Avalanche Photodiode-Based γ-Photon Scintillation Detectors for Personalized Dosimetry in Targeted Radionuclide Therapy

Abstract

<h3>Purpose/Objective(s)</h3> Targeted radionuclide therapy holds great promise as shown in neuroendocrine and prostate cancer, but highly variable biodistribution and tumor uptake requires personalized dosimetry for each patient. Optimal dosimetry requires the measurement of total integrated dose, demanding dose measurements over multiple half-lives of the radionuclide. The current state-of-art, SPECT, is often only available to a small subset of patients and is rarely done more than once. Therefore, a method for continuous radionuclide dosimetry is needed. Highly-sensitive γ-photon detectors placed on the body could solve this problem by monitoring γ decays indicative of the activity within tumors and organs at risk (OAR). We have developed a system capable of detecting γ-photons externally, in real time, with high sensitivity and dynamic range. Here we present our results for photon detection as well as a methodology to use these signals to monitor the activity in tumors and OAR in real time. <h3>Materials/Methods</h3> Y₂O₃ Eu-doped phosphor was compacted at the interface of a bare optical fiber to scintillate incoming γ-photons into 610 nm (red) light. The optical fiber was coupled to avalanche photodiodes (APDs) with a 60% peak detection efficiency at 610 nm and dead time of 50ns. The transistor-transistor logic signals from each APD are conveyed to an FPGA to compute counts per minute (CPM). The system was tested by measuring the emitted γ-photons from a titration of ¹⁷⁷Lu. Six vials were prepared by serially diluting ¹⁷⁷Lu to activities per mL ranging from 0.6∼566.2µCi/mL. The detector was placed on the outside of the vials, and the vials were incrementally changed. The CPM were recorded in real time. To determine the ability to reconstruct a SPECT image in tumors and OAR, behavioral simulations were conducted. %ID/g in these organs, paralleling the measured activity in the ¹⁷⁷Lu dilutions, was varied and the ability of the sensors to track these variations was evaluated. <h3>Results</h3> The mean CPM for the six vials ranged from 30.8 CPM (0.6µCi/mL) to 47,240 CPM (566.2µCi/mL) and was highly linear with activity. The least-squares regression fit between µCi/mL and CPM was CPM=82.28 ⨉ Concentration(µCi/mL) with an R² value of 0.9970. This range of activities is consistent with tumor and kidney uptake of an 800µCi ¹⁷⁷Lu injection in a mouse model. With this mapping from µCi/mL to CPM, the behavioral simulation demonstrated that the temporally varying trend of %ID/g of tumors and OAR could successfully be tracked in real time. It was shown that sensor placement can be optimized to predictably quantify and prevent excessive cross-talk between signals from different organs. <h3>Conclusion</h3> An APD-based γ-photon scintillation detector was developed and verified with a ¹⁷⁷Lu titration. We envision that the proposed work could be used to create a sensor network on the body capable of real time dosimetry of tumors and OAR.