Room: AAPM ePoster Library
Purpose: Previous studies have demonstrated that mixed electron-photon beam radiation therapy (MBRT) treatment plans could provide superior normal tissue sparing while maintaining similar target coverage to photon or electron-only plans in superficial tumour sites. In this study, a quality assurance protocol was developed to validate electron applicator-free delivery of MBRT plans. The aim of this study was to demonstrate the feasibility of patient-specific dose verification in the context of MBRT.
Methods: A MBRT plan was optimized for a flat Solid Water phantom using a single electron beam angle and a photon arc. The plan was delivered on a Varian TrueBeam linac with a Gafchromic film positioned at 2 cm depth. Another clinically representative plan was optimized on a cylindrical acrylic phantom. Electron beams (6,9,12,16,20 MeV) were delivered from three angles while 6 MV photons were delivered in an arc. An Exradin A1SL ionization chamber was used to measure the dose at 1.4 cm depth. The difference in chamber response in the MBRT beam compared to reference conditions was accounted for by calculating the ratio of stopping power ratios of water to air in these two conditions using a modified EGSnrc user-code. Finally, the same treatment plan was delivered on a second identically shaped phantom which allowed to position a film at 2 cm depth.
Results: By performing a gamma analysis using a 2%/2mm criterion and 10% low-dose threshold, the MBRT deliveries were found to have good relative dose agreement with pass rates over 96.7% on the flat phantom and over 92.2% on the cylindrical phantom. The point dose measurements performed on the cylindrical phantom agreed with MC-calculated doses within 0.1% when summing over all electron components and within 0.6% overall.
Conclusion: The agreement between measured and calculated doses demonstrates the feasibility of accurate MBRT deliveries using photon-MLC-shaped applicator-free electron beams.
Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge support from the Fonds de recherche du Quebec Nature et Technologie (FRQNT), the CREATE Medical Physics Research Training Network grant (number 432290) of the Natural Sciences and Engineering Research Council (NSERC) and the Canadian Institute of of Health Research Foundation Grant (FDN-143257).