A new paradigm for calculating skin dose

Most institutions model breast epidermis with a surface contour and record the maximum dose on the external surface of the patient. The objective of this study was to compare the external surface contour (ext) model of the skin with our current volumetric model for skin for radiation treatment planning in accelerated partial breast irradiation brachytherapy.

A literature search was conducted to identify studies measuring breast epidermal thickness. Clinical plans were performed with a 2-mm contraction of the external surface contour. This 2-mm contraction of the external surface contour was used to approximate breast epidermis thickness. Then, the external surface contour was expanded 5mm outside the external contour of the patient for the second skin model. Maximum doses from the two models were recorded and compared.

The average breast epidermal thickness from five studies was 1.68mm. Mean percent difference between skin and ext+5mm for balloon plans, strut plans, and all plans was 10.1%, 14.5%, and 12.5%, respectively. Differences in doses between the two skin models were statistically significant (p<0.0001).

The volumetric skin model was validated because the average breast epidermal thickness was 1.68mm. The surface model for skin may underestimate the dose delivered to the epidermis by as much as 23.8%. The external surface contour method does not accurately represent the dermatologic skin thickness of the breast as the skin is modeled as a surface rather than a volume. These discrepancies may skew correlations of dose to skin and toxicity determinations.