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Assessment of Dosimetric Accuracy for a 3D Gel-Based Dosimetry Service
B Rosen*, K Lam , J Moran , University Michigan Medical Center, Ann Arbor, MI
Presentations
SU-F-T-565 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To assess the 3D dosimetric accuracy when using a mail-in service for square and stereotactic fields in a clinical environment.
Methods: The 3D dosimetry mail-in service (3DDaaS), offered by Modus QA (London, ON), was used to measure dose distributions from a 6 MV beam of a Varian Clinac. Plastic jars filled with radiosensitive ClearView™ gel were received, CT scanned (for registration and density information), irradiated, and then mailed back to the manufacturer for optical CT readout. Three square field irradiations (2x2, 4x4, and 10x10 cm²) were performed with jars immobilized in a water tank, and a composite small-field stereotactic delivery was performed using an in-air holder. Dosimetric properties of the gel were quantified within the 25-50 Gy dose range using 3D optical attenuation (OA) distributions provided by the manufacturer. OA was normalized to 100% at the position of isocenter, which received 40Gy. Percentage depth dose, profiles, and 3D gamma distributions (3%/1mm criteria) were calculated to quantify feasibility for relative dosimetry.
Results: Mean CT-measured density in the central (3x3x3) cm³ gel region was 40 ± 3 HU, indicating good homogeneity and near-water-equivalence. Measured and calculated central axis doses agreed to within ±3% in the 25-50 Gy dose range. For the square field irradiations, dose profiles agreed to within 1mm. Gamma analysis of the composite irradiation yielded 99.8%, 91.4%, and 79.1% passing rates for regions receiving at least 10, 5, and 2 Gy, respectively, indicating feasibility for use in high-dose regions. Absolute response varied by up to 16% between jars, indicating limitations for absolute dosimetry under the mail-in conditions.
Conclusion: 3DDaaS is a novel near-water-equivalent dosimetry system accurate to within 3% dose and 1mm 3D spatial resolution, and is straightforward to use in a clinical setting. Future investigations are warranted to improve dosimeter response in low-dose regions.
Funding Support, Disclosures, and Conflict of Interest: The authors would like to thank ModusQA (London, ON) for providing the gels and optical readouts used this work. This work was partially funded by NIH P01CA059827.
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