Rectal and Urethro-vesical Subregions for Toxicity Prediction After Prostate Cancer Radiotherapy: validation of voxel-based models in an independent population

Abstract Purpose /Objectives: Recent voxel-based studies have shown that the dose to specific rectal and urethro-vesical subregions is predictive of toxicities, following prostate cancer IMRT. The objective of this study was to validate the discriminatory power of these subregions with respect to the whole organs, in a large independent population. Material/Methods The validation cohort consisted of 450 patients, from the XXX(BLIND) trial, treated with 3D-CRT at 66-74 Gy. Previous voxel-based analyses identified an infero-anterior rectal subregion as predictive of rectal bleeding and five subregions in the urethra and the posterior and superior part of the bladder as predictive of urinary incontinence, dysuria, retention and hematuria. In the validation cohort, these subregions were segmented in each patient’s anatomy. DVHs of the whole organs and the six subregions were compared bin-wise between patients with and without toxicities. The discriminatory power of DVHs for grade≥2 toxicity endpoints was assessed using the area under the ROC curve (AUC). Results Subregion DVHs were significantly different between patients with and without toxicities for late rectal bleeding (V44-V74), acute urinary incontinence (V68-V72), late dysuria (V56-V68) and late retention (V14-V64). The dose to the rectal subregion and the whole rectum were equally predictive of rectal bleeding (V68;AUC=0.61). The doses to three out of the five urethro-vesical subregions were found to be more predictive than the dose to the whole bladder: in the urethra for acute incontinence (V71;AUC=0.69 versus V71;AUC=0.66), in the posterior part of the bladder for late dysuria (V65;AUC=0.66 vs V68;AUC=0.59) and late retention (V39;AUC=0.74 vs no significant AUC). Conclusion Three subregions located in the urethra and the bladder were successfully validated as more predictive of urinary toxicity than the whole bladder, for urinary incontinence, retention and dysuria. Sparing in particular the posterior part of the bladder in treatment planning may reduce the risk of late urinary retention.