Matched Fields Technique for $^{90}{\rm Sr}/^{90}{\rm Y}$ Low-Dose Rate Source for Keloid Treatments

⁹⁰
Sr/
⁹⁰
Y low-dose rate sources are commonly used in pterygium, keloid, and skin postsurgery treatments to avoid reincidence. The beta radioactive materials are deposited on metal plates, in geometries, such as square, rectangular, and circular, to treat lesions up to 2 mm depth from the skin surface. Although it has a limitation for deeper lesion treatments, beta application presents an advantage of minor healthy tissue damage. The chemical dosimeter Fricke Xylenol Gel (FXG), which presents high spatial resolution and effective atomic number (Z
_eff
) similar to the soft tissue, can determine the depth dose distribution generated under a source and simultaneously simulate the soft tissue. In this paper, the FXG was used to evaluate the depth dose distributions, along two adjacent square
⁹⁰
Sr/
⁹⁰
Y sources, to verify the radiation homogeneity at the interface region. From the results, one could infer a subdose region under the interface area, when the metal plates are matched (ordinary procedure for treatments using adjacent metal plates). Absorbed dose percentages of 53% and 32% were found at the surface and at 1 mm depth, respectively, which are lower than those expected for the interface region. In this sense, the results suggest that the radioactive areas must be adjacent instead of the physical ones, which means that the metal plates should be overlapped by 5 mm, approximately.