To examine the effect the number of biopsy cores taken has on the rate of clinically significant Gleason sum upgrading (GSU) in patients with low-risk prostate cancer.

We analyzed the data from 301 patients with low-risk prostate cancer (clinical Stage T1c-T2a, prostate-specific antigen ≤10 ng/mL, and biopsy Gleason sum ≤6) who underwent an extended (≥10-core) prostate biopsy. Prostate-specific antigen level, clinical stage, biopsy Gleason sum, prostate volume, year of diagnosis, number of biopsy cores taken, and number of positive cores were used as predictors in logistic regression models addressing the rate of clinically significant GSU (defined as upgrading from biopsy Gleason sum 5-6 to radical prostatectomy Gleason sum of ≥7). Regression coefficients were used to estimate the predictive accuracy of individual variables, as well as their combined effect in the prediction of GSU.

The median number of biopsy cores taken was 18. Upgrading was recorded in 96 (31.9%). In men assessed with 10-12 cores, the rate of GSU was 47.9% compared with 23.5% if >18 cores were taken (P < .001). In multivariate analyses, the consideration of the variable defining the number of cores added 9.0% (P < .001) to the ability to predict GSU.

Patients with low-risk prostate cancer assessed with fewer biopsy cores are at a substantially greater risk of GSU. The number of biopsy cores taken represents one of the foremost predictors of GSU and should be taken into consideration during clinical decision-making in patients who are candidates for watchful waiting, active surveillance, or brachytherapy.