of breast volumes receiving intermediate or high doses compared to 3D-CRT (3D-CRT Dmax left and right breast: 16.31 Gy and 15.21 Gy versus VMAT Dmax left and right breast: 10.46 Gy and 10.28 Gy, p=0.03). No significant difference between the two techniques was observed in terms of mean dose received to both the breasts and the lungs. Dmax for spinal cord was 18.04 Gy (3D-CRT) and 13.29 Gy (VMAT), respectively (p= 0.0005). Conclusion VMAT resulted in a substantial sparing of thyroid and spinal cord. Mean doses to heart, breasts and lungs did not significantly differ between the two techniques, while VMAT allowed a better sparing of breast volumes receiving intermediate/ high doses potentially involved in the risk of second malignancies.As lymphoma therapy continues to evolve, with an emphasis on treatment reduction, radiation oncologists should use at best the available tools to minimize the dose to organs at risk. An individual approach considering age, gender, and finality of RT has been strongly recommended. PO-1467 Comparison of planning techniques in randomised radiotherapy trials for localised prostate cancer I. Gleeson 1 1 Addenbrooke's Hospital - Oncology Centre, Medical Physics, Cambridge, United Kingdom Purpose or Objective Moderate hypofractionated radiotherapy remains standard of care in many countries as SBRT data continues to mature. Heterogeneity exists across trials for techniques in localised prostate radiotherapy. Most trials report similarly high efficacy albeit with different toxicity. This work compares radiotherapy techniques used in the randomised external beam trials CHHiP, CHHiP IGRT sub- study, PACE B, PIVOTALboost arm A (PIV B) and PROFIT. Material and Methods 5 patients were planned by the same planner using 6 MV VMAT following trial protocols CHHiP, CHHiP IGRT sub study, PACE B, PIVOTAL boost arm A and PROFIT. Target volumes were grown as per their respective trial protocol in Table 1 and one set of OAR were used for all plans which was outlined as per CHHiP. Plans were prescribed 60 Gy in 20 fractions except PACE plans which was 62 Gy in 20 fractions. Target coverage was achieved and doses to the rectum, bladder and penile bulb were compared using one- way ANOVA Bonferroni’s multiple pairwise comparison test.

Conclusion Automatically generated LINAC-based SRS plans for multiple BM with one isocenter can make the number of MU and irradiation time smaller with a comparable or even better plan quality to manual DCA plans with the separate- isocenter approach. Based on all compared parameters, DCA plans with one isocenter were the best and comparable with manual DCA plans with separate isocentres for each metastasis. Moreover, we can conclude that automated DCA plans with v2.0 performed better than v1.6. PO-1466 Dosimetric comparison 3D vs VMAT tecniques in a pediatric population treated for Hodgkin's Lymphoma S. Meregalli 1 , S. Baroni 2 , C. Julita 3 , E. Bonetto 1 , D. Panizza 4 , G. Montanari 4 , F. Colangelo 2 , S. Arcangeli 2 1 Ospedale S. Gerardo, Radiation Oncology Department, Monza, Italy ; 2 Università Milano-Bicocca, Radiation Oncology Department, Monza, Italy ; 3 Ospedale S. Gerardo, Radiation Oncology Department, Monza, Italy ; 4 Ospedale S. Gerardo, Physics Department, Monza, Italy Purpose or Objective To retrospectively compare the dose distribution to non- targeted tissues in pediatric patients treated for mediastinal Hodgkin's lymphoma (HL) using 3D conformal radiotherapy (3D-CRT) and Volumetric Arc Therapy (VMAT). Material and Methods