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A Comprehensive Patient Specific Quality Assurance Scheme Developed for Nasopharynx Brachytherapy

M Mohammadi1*, M Gholami2, R Jaberi3, A Sedighi4, (1) Royal Adelaide Hospital, Adelaide, SA, AU, (2) Mahdieh Radiotherapy Centre, Hamadan, (3) Cancer Institute, Tehran University Of Medical Sciences, (4) Mahdieh Radiotherapy Centre, Hamadan, Iran,

Presentations

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

Room: AAPM ePoster Library

Purpose: To verify the radiation dose delivery to potential point of interests, a comprehensive Patient Specific Quality Assurance (PSQA) approach was established for brachytherapy.

Methods: A home-made phantom was designed to characterise the procedure of the PSQA developed for brachytherapy applicators. The PSQA procedure’s sensitivity to source positional errors and the delivered radiation dose were investigated through intentional and controlled modifications in patient’s plans.
A three-dimensional radiation dose distribution was provided through a commercial Treatment Planning System (TPS) using AAPM-TG-43 formalism. The radiation dose was also measured at patient’s surface area using Gaf-Chromic films, fibre optic dosimeters and glass bid. Applying an accurate in-house back-projection algorithm, corrected for inhomogeneities’ effects, the calculated dose distributions were compared with those resulted from TPS. Results were then compared with those achieved by a Monte Carlo simulation through Geant-4 code. 3D dose maps were compared through gamma function with the same passing criteria, used for external beam radiotherapy PSQA (3%/3mm) through global and local gamma indices.
Results: The maximum dose difference between film, fibre, glass bid measurements and TPS were found to be 6.1%, 8.3% and 8.7% respectively. Gamma pass rates with 3%/ 3 mm for measured, calculated and simulated for a 3D dose distribution were found 93%, 90.3% and 91.7% respectively. The gamma pass rates were also evaluated with 4%/ 2 mm criteria to achieve the results of more than 90% as safe pass rate.
Conclusions: Although the uncertainty in dose calculation is considered, there is still an opportunity to increase the precision of the dose delivery through a PSQA program. The application of gamma index instead of point dose reports is recommended for brachytherapy. The precision and accuracy of dose delivery can be increased by the evaluation of the dose distribution and understanding of the treatment planning limits through a PSQA plan.

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