Dosimetry Dust: An Ultrasonic Backscatter Implantable Dosimetry Device

Proton beam therapy is a well-established medical procedure for treating certain kinds of cancer, and is uniquely suited for treatment of head, neck, and eye tumors. Despite the continuous improvements in medical physics treatment plan simulations, improper tissue irradiation can easily occur if there is a shift in the tumor and/or critical organs during the irradiation process. A fully implantable dosimeter for in vivo dose measurements can provide closed loop treatment feedback to a physician during radiation treatments, and assist in enabling full irradiation of a tumor. This work details the theory behind a fully implantable, 1 mm3 MOSFET dosimeter mote, and provides a set of instructions for characterizing this mote for in vivo work. An o↵ the shelf nMOS, ALD1106, was irradiated at Crocker Nuclear Laboratory at UC Davis using an ocular melanoma treatment plan. The ALD1106 devices were further characterized with a piezoelectric transducer and a water tank in order to determine backscatter amplitude shifts as a function of dose received, a precursor to future in vivo work with an nMOS ASIC. The experimental method used and improvements for the fully implantable device are also described.