Relationship among different skin dose definitions in high-dose-rate (HDR) balloon breast brachytherapy

Abstract

Objective

To establish the relationship among various skin dosimetric indices and different volumetric definitions of skin in high-dose-rate (HDR) balloon breast brachytherapy.

Methods

Fifty breast cancer patients were treated with HDR balloon brachytherapy. The MammoSite® applicator was used for 40 patients and the Contura® applicator for 10 patients. Skin structure was retrospectively defined by expanding the skin surface internal to the body with a thickness of 1, 2, 3, 4, or 5 mm in one method. In another method, the skin was defined by expanding its external to the body to demonstrate the maximum point dose on the skin surface. For each skin structure defined by six different methods, three dosimetric data points extracted from dose-volume histograms were compared. Dmax was defined as the maximum point dose, and D1cc and D0.1cc were defined as the minimum dose to 1 cm³ and 0.1 cm³ of the most irradiated skin volume, respectively. The relationship among 18 dosimetric parameters was presented in graphs, and linear curve fitting was performed to provide mathematical formulas.

Results

For each skin definition, the Dmax, D1cc, and D0.1cc values show a linear relationship such that Dmax is the largest, D0.1cc is the next, and D1cc is the smallest value. For each dosimetric parameter, there was a linear relationship among the dosimetric indices for 6 different skin definitions. For clinical use, all linear relationships were displayed in graphs and two parameters for linear fitting were provided. Average R² value for curve fitting was 0.978.

Conclusion

The presented relationships can be developed in each individual institution and convert one dosimetric index to another for different skin definitions.

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