Biological-effective versus conventional dose volume histograms correlated with late genitourinary and gastrointestinal toxicity after external beam radiotherapy for prostate cancer: a matched pair analysis

Background: To determine whether the dose-volume histograms (DVH's) for the rectum and bladder constructed using biological-effective dose (BED-DVH's) better correlate with late gastrointestinal (GI) and genitourinary (GU) toxicity after treatment with external beam radiotherapy for prostate cancer than conventional DVH's (C-DVH's). Methods: The charts of 190 patients treated with external beam radiotherapy with a minimum follow-up of 2 years were reviewed. Six patients (3.2%) were found to have RTOG grade 3 GI toxicity, and similarly 6 patients (3.2%) were found to have RTOG grade 3 GU toxicity. Average late C-DVH's and BED-DVH's of the bladder and rectum were computed for these patients as well as for matched-pair control patients. For each matched pair the following measures of normalized difference in the DVH's were computed: (a) delta(AUC) = (Area Under Curve [AUC] in grade 3 patient - AUC in grade 0 patient)/(AUC in grade 0 patient) and (b) delta(V60) = (Percent volume receiving = 60 Gy [V60] in grade 3 patient - V60 in grade 0 patient)/(V60 in grade 0 patient). Results: As expected, the grade 3 curve is to the right of and above the grade 0 curve for all four sets of average DVH's - suggesting that both the C-DVH and the BED-DVH can be used for predicting late toxicity. delta(AUC) was higher for the BED-DVH's than for the C-DVH's - 0.27 vs 0.23 (p = 0.036) for the rectum and 0.24 vs 0.20 (p = 0.065) for the bladder. delta(V60) was also higher for the BED-DVH's than for the C-DVH's - 2.73 vs 1.49 for the rectum (p = 0.021) and 1.64 vs 0.71 (p = 0.021) for the bladder. Conclusions: When considering well-established dosimetric endpoints used in evaluating treatment plans, BED- DVH's for the rectum and bladder correlate better with late toxicity than C-DVH's and should be considered when attempting to minimize late GI and GU toxicity after external beam radiotherapy for prostate cancer.