SU-E-T-299: Sensitivity Analysis of the Radiobiological Parameters Used to Examine the Effectiveness of Different Fractionation Schedules in Prostate Cancer HDR Brachytherapy

Purpose: The aim of the present study is to evaluate the robustness of the effectiveness of different fractionation schemes of HDR brachytherapy (to be applied as monotherapy for low-risk prostate cancer) by using radiobiological models and dosimetric measures. Methods: In this study, the treatment plans of 12 patients who had undergone clinical implants for HDR brachytherapy of prostate cancer were evaluated using the response probabilities of the individual organs, the complication-free tumor control probability (P+), the total tumor control probability (PB), the total normal tissue complication probability (PI), the biologically effective uniform dose (BEUD) and the generalized equivalent uniform dose (gEUD) as measures. These indices use the D50, γ, s, α/β and alpha radiobiological parameters together with the corresponding tissue DVH data. The values of those parameters were varied around their reference values in order to examine the impact of their uncertainty on the values of the radiobiological measures in the evaluation of four fractionation schemes. Results: For the four examined fractionation schemes, the tumor control probabilities are 98.5% in 1×20Gy, 98.6% in 2×14Gy, 97.5% in 3×11Gy and 97.8% in 4x9.5Gy. The corresponding total normal tissue complication probabilities are 9.7% in 1×20Gy, 14.6% in 2×14Gy, 11.5% in 3×11Gy and 15.6% in 4×9.5Gy. The complication-free tumor control probabilities are 88.8% in 1x20Gy, 83.9% in 2×14Gy, 86.0% in 3×11Gy and 82.3% in 4×9.5Gy. The sensitivity analysis showed that the values of P+, PB, PI and BEUD varied by 6.1–7.3%, 5.8–9.3%, 5.6–7.2% and 7.6–9.0 Gy, respectively. Conclusion: The present analysis indicates that among the fractionation schemes of 1×20Gy, 2×14Gy, 3×11Gy and 4×9.5Gy the first scheme is more effective. The sensitivity analysis shows that the variation of the radiobiological parameters considerably affects the values of the response indices, especially the α/β parameter.