This paper describes the functionality, radiation characteristics, and clinical implementation of an implantable MOSFET radiation detector (dosimeter). The dosimeter is powered by radio frequency telemetry eliminating the need for a power source inside the dosimeter. The data can be accessed telemetrically for each treatment day during the course of therapy. The detector has been validated in vitro to confirm its accuracy. Variance between predicted and measured dose in patients is discussed. Factors such as patient setup, treatment plan error, and physiologic motion can affect the accuracy of dose delivery in moving from in vitro to in vivo dose measurements. The initial data suggests that the dosimeters can play a useful role in tracking dose discrepancies, both random and systematic, in patients treated with external beam radiation therapy. The implantable dosimeter can be used, together with the current radiation delivery and planning techniques, to optimize radiation treatment on an individual basis.