Author: Michael Butkus, Camilo M. Correa Alfonso, Olga M. Dona Lemus, James Arthur Perez-Sanchez 👨🔬
Affiliation: University of Miami 🌍
Purpose: Accurate skin dose calculation is essential for treating superficial lesions such as those of the breast, chest wall, and scalp. Clinical treatment planning systems often rely on approximations for lateral scattering and particle contamination, which, coupled with external body contour definitions, may lead to inaccuracies in surface dose estimation. This study evaluates the impact of body contouring uncertainties on surface dose calculations.
Methods: The mean skin dose, defined as a 4-mm inner margin from the external body contour, was calculated using a solid water slab phantom. Contours were generated using various Hounsfield Unit (HU) thresholds: -100 to -950 (in 100-HU increments). Two specific contour definitions were also assessed: the (-327 HU) default body contour (DBC) and an extended body contour (EBC), which was created by adding a 0.5 cm external margin to the DBC. Dose calculations were performed using the Eclipse 16.1 Proton Convolution Superposition (PCS) algorithm and the AcurosPT 16.1 Monte Carlo implementation.
Results: Extending the DBC to define the EBC resulted in percentage differences in calculated skin dose of 2.1% (AcurosPT) and 1.5% (PCS). For oblique incidence, the PCS difference increased to 1.7%. Across HU thresholds, deviations from DBC-based plans ranged from -5.08% to +1.65% (PCS) and -7.32% to +3.90% (AcurosPT), with the largest errors at -100 HU. AcurosPT overestimated mean skin dose by an average of 6.7% compared to PCS.
Conclusion: Substantial variability in skin dose calculations arises from differing external body contour definitions. Contours using HU thresholds of -100 and -200 significantly underestimate dose and are unsuitable for clinical use. These findings highlight limitations in superficial dose modeling. Future validation through Monte Carlo simulations and radiochromic film measurements will confirm these findings and help to refine clinical practice to enhance treatment accuracy.