Pathological predictors for site of local recurrence after radiotherapy for prostate cancer

Purpose: Rational design of targeted radiotherapy (RT) in prostate cancer (Pca) hinges on a better understanding of spatial patterns of recurrence. We sought to identify pathological factors predictive for site of local recurrence (LR) after external beam RT. Methods and Materials: Prospective databases were reviewed to identify men with LR after RT from 1997 through 2009. Patients with biochemical failure and biopsy-confirmed Pca more than 2 years after RT were evaluated. Prediction for site of recurrence based on the following pretreatment factors was determined on independent and cluster-sextant basis: presence of malignancy, dominant vs. nondominant percentage core length (PCL) involvement, PCL ≥ or <40%, and Gleason score. Sites of dominant PCL were defined as sextants with peak PCL involvement minus 10%, and >5% for each patient. Results: Forty-one patients with low-intermediate risk Pca constituted the study cohort. Median time to biopsy after RT was 51 months (range, 24-145). Of 246 sextants, 74 were involved with tumor at baseline. When sextants are treated as independent observations the presence of malignancy (77% vs. 22%, p = 0.0001), dominant PCL (90% vs. 46%, p = 0.0001), and PCL ≥40% (89% vs. 68 %, p = 0.04) were found to be significant predictors for LR, although PCL ≥40% did not retain statistical significance if sextants were considered correlated. The vast majority of patients (95%) recurred at the original site of dominant PCL or PCL ≥40%, and 44% also recurred in regions of nondominant PCL <40% (n = 8) and/or benign sampling (n = 14) at baseline. Conclusions: LR after RT predominantly occurs in regions bearing higher histological tumor burden but are not isolated to these sites. Our data highlights the value of spatially resolved baseline pathological sampling and may assist in the design of clinical trials tailoring RT dose prescriptions to subregions of the prostate gland.