Skin cancer is the most common form of cancer in the United States today and safer, more accurate treatment methods are needed to combat this disease. Radiation therapy has proven to be one of the best options for managing this disease. Specifically, high-dose rate (HDR) brachytherapy utilizing surface applicators provides an excellent choice for physicians in treating small superficial lesions, limiting the irradiation to the target area. The Leipzig and Valencia HDR applicators (accessories of the Varian microSelectron-HDR afterloader) are designed to be used on a flat skin surface, for which case the isodose distributions have been well characterized. However, surface irregularities arising from natural facial contours (e.g., nose or ears) may introduce air gaps affecting the dose distribution. Investigation of the dosimetric impact of air gaps between skin and an applicator, for both cases of the Leipzig and Valencia applicators, is the main objective of this study.
The systematic evaluation of dose distributions resulting from air gaps of various thickness (1, 3, and 5 mm) introduced under half of the applicator area was performed both experimentally and through Monte Carlo modeling. The latter approach allows for calculation of a detailed dose distribution under more complex surfaces. Utilizing a well developed model we have obtained dose distributions for situations that are possible in the clinic yet difficult to measure.
Our simulations suggest that a significant amount of the dose (~20%) is contributed by scatter from applicator. This mitigates the dose change at the edges of air gaps, resulting in partial “filling” of gaps evident in the shape of dose profiles.
We have found that use of the Leipzig and Valencia applicators over non-flat areas with air gaps up to 5mm deep, and less than 5mm wide should be clinically acceptable, as these surface irregularities do not generate significant changes in dose distribution. Detail results of this investigation will be presented.