Quantitative assessment of protein content in irradiated human skin.

PURPOSE

Radiation-induced fibrosis is a common late reaction of radiation therapy. Due to a lack of feasible noninvasive techniques to assess this reaction, the long-term development of radiation fibrosis is not well described. In order to develop quantitative means for the purpose, subcutaneous fibrosis of breast cancer patients after postmastectomy radiotherapy was evaluated by clinical scoring and a new technique based on dielectric properties of the skin.

METHODS AND MATERIALS

Dielectric properties of biological tissues at radiofrequencies are principally determined by tissue water content. The major skin components are proteins, proteoglycans, and water either free or bound to the surface of proteins and proteoglycans. Since the MR studies have shown that bound water is tightly attached onto the surface of collagen, a dielectric measurement sensitive to bound water could be related to the protein content. Therefore, the dielectric constant of human skin was measured in vivo with an open-ended coaxial probe at electromagnetic (EM) frequencies in the range of delta-dispersion. Since the in vitro experiments with protein-water solutions have indicated that the slope of the dielectric constant vs. the EM frequency is a measure of the protein concentration, a respective slope was determined with irradiated skin of 14 breast cancer patients 2 years after postmastectomy radiotherapy at 63, 100, 300, and 500 MHz. Irradiated skin sites were clinically scored for subcutaneous fibrosis using a scale: none, slight, moderate, or severe fibrosis.

RESULTS

A statistically significant correlation was found between the slope and the clinical score of subcutaneous fibrosis at 63, 100, and 300 MHz but not at 500 MHz. The correlation was best at 100 and 300 MHz.

CONCLUSIONS

Considerable changes in the dielectric constant of the irradiated skin were found. The correlation between the dielectric constant and clinical score suggests that this novel technique is a potential tool for the follow-up and quantitative assessment of radiation-induced subcutaneous fibrosis.