P259] Radiobiological method for the clinical validation of models dose-to-water versus dose-to-meduim in radiotherapy

Abstract

Purpose A present challenge in medical physics is to implement the most accurate dose calculation algorithm, close to Monte Carlo simulations, with the highest safety to obtain the expected tumour control probability (TCP) and acceptable normal tissue complication probabilities (NTCP). The purpose is to propose dosimetric and radiobiological criteria to safely implement new algorithms in radiotherapy. Methods Lung tumors are taken as examples to demonstrate this transition. The doses were calculated using three types of algorithm: type (a) as pencil beam kernels model, like PBC; type (b) as point kernels model, like AAA and type (c) as Acuros-XB based on the deterministic resolution of the Boltzmann linear transport equation. Acuros-XB offers results close to Monte Carlo simulations and propose two options: dose to water (AXB-Dw) and dose to medium (AXB-Dm). The monitor units (MUs) are retrieved from the reference plan to recalculate the dose distribution with the new one. The equivalent uniform dose (EUD), TCP, NTCP and Uncomplicated Tumor Control Probability (UTCP) were calculated to measure the clinical benefit – toxicity. Results The more advanced algorithms showed that irradiating the lung tumor according to the same prescription, would increase the MUs (5–10%) when moving from pencil kernel to point kernel, and ∼ 2% when moving from AXB-Dw to AXB-Dm, p Conclusion The adjustment of prescribed dose depends on transition types and prescription method. In the lung, a reduction of prescription is advisable when moving from pencil beam model to point kernels model (e.g, 3 × 18 Gy instead of 3 × 20 Gy for SBRT and 5% for 3DCRT). In addition, an optimization should be carried out to obtain the same UTCP. To keep the same TCP and NTCP, we recommend respectively that the EUD for the target ⩾ prescription dose and EUD for healthy lung