SU-E-T-93: A CT Polymer Gel Dosimetry System for End-To-End Dosimetry

Abstract

Purpose: To design and test an x‐ray CT polymer gel dosimetry system for 3D end‐to‐end dosimetry of brain and head and neck radiotherapy Methods: A head and neck phantom has been designed to undergo the entire RT process from immobilization and planning CT through to treatment. The phantom houses a 1L polymer gel in one of two locations: cranial and inferiorly in the neck region allowing for quality assurance of both brain and head and neck treatment processes. The gel dosimeter measures the delivered radiation dose in 3D, and following CT read‐out, provides a dose record of the entire treatment process, i.e. “end‐to‐end” dosimetry. Enhanced dose‐sensitivity N‐isopropylacrylamide (NIPAM) based gels along with an optimized CT protocol were used to minimize uncertainty in image CT numbers (H). Tests of phantom set‐up were performed to quantify image noise, uniformity and positioning reproducibility using locking bar (ideal conditions) and aquaplast mask (clinical conditions). The end‐to‐end dosimetry capability was tested by undertaking the full RT process (immobilization, planning CT, treatment planning, treatment set‐up and delivery) and comparing delivered and planned doses for a simple “star‐pattern” irradiation. Results: Removing the phantom head for gel read‐out minimized noise and artifacts (63% noise reduction), produced uniform images and has minimal impact on phantom re‐positioning (less than 0.5mm). Full phantom re‐positioning reproducibility was also excellent: less than 0.9 mm for both locking bar and mask immobilization. Initial end‐to‐end dosimetry tests indicate accurate localization of treatment dose to within 1 mm. Conclusions: An x‐ray CT polymer gel dosimetry system for performing 3D end‐to‐end dosimetry has been designed, tested and is demonstrated to provide accurate 3D localization of delivered radiation dose. Future work will assess clinical processes as undertaken by appropriate RT staff and look at other treatment sites.

Evaluation of accuracy and precision in polymer gel dosimetry

“…Gels were set up on the CT scanner bed using the head and neck phantom to ensure synchronized localization of the gel for imaging and irradiation. For imaging, the head was removed, which has been shown to reduce imaging noise and artifacts without significantly affecting position reproducibility . Prior to the imaging of blank and irradiated gels, a short warm‐up scan was performed, as recommended by Johnston et al …”

“…The base was locked into place on the treatment bed using a locking bar and set of weights which ensure complete immobilization of the gel and phantom and provides submillimeter position reproducibility. 27 The temperature of the water in the head and neck phantom was measured before and after irradiation to ensure a consistent temperature of the gel dosimeter when irradiated. After irradiation, the gel was returned to the refrigerated water bath and allowed to polymerize for 24 h prior to imaging.…”

An x-ray CT polymer gel dosimetry prototype: II. Gel characterization and clinical application

“…Once screwed on, the phantom head is water tight and the phantom can be filled with water for irradiation. The design also allows for removal of the head while maintaining gel position, a feature required to optimally reduce image noise during CT read-out (Hilts et al 2011). Finally, the phantom can be precision positioned on the treatment and CT couches using a locking bar or alternately using a clinical mask-type immobilization system.…”

Evaluation of an x-ray CT polymer gel dosimetry system in the measurement of deformed dose

“…The jars were fit into an acrylic base and attached to the treatment couch using a locking bar. This apparatus and setup technique has been shown to be reproducible for x-ray CT gel dosimetry producing sub-millimetre setup errors (Hilts et al 2011, Maynard et al 2018. For static measurements, the gels were simply set up in the acrylic base apparatus and irradiated and for the deformable measurement the gel was compressed externally by 25 mm using an acrylic piston during irradiation.…”