Tissue permittivity: a monitor for progressive tissue fibrosis as observed in bystander tissues following experimental high dose rate irradiation.

Fibrosis is a pathological condition resulting from radiation injury which often limits the prescription of higher (or boost) doses of radiation, risking inadequate tumor control in patients. Recent studies have documented reduction in fibrotic lesions after administration of pentoxyfilline and tocopherol combinations to breast cancer patients receiving adjuvant radiation therapy. Despite the promise of these findings, no techniques or markers are available which can be used to identify the onset or progression of fibrosis in such patients at stages early enough to allow maximum benefit from these types of pharmacological agents. Relative permittivity of skeletal muscle has been investigated in an animal model utilizing high dose rate radiation both at the treatment site as well as on the contralateral site, and was found to be directly related to the formation and progression of fibrotic lesions. A cubic increase in the quantified fibrotic fraction of the tissue (2.7%-13.9% over 11 w post irradiation) was reflected in a linear increase in the tissue's relative permittivity (epsilon(r) = 6.3-8.8 over 11 w post irradiation). These findings mandate further investigation of the relationship between tissue's relative permittivity and subcellular injury leading to fibrosis using electrical impedance spectroscopy (EIS).