Exploring Factors Associated with Late Urinary Toxicity After Prostate Stereotactic Body Radiotherapy: Findings from the PACE-B Study.

BACKGROUND AND

OBJECTIVE

Stereotactic body radiotherapy (SBRT) is increasingly used for localised prostate cancer but is associated with higher urinary toxicity within 2 yr. This study identifies factors associated with late urinary toxicity after SBRT.

METHODS

PACE-B SBRT patients with dose-volume histogram data were analysed. All had low/intermediate-risk prostate cancer treated with SBRT alone (36.25 Gy per five fractions to planning target volume and 40 Gy per five fractions to clinical target volume). Doses to the surrogate/contoured urethra, bladder trigone, and bladder were extracted. Logistic regression identified factors associated with late toxicity. Outcomes of interest were Common Terminology Criteria for Adverse Events (CTCAE) grade 2+ urinary toxicity and International Prostate Symptom Score (IPSS; with imputation) +7 at 2 yr (a ≥7-point rise above baseline).

KEY FINDINGS AND LIMITATIONS: Of 390 patients, 357 had 2-yr CTCAE data; 12% (n = 44) experienced grade 2+ urinary toxicity. Associated factors included higher baseline IPSS (odds ratio [OR] 1.11, 95% confidence interval or CI [1.05-1.18], p = 0.001), baseline urinary medication use (OR 2.32, 95% CI [1.09-4.95], p = 0.03), and grade 2+ acute urinary toxicity (OR 3.15, 95% CI [1.35-7.37], p = 0.008). Conventional LINAC (CL)-SBRT without fiducials and CyberKnife SBRT were associated with lower odds of grade 2+ urinary toxicity at 2 yr compared with CL-SBRT with fiducials; however, this association was not observed when using IPSS as the endpoint. No association was seen between urinary substructure dose and late toxicity.

CONCLUSIONS AND CLINICAL IMPLICATIONS: Baseline urinary symptoms are associated with late urinary toxicity; patients with worse symptoms should be counselled on their elevated risk and consider moderate hypofractionation. The observed association between treatment technique and toxicity may reflect variations in urinary medication prescribing practices and warrants further validation.