Three-dimensional summation of rectal doses in brachytherapy combined with external beam radiotherapy for prostate cancer

Abstract Background and purpose To determine the dose constraints for rectal bleeding in brachytherapy (BRT) combined with external beam radiotherapy (EBRT). Materials and methods Post-BRT, pelvic computed tomography images were used for subsequent EBRT planning and BRT postplans in 37 patients. The physical doses for each plan were converted to biologically effective doses, and corresponding voxel doses were integrated to plot the summed dose-volume histogram (sum-DVH). Between 5 patients with (bled-pts) and 32 without (spared-pts) grade 2 or 3 rectal bleeding, the differences in the mean minimal dose (rDn) covering the rectal volume of 0.5–10.0 cc and the rectal volume (rVn) receiving the calculated dose of 20–150 Gy were compared. Results The differences in the summed-rDn were determined by BRT exposure, while those of the summed-rVn were determined in the low-dose range and superimposed in the high-dose range by EBRT exposure. Of the 13 patients with rV150 of >1.2 cc, 4 were bled-pts (30.8%). Of the 24 patients with rV150 of ≦ 1.2 cc, 1 was a bled-pts (4.2%) ( p = 0.024; odds ratio, 10.2; CI (95%), 1.0–104.3). Conclusions The mono-scale DVH analysis is a promising method for exploring the threshold for rectal bleeding in combined radiotherapy.