Abstract Purpose Despite the existence of guidelines for permanent prostate brachytherapy, it is unclear whether there is interinstitutional consensus concerning the parameters of an ideal implant. Methods and material Three institutions with extensive prostate brachytherapy expertise submitted information regarding their implant philosophy and dosimetric constraints, as well as data on up to 50 radioiodine implants. Regression analyses were performed to reflect each institution's utilization of seeds and implanted activity. Results Despite almost identical implant philosophy, target volume, and dosimetric constraints, there were statistically significant interinstitutional differences in the number of seeds and total implant activity across the range of prostate volumes. For larger volumes, the variation in implanted activity was 25%; for smaller glands, it exceeded 40%. Conclusions There remain wide variations in implanted activity between institutions espousing seemingly identical implant strategies, prescription, and dosimetry constraints. Brachytherapists should therefore be wary of using nomograms generated at other institutions.