ESTRO 37 Abstract book

S1258

ESTRO 37

Conclusion These results demonstrated that heavy ion ( 12 C 6+ ) presented special advantages in terms of treating malignant melanoma. EP-2278 Second malignancy risk in cranio-spinal irradiation with protons compared to conventional techniques J. Selvaraj 1 , C. McKenzie 2,3 , V. Sakthivel 4,5 1 ACT Govt Health Directorate, Medical Physics, Canberra, Australia 2 Miami Cancer Institute, Department of Radiation Oncology, Miami, USA 3 RaySearch Laboratories, New York, USA 4 Advanced Medical Physics, Medical Physics, Houston, USA 5 Bharatiyar University, Department of Medical Physics, Coimbatore, India Purpose or Objective Cranio-spinal irradiation (CSI) is widely used for treating medulloblastoma cases in children. Radiation- induced second malignancy is of grave concern; especially in children due to their long-life expectancy and higher radiosensitivity of tissues at young age. Several techniques can be employed for CSI including 3DCRT, IMRT, VMAT and tomotherapy. However, these techniques are associated with higher risk of second malignancy due to the physical characteristics of photon irradiation which deliver moderately higher doses to normal tissues. On the other hand, proton beam therapy delivers substantially lesser dose to normal tissues due to the sharp dose fall off beyond Bragg peak compared to photon therapy. The aim of this work is to quantify the relative decrease in the risk with proton therapy compared to other photon treatments for CSI. Material and Methods Ten anonymized patient DICOM datasets treated previously were used for this study. 3DCRT, IMRT, VMAT, tomotherapy and proton therapy with pencil beam scanning (PBS) plans were generated. The prescription dose was 36 Gy in 20 fractions. PBS was chosen due to substantially lesser neutron dose compared to passive scattering. An RBE value of 1.1 was used for the proton plans. The age of the patients ranged from 3 to 12 with a median age of 8 with 6 male and 4 female patients. Commonly used linear and a mechanistic dose-response models (DRM) were used for the analyses. Dose-volume histograms (DVH) were calculated for critical structures to calculate organ equivalent doses (OED) to obtain excess absolute risk (EAR), life-time attributable risk (LAR) and other risk relevant parameters. A α’ value of 0.018 Gy -1 and a repopulation factor R of 0.93 was used in the mechanistic model for carcinoma induction. Gender- specific correction factor of 0.17 and -0.17 for females and males were used for the EAR calculation. Results The relative integral dose of all critical structures averaged were 4.7, 4.5, 4.8 and 6.3 times higher in 3DCRT, IMRT, VMAT and tomotherapy respectively compared to proton therapy. The mean relative LAR calculated from the mean EAR of all organs with linear DRM were 3.3, 2.7, 2.3,2.3 higher for male and 3.8,2.8,2.8 and 2.5 times higher for female patients compared to proton therapy. The same values with the mechanistic model were 1.0,2.0,2.0,2.0 and 2.0,3.0,3.0,3.0 times higher compared to proton therapy for male and female patients respectively. All critical

structures except lungs and kidneys considered in this study had a substantially lower OED in proton plans.

Conclusion Risk of radiation-induced second malignancy in Proton PBS compared to conventional photon treatments were up to three and four times lesser for male and female patients respectively with the linear DRM. Using the mechanistic DRM these were up to two and three times lesser in proton plans for male and female patients respectively. EP-2279 Infidelity in proton therapy: Getting double strand breaks back together N. Henthorn 1 , J. Warmenhoven 1 , M. Sotiropoulos 1 , R. Mackay 2 , K. Kirkby 1 , M. Merchant 1 1 University of Manchester, Divisioion of Cancer Sciences, Manchester, United Kingdom 2 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective The relative biological effectiveness (RBE) of protons relative to photons is 1.1. However, large variability for the measured RBE has been seen in the literature, depending on several factors such as dose, linear energy transfer (LET), cell type etc. Failure to understand, and properly utilise, this variable RBE limits the efficacy of proton beam therapy. It has been observed that RBE increases with LET, where the greatest cell kill is at the end of the proton range. However, there is no mechanistic understanding of this phenomenon. It is thought that the link between LET and increased RBE could be related to the proton track structure, where higher LET particles produce: a greater yield of DNA double strand breaks (DSB), more complex DSBs, and In this work we score DSBs within a realistic DNA model irradiated by various LET protons, simulated with the Monte Carlo toolkit 'Geant4-DNA”, including direct and indirect damage. The DNA damage patterns produced are used as an input for our mechanistic diffusion controlled non-homologous end joining repair simulations, helping to understand the initial biological response. more proximal DSBs. Material and Methods