(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: In prone breast radiation, the treatment beam may pass through additional scatter material (i.e. breast board), which can contribute to an increased skin dose. Monte Carlo (MC) simulations are expected to provide better computational accuracy in the buildup region than the conventional model-based treatment planning algorithms. We compared the surface/shallow depth dose enhancement effect between measured and MC results in this study.
Methods: The prone breast board studied is made of carbon fiber material. Surface and shallow depth measurements were performed using a parallel plate chamber in a phantom for air gaps 0-10 cm between the phantom and the board. The air gaps were chosen to represent a range of breast-to-board distances among different breast sizes. Simple geometric modelling was carried out in EGSnrc to mimic the same experimental setup. A custom density correction file was created to represent the carbon fiber board density of 0.3 g/cm³. The thickness used to model the board base was 1.2 cm. Point doses were collected on the central axis for comparison.
Results: Measured and simulation results were both normalized to the dose values at 10 cm air gap to demonstrate the dose enhancement effect at the surface and shallow depths due to additional scatter from the board base. Results showed consistency between measurements and simulations at 3 mm (<1%) and 5 mm (<1%) depth. Larger discrepancies (up to 25%) were observed at the surface, which could be due to limitations of surface dose measurements using an ion chamber.
Conclusion: We were able to model the prone breast board base using a uniform effective density for MC simulations in this phantom study. By applying the same approach in clinical prone breast radiation treatment plans, additional skin dose information may be provided for potential skin toxicity evaluation.
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