Room: Exhibit Hall
Purpose: Evaluate the 3D dosimetry system from ModusQA consisting of current Clear View radiochromic gel formulation with Vista16 optical cone beam CT scanner and commercial blow molded polyethylene terephthalate vessels.
Methods: Gel samples from two 20 liter batches were evaluated in 10 and 15 cm diameter vessels. Scans included 1000 projections over 360 degrees acquired in two minutes. Samples were irradiated at 20 C with a 6 MV photon beam, uniform doses and small fields ranging from 3x3 to 0.6x0.6 cm and by 12 MeV electron beams. The impact of storage time at 4 C was one focus of this study. Irradiation of samples with a uniform dose or a known gradient, such as a depth dose, allowed generation of a spatial dose sensitivity calibration matrix for application to subsequent irradiations.
Results: This gel formulation appears colorless and nearly transparent and forms a stable purple dose image within minutes of irradiation. The dose response of ~0.004 cm-1 Gy-1 at 530nm, was linear to > 40 Gy. No dose rate effects were observed in beam profilesor depth doses. Irradiating to same dose with 12 MeV beam at 100, 400 and 600 MU/min resulted in same optical attenuation coefficient. Dose sensitivities were within 5% of mean value between the two batches and intra-batch samples evaluated up to 3 months after preparation. Samples prepared with an air head space showed up to 5% increase in dose response as the distance from the air interface increases. This spatial gradient in sensitivity increased with storage time.
Conclusion: This formulation of Clear View gel demonstrated: reproducible dose sensitivity between consecutive batches, linear dose response and no dose rate effects. Spatial variations in dose response may be related to the oxygen permeability of the PET jar lids and is under investigation.
Funding Support, Disclosures, and Conflict of Interest: K Jordan has a licensing agreement with ModusQA concerning optical computed tomography and gel dosimetry. R Hazarika was funded from a CHRP grant.