An Investigation of Dosimetric Correlates of Acute Toxicity in Prostate Stereotactic Body Radiotherapy: Dose to Urinary Trigone is Associated with Acute Urinary Toxicity.

Abstract

AIMS: There are limited data on dosimetric correlates of toxicity in stereotactic body radiotherapy (SBRT) for prostate cancer. We aimed to identify potential relationships between dose and toxicity using conventional dose-volume histograms (DVHs) and dose-surface maps (DSMs). MATERIALS AND METHODS: Urinary bladder trigone and rectum DSMs were produced for a single-institution service evaluation cohort of 50 patients receiving SBRT for localised prostate cancer, together with conventional DVHs for bladder and rectum. Patients had been prospectively recruited to this cohort and treated according to a pre-defined protocol to a dose of 36.25 Gy in five fractions. Radiation Therapy Oncology Group (RTOG) and International Prostate Symptom Score (IPSS) toxicity data were recorded prospectively. Logistic regression was used to identify dosimetric predictors of acute IPSS+10 (rise of 10 points or more above baseline) and grade 2+ RTOG toxicity. RESULTS: On univariate analysis, trigone area receiving 40 Gy and trigone Dmax were associated with IPSS+10 (odds ratio 1.06 [1.02-1.11], P = 0.007 and odds ratio 1.54 [1.06-2.25], P = 0.024, respectively). These two variables were highly correlated. In a multivariate model, including all baseline variables, trigone Dmax remained associated with IPSS+10 (odds ratio 1.91 [1.13-3.22], P = 0.016). These findings were not significant with Holm-Bonferroni correction for multiple testing (corrected P value threshold 0.006). No associations were seen between rectal toxicity and DVH or DSM parameters. CONCLUSIONS: Our study suggests a potential relationship between high doses to the urinary bladder trigone and patient-reported urinary toxicity in prostate SBRT, and is consistent with previous studies in conventionally fractionated radiotherapy, justifying further evaluation in larger cohorts.