Dosimetric predictors of biochemical control of prostate cancer in patients randomised to external beam radiotherapy with a boost of high dose rate brachytherapy.

Background: To correlate dose and volume dosimetric parameters (D₉₀ and V₁₀₀) with biochemical control in advanced prostate cancer treated with high-dose rate brachytherapy (HDR-BT).

Methods: One hundred and eight patients received external beam radiotherapy (EBRT) to 35.75 Gy in 13 fractions followed by HDR-BT of 2 × 8.5 Gy. Kaplan-Meier freedom-from-biochemical relapse (FFbR; nadir+2 μg/L) fits were grouped by the first (Q1), second (Q2) and third (Q3) D₉₀ and V₁₀₀ quartiles. Groups were compared with the log-rank test. Univariate and multivariate Hazard Ratios (HR) for D₉₀ and V₁₀₀ and other co-variates (PSA, androgen deprivation therapy (ADT) were obtained using Cox's proportional hazard model.

Results: FFbR was significantly higher in patients whose D₉₀ and V₁₀₀ were at or above the second and third quartile (log rank p ≤ 0·04). In multivariate analysis D₉₀, V₁₀₀ were significant covariates for risk of relapse.

Conclusions: Dichotomising the data using 6 levels of response (above and below Q1, Q2 and Q3) showed a progressive and continuous improvement in biochemical control of disease across the entire dose (and volume) range. The data show that a minimum D₉₀ of 108% of the prescribed dose should be the target to achieve.