Technical Evaluation of Radiation Dose Delivered in Prostate Cancer Patients as Measured by an Implantable MOSFET Dosimeter

Purpose To perform a comparison of the daily measured dose at depth in tissue with the predicted dose values from treatment plans for 29 prostate cancer patients involved in a clinical trial. Methods and Materials Patients from three clinical sites were implanted with one or two dosimeters in or near the prostatic capsule. The implantable device, known as the DVS, is based on a metal-oxide-semiconductor field effect transistor (MOSFET) detector. A portable telemetric readout system couples to the dosimeter antenna (visible on kilovoltage, computed tomography, and ultrasonography) for data transfer. The predicted dose values were determined by the location of the MOSFET on the treatment planning computed tomography scan. Serial computed tomography images were taken every 2 weeks to evaluate any migration of the device. The clinical protocol did not permit alteration of the treatment parameters using the dosimeter readings. For some patients, one of several image-guided radiotherapy (RT) modalities was used for target localization. Results The evaluation of dose discrepancy showed that in many patients the standard deviation exceeded the previous values obtained for the dosimeter in a phantom. In some patients, the cumulative dose disagreed with the planned dose by ≥5%. The data presented suggest that an implantable dosimeter can help identify dose discrepancies (random or systematic) for patients treated with external beam RT and could be used as a daily treatment verification tool for image-guided RT and adaptive RT. Conclusion The results of our study have shown that knowledge of the dose delivered per fraction can potentially prevent over- or under-dosage to the treatment area and increase the accuracy of RT. The implantable dosimeter could also be used as a localizer for image-guided RT.