Evaluation of external beam radiotherapy and brachytherapy for localized prostate cancer using equivalent uniform dose.

Various radiotherapy (RT) modalities, such as external beam radiotherapy (EBRT) and permanent/high-dose-rate (HDR) brachytherapy, have been used for the management of localized prostate cancer. Using the linear-quadratic (LQ) model, we compared the relative merits of these modalities in terms of equivalent uniform dose (EUD) and tumor control probability (TCP). The LQ parameters (alpha = 0.15 Gy(-1) and alpha/beta = 3.1 Gy) determined recently from compiled clinical data, as well as other sets of LQ parameters for prostate cancer, were used to carry out the EUD and TCP calculations. A computer code was developed for this purpose. We calculate the EUD for some common RT modalities, and present the corresponding TCP data predicted for a sample patient group (high-risk). Biological equivalence of treatment outcome among various RT modalities is demonstrated. The model suggests that the hypofractionation is preferred in terms of tumor control, due to the lower alpha/beta ratio. Also, the current combined treatment schemes (initial EBRT + permanent/HDR brachytherapy boost) provide higher EUD and TCP than these monotherapies. The study shows that EUD is less sensitive to model parameters than TCP, and EUD can be used to compare and to optimize treatment plans involving different RT modalities. Techniques to further optimize and/or to combine external beams with brachytherapy for better treatment outcomes are proposed.