The Loma Linda University Proton Treatment Facility has treated over 5,000 patients for prostate cancer. Other institutions may find information regarding field size and range requirements for this population of patients useful for designing new proton beamlines. The maximum range, range modulation, and maximum field radius for 240 fields of prostate patients undergoing treatment were sampled and analyzed. Most fields required a range less than 290 mm of water, a modulation width less than or equal to 120 mm, and a radius less than 75 mm.

Key words: Proton, Range, Modulation, Field size.