Radiobiological analysis of the response of prostate cancer to different fractionations

Purpose To investigate the response of prostate cancer to different radiotherapy schedules, including hypofractionation, and to evaluate potential departures from the linear-quadratic (LQ) response. To obtain best-fitting parameters for low (LR), intermediate (IR), and high risk (HR) prostate cancer. Methods and Materials We have constructed a dataset of dose-response containing 87 entries (35 LR, 32 IR, 20 HR), with doses per fraction ranging from 1.8 to 10 Gy. These data were fitted to tumor control probability models based on the LQ model, linear-quadratic-linear (LQL), and a modification of the LQ (LQmod) accounting for increasing radiosensitivity at large doses. Fits were performed with the maximum likelihood expectation methodology, and the Akaike-Information-Criterion (AIC) was used to compare models. Results The AIC shows that the LQ model is superior to the LQL and LQmod for all risks, except for IR where the LQL outperforms the other models. The analysis shows a low α/β for all risks: 2.01 Gy for LR (95% confidence interval 1.74-2.26), 3.44 Gy for IR (2.99-4.02), and 2.78 Gy for HR (1.43-4.18). Best-fits do not show proliferation for LR, and only moderate proliferation for IR/HR. Conclusions In general, the LQ model describes the response of prostate cancer better than the alternative models. Only for IR the LQL outperforms the LQ. This study confirms a low α/β for all risks, with doses per fraction ranging from <2 Gy up to 10 Gy.