Magnetic resonance imaging (MRI)-targeted, 3D transrectal ultrasound (TRUS)-guided "fusion" prostate biopsy aims to reduce the 21–47% false negative rate of clinical 2D TRUS-guided sextant biopsy, but still has a substantial false negative rate. This could be improved via biopsy needle target optimization, accounting for uncertainties due to guidance system errors, image registration errors, and irregular tumor shapes. As an initial step toward the broader goal of optimized prostate biopsy targeting, in this study we elucidated the impact of biopsy needle delivery error on the probability of obtaining a tumor sample, and on the core involvement. These are both important parameters to patient risk stratification and the decision for active surveillance vs. definitive therapy. We addressed these questions for cancer of all grades, and separately for high grade (≥ Gleason 4+3) cancer. We used expert-contoured gold-standard prostatectomy histology to simulate targeted biopsies using an isotropic Gaussian needle delivery error from 1 to 6 mm, and investigated the amount of cancer obtained in each biopsy core as determined by histology. Needle delivery error resulted in variability in core involvement that could influence treatment decisions; the presence or absence of cancer in 1/3 or more of each needle core can be attributed to a needle delivery error of 4 mm. However, our data showed that by making multiple biopsy attempts at selected tumor foci, we may increase the probability of correctly characterizing the extent and grade of the cancer.