Modification of an Implantable MOSFET Dosimeter to Act as a Fiducial Marker

In radiation therapy, the tumor and normal tissue response is highly dependent on accurate absorbed dose delivery to the intended target. Although ex vivo techniques are widely used to improve the accuracy of the dose delivered, it is impossible to verify that the dose has been delivered correctly without measuring the dose within the patient. The Dose Verification System (Sicel Technologies, Inc., Morrisville, NC), which consists of an implantable MOSFET dosimeter and a wireless reader, is currently the only commercial dosimetry system capable of measuring this in situ dose. Since the MOSFET dosimeter is implanted in the patient, it has been suggested that it could be used as not only a dosimeter but also a fiducial marker for image-guided radiation therapy (IGRT) provided that it could be seen on radiographs. Though visible on kilovoltage radiographs, the current design of the dosimeter is not visible on megavoltage radiographs limiting its usefulness in IGRT treatment. In this work, we modified the MOSFET dosimeter to increase its visibility on megavoltage radiographs. We then compared the visibility of the modified dosimeter with that of the commercial (unmodified) dosimeter and a traditional gold fiducial marker in an anthropomorphic phantom on an IGRT system. To ensure that the modification did not affect the dosimeter´s ability to measure and transmit dose data, we compared the modified dosimeter´s wireless read range, energy dependence, and angular dependence with those of the commercial dosimeter. We conclude that modifying the MOSFET dosimeter increased its visibility without affecting its dosimetric properties, thereby allowing a single implanted device to serve two critical roles in IGRT.