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To estimate the limits of dose escalation for prostate cancer as a function of planning target volume (PTV) margins, the maximum achievable dose (MAD) was determined through iterative plan optimizations from data sets of 18 patients until the dose constraints for rectum, bladder and PTV could no longer be met. PTV margins of 10, 5 and 3 mm yielded a mean MAD of 83.0 Gy (range, 73.8-108.0 Gy), 113.1 Gy (range, 90.0-151.2 Gy) and 135.9 Gy (range, 102.6-189.0 Gy), respectively. All comparisons of MAD among margin groups were statistically significant (P < 0.001). Comparison of prostate volumes of 30-50 mL (n = 8) with volumes of 51-70 mL (n = 7) and 71-105 mL (n = 3) showed an inverse relationship with MAD. Decreases in PTV margin significantly decreased the PTV overlap of the rectum (P < 0.001 for all margin comparisons). With decreases in the PTV margin and maintenance of identical dose constraints, doses well above those currently prescribed for treatment of localized prostate cancer appear feasible. However, the dose escalation suggested by these findings is a theoretical estimate, and additional dose constraints will likely be necessary to limit toxicity to normal tissue.