In vivo electrical impedance spectroscopic monitoring of the progression of radiation-induced tissue injury.

This study evaluates the potential of electrical impedance spectroscopy (EIS) as a noninvasive technique for tracking the progression of radiation-induced damage in normal muscle tissue. Male Sprague-Dawley rats were irradiated locally to the gastrocnemius and biceps femoris muscle. Single doses were administered using a procedure that spares skin and bone. Complex impedance spectral measurements (taken at 50 frequency points between 1 kHz and 1 MHz) were made at monthly intervals using recessed disk electrodes applied to the skin. A histological scoring scheme was developed for evaluation of injury. A strong dose-dependent progression of injury evident in both spectral measurements and histological scoring has been observed. Latent time also appears to be dependent on dose with changes induced by 70 Gy evident by 2 months, changes induced by 90 Gy observed by 1 month, and dramatic changes found within 3 weeks at 150 Gy. Injury was morphologically comparable to the type of damage that occurs in response to small, fractionated doses, but on a much shorter time scale. Increased spectral shift was a consistent indicator of the extent of tissue injury at the time of measurement. The use of a large single dose resulted in an excellent model in terms of inducing a significant progression in tissue injury over a short post-treatment follow-up period in the muscle mass while also providing a consistent location for in vivo electrical impedance measurements. The results show that EIS can follow radiation-induced tissue change, suggesting that EIS has the potential to monitor the types of injury observed in late radiation damage of muscle tissue noninvasively.