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High Resolution Optical Imaging of 4 & 5 Millimeter Beams: A Small Field Dosimetry Technique

M Ashraf1*, P Bruza2, R Zhang3, M Rahman4, B Williams5, B Pogue6, D Gladstone7, (1) Dartmouth College, Hanover, NH, (2) Dartmouth College, Hanover, NH, (3) Dartmouth College, Lebanon, NH, AF, (4) Dartmouth College, West Lebanon, NH, (5) Dartmouth Hitchcock Medical Center, Lebanon, NH, (6) Thayer Engineering, Hanover, NH, (7) Dartmouth-Hitchcock Med. Ctr., Lebanon, NH


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

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.


Radiosurgery, Scintillators, Small Fields


TH- Radiation Dose Measurement Devices: 3D solid gel/plastic

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