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 re action, the long-term development of radiation fibrosis is not well describ ed. In order to develop quantitative means for the purpose, subcutaneous fi brosis of breast cancer patients after postmastectomy radiotherapy was eval uated by clinical scoring and a new technique based on dielectric propertie s of the skin. Methods and Materials: Dielectric properties of biological tissues at radio frequencies are principally determined by tissue water content. The major s kin components are proteins, proteoglycans, and water either free or bound to the surface of proteins and proteoglycans. Since the MR studies have sho wn that bound water is tightly attached onto the surface of collagen, a die lectric measurement sensitive to bound water could be related to the protei n content. Therefore, the dielectric constant of human skin was measured ir t vivo with an open-ended coaxial probe at electromagnetic (EM) frequencies in the range of delta-dispersion. Since the in vitro experiments with prot ein-water solutions have indicated that the slope of the dielectric constan t vs. the EM frequency is a measure of the protein concentration, a respect ive slope was determined with irradiated skin of 14 breast cancer patients 2 years after postmastectomy radiotherapy at 63, 100, 300, and 500 MHz. Irr adiated 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 slop e and the clinical score of subcutaneous fibrosis at 63, 100,and 300 MHz bu t 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 an d quantitative assessment of radiation-induced subcutaneous fibrosis. (C) 1 999 Elsevier Science Inc.