Room: AAPM ePoster Library
Accurate dosimetric characterization of small fields used in external beam radiotherapy is challenging. In this study, we introduce optical imaging as a quality assurance tool for small field dosimetry.
A rectangular water phantom, doped with a fluorophore, was used as the dose deposition medium. Radioluminescence was captured using an intensified blue sensitive camera. 10 MV FFF 10 x 10 cm², conical 5 mm and 4mm beams were delivered to the water phantom. Assuming symmetry, multiple projection images of the beam were approximated by rotating the collimator. Using filtered back projection, a 3D volume was reconstructed. Important central axis dosimetric quantities were extracted and compared to data acquired with a 1 mm stereotactic diode, Radiochromic film and Monte Carlo (MC) simulations.
The final 3D volume was reconstructed at a voxel size of 0.5 mm³. The limiting resolution, defined as the full-width half maximum of the line spread function, was measured to be 1.10 mm. Output factors (OF) for the 5 mm and 4 mm beam were found to be 0.473 and 0.375 cGy/MU at point of maximum dose, respectively. In comparison, OF obtained from film data were 0.435 and 0.375 cGy/MU for the 5- and 4-mm beam, respectively. Applying a 5% dose threshold, optical off-axis ratios (OAR) at depth 1cm and 2 cm exhibited a 100 % passing rate for the 1%/0.5 mm gamma criteria when compared to OAR ratios measured by film and the diode. Beyond the buildup region, the optical central-axis percentage depth dose curve for the 5- and 4-mm beam exhibited a 100% passing rate (1.5%/0.5 mm), when compared to the PDD obtained by diode and MC simulations.
Optical imaging was presented as a high-resolution dosimetric QA tool for small fields. The technique can be used to accurately measure OF and central-axis beam profiles.
Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge the Irradiation Shared Resource at the Norris Cotton Cancer Center at Dartmouth with NCI Cancer Center Support Grant P30 CA023108 and the NIH Grant: R01 EB023909. Brian Pogue reports financial interest in DoseOptics LLC, a company developing cameras and software for the use of Cherenkov imaging.