Purpose: Accumulating evidence demonstrates that prostate cancer has a low alpha/beta ratio. However, several challenging issues have been raised from previous studies, including the biologic equivalence between external beam radiotherapy (EBRT) and brachytherapy, the effect of relative biologic effectiveness (RBE) for permanent implantation, and the systematic uncertainties of multi-institutional and multi-modality clinical data. The purpose of this study is to address these issues by reexamining a reported clinical outcome of high-dose-rate (HDR) brachytherapy and to confirm the low alpha/beta ratio for prostate cancer.
Methods and materials: The generalized linear-quadratic (LQ) model with considerations of sublethal damage repair and clonogen repopulation was used to calculate the cell-killing efficiency of radiotherapy treatments for prostate cancer. Standard models of tumor cure based on Poisson statistics were used to bridge cell killing to treatment outcome. The data collected in a clinical trial using EBRT plus HDR brachytherapy boost for prostate cancer at William Beaumont Hospital (WBH) were reanalyzed. A 4-year post-treatment time endpoint was chosen as compared to the 3-year endpoint used in the previous study because of better maturity and stability of the data. The least chi-square method was employed to fit the clinical data to estimate the LQ parameters as well as their confidence intervals. The number of clonogens for prostate tumors derived in a separate study was used as a constraint for the data modeling to improve the confidence level.
Results: Our analysis demonstrates that only relationships among the LQ parameters, not their definitive and unique values, can be derived from the WBH data set alone. This is due to the large statistical uncertainties, i.e., the small numbers of sampled patients. By combining with the results obtained with the clinical data from Memorial Sloan-Kettering Cancer Center (MSKCC), a new set of LQ parameters (alpha = 0.14 +/- 0.05 Gy(-1), alpha/beta = 3.1(-1.6)(+2.6) Gy) was obtained from the current analysis of the WBH data without dealing with data from permanent implants. The results are consistent with a previous study based on the biologic equivalence between EBRT and permanent implants with a consideration of tumor repopulation. This set of LQ parameters provides a consistent interpretation of clinical data currently available for prostate cancer.
Conclusions: This study provides further evidence to support that prostate cancer has a low alpha/beta ratio of about 3.1 Gy. This study shows that the RBE effect in permanent implantation may not be clinically significant for prostate cancer. The consistency found between this analysis and the previous reported study supports the general biologic equivalence between EBRT and brachytherapy treatments for prostate cancer. The low alpha/beta ratio opens the door to search for more effective radiotherapeutic approaches for prostate cancer, e.g., hypofractionation radiotherapy.