(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To compare measured electron doses under thermoplastic mask delivered by an Elekta VersaHD linear accelerator to calculated doses in Pinnacle following hot spots shown under head masks in patient treatment plans.
Methods: A phantom was constructed using solid water, a head cushion, and a thermoplastic mask. Four arrangements of the phantom explored the interfaces between the mask and mold, the mold and solid water, and the mold and air. Each arrangement was imaged with a head and neck protocol with 1.25mm axial slice CTs and planned on Pinnacle TPS. An AP 12 MeV electron beam delivering 400 MU at 110 SSD with a 10x10 electron cone applicator was planned for each of the setups. This setup was then measured with Gafchromic EBT3 film. The film was analyzed using RIT software and compared to the doses in corresponding positions on the plan.
Results: Percent differences for the film and Pinnacle doses ranged from 1.9% to 25.1%, with the exception of the two greatest differences, which showed 88.9% and 146.1% difference. The previously mentioned greatest differences were doses measured in the phantoms utilizing air under the mask and under the solid water at the water-air interface.
Conclusion: This work indicates that Pinnacle and film doses are comparable for stable interfaces, while being inaccurate at interfaces with lack of electron equilibrium. The differences in the doses are presumed to be due to the uncertainties in Pinnacle’s dose calculation algorithm.
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