Improving the Safety and Quality of Eye Plaque Brachytherapy by Assembly with Intensity Modulated Loading

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Project summary / Abstract Intraocular cancers are optimally treated with eye plaque brachytherapy (EPB), involving surgical implantation of a carrier (“plaque”) loaded with radioactive sources (called seeds) on the scleral surface over the tumor base. While it is critical to ensure the strength and exact positions of the radioactive seeds for accurate treatment, current quality assurance (QA) practice according to the AAPM Task Group (TG) report 129 only dictates the plaque assembly be visually inspected due to the lack of an effective and practical technique to measure the seed radioactivity distribution. Clinically, this often limits treatments to using uniform seed intensity, otherwise we have to assume the correct loading without verification. This is especially suboptimal for plaques preloaded by the vendor, as seeds with different activities cannot be distinguished visually. Therefore, unlike the common practice of employing intensity modulation in external beam radiation therapy, we cannot further optimize treatment by using differential seed strengths to provide more personalized dosing based on the tumor itself and the location of normal organs. To overcome this fundamental issue hindering EPB advancement and solve the problems of previous EPB QA designs, our goal is to develop a fast, accurate, and low-cost QA system to allow safe integration of differentially loaded EPB. We constructed a proof-of-concept system consisting of a flat scintillator sheet and a lens/camera system mounted together, aiming to quantitatively measure the discrepancy in radioactivity/dose distribution of the plaque assembly from the originally planned distribution through the gamma evaluation commonly used for the QA of intensity modulated radiotherapy. The radioluminescence signal generated by a loaded plaque near the scintillator will be collected by the camera and processed in real- time to verify correct loading of the seeds. The measurement and analysis only require a few minutes, significantly shorter than the time to assay the extra verification seeds the physicist already must perform as recommended by TG129. Our preliminary data are promising, while we hypothesize that the performance can be improved by using a hemispherical scintillator conformal the plaque inner surface to enhance the system sensitivity and identify each individual seed in the plaque. Our initial Monte Carlo (MC) simulation demonstrated the feasibility. Specific aims: Design and construct a novel, functional prototype QA system with associated software enabling us to offer seed loading modulation to patients with ocular tumors, and commission/evaluate/validate the system through extensive phantom experiments and MC simulation. Study design: Systematic MC simulations will be performed to optimize the design before building the improved device. Software tools will be developed to analyze the data and document the measurement results. Extensive phantom measurements will be performed to compare and tune the MC and optical system models. Factors affecting the QA uncertainties will be studied. Health relatedness: This proposal aims to address the unmet need of intensity modulated, personalized EPB delivery based on the lesion shape and critical structure sparing.