ESTRO 36 Abstract Book

S526 ESTRO 36 2017 _______________________________________________________________________________________________

the use of predictive biomarkers for assessing radiotherapy related toxicity.

Poster: Radiobiology track: Radiobiology of proton and heavy ions

PO-0960 Radiobiological effectiveness and its role in modelling secondary cancer risk for proton therapy A. Madkhali 1,2 , C. Timlin 1 , M. Partridge 1 1 University of Oxford, Oncology, Oxford, United Kingdom 2 King Saud University, Medicine, Riyadh, Saudi Arabia Purpose or Objective In proton therapy, a radiobiological effectiveness ratio (RBE) of 1.1 (RBE 1.1 ) is often used. In reality, RBE depends on dose, linear energy transfer (LET), biological end point, and tissue type. Using a value of RBE that may be not accurate may affect dose calculation and hence, outcome. Material and Methods We used an in-house built code for modelling malignant induction probability (MIP) from voxel-by-voxel dose map (Timlin 2014) and implement a published model to calculate structure-specific RBE, recalculate dose and MIP, and compare the outcomes with initial calculations using RBE 1.1 . MIP was calculated using linear quadratic (LQ), linear (LIN), and linear-no-threshold (LNT) models for proton therapy plans for an adult and a teenage patient diagnosed with medulloblastoma (MB). The MIP was then re-calculated using the RBE model by Dale and Jones which is a function of dose (d), α and β and RBE min and RBE max:

Results Results are shown in Table 1. The difference in MIP by using RBE 1.1 and RBE MinMax is ~2-3%. The effect on mean dose varies between different organs and is between 6% and 8%. Clinical implications due to difference in RBE depend on beam characteristics, dose, structures concerned, and the volume irradiated.

Conclusion Using RBE 1.1

makes proton therapy dose and dose- dependent predictions less accurate. Our results using a RBE calculation model show that decreased accuracy may have clinical implications, which agrees with published literature (Jones 2012; Jones, 2014), and may affect secondary cancer risk and normal tissue complication probability calculations as well. PO-0961 DNA damage and repair influence tumor sensitivity to diffusing alpha emitters radiation therapy Y. Keisari 1 , R. Etzyoni 1 , H. Bittan 2 , E. Lazarov 2 , M. Efrati 1 , M. Schmidt 2 , T. Cooks 1 , L. Arazi 2 , I. Kelson 2 1 Tel-Aviv University / Faculty of Medicine, Clinical Microbiology and Immunology, Tel-Aviv, Israel 2 Tel Aviv University, School of Physics and Astronomy- Sackler Faculty of Exact sciences, Tel Aviv, Israel Purpose or Objective We developed an alpha radiation based brachytherapy, which provides efficient ablation of solid tumors by alpha radiation. This treatment termed, Diffusing Alpha emitters Radiation Therapy (DaRT) utilizes radium-224 loaded wires, which when inserted into the tumor release by recoil short-lived alpha-emitting atoms. These atoms disperse in the tumor, and spray it with highly destructive alpha radiation. DaRT achieved substantial tumor growth retardation, extended survival, and reduced lung metastases in mice bearing various mouse and human derived tumors. Better tumor control was achieved when DaRT was applied with chemotherapy. Furthermore, tumor ablation by DaRT boosted anti-tumor immune responses.