Acute genitourinary toxicity of pencil beam scanning proton therapy for localized prostate cancer: utility of the transition zone index and average urinary flow rate in predicting acute urinary retention.

The incidence of acute genitourinary toxicities was lower in the moderately hypo-fractionated proton therapy compared with the normo-fractionated proton therapy. Lower pretreatment average flow rate and a higher transition zone index were useful predictors of grade 2 acute urinary retention.