Tissue response of radiation therapy assessed by electrical impedance spectroscopy (EIS) in subcutaneous tumours in rats.

The present investigation aims to evaluate the possibility of using bio-impedance spectrometry to measure tumour and tissue response to radiation therapy. Bio -impedance measurements performed with CythorLab™ equipped with a signal generator with a known high output impedance and signal measuring device able to measure the voltage applied by the generator. The control unit triggers the signal generator that generates an MLS-sequence. The same control unit process the signal that simultaneously measured by the voltage-recording device. An FFT analysis performed to obtain the magnitude of the real and imaginary parts of the impedance spectrum. The effect of various numbers of fractions of radiation therapy (RT) on the impedance measured with surface plate electrodes investigated in male rats of the Fischer-344 strain with rat glioma N32 tumours implanted subcutaneously on the flank. Tumours produced by injecting 100 000 N32 tumour cells just below the skin. Tumours were treated about four weeks after injection when a solid tumour has developed with a diameter of 1-1.5 cm. Before treating the tumours, animals anaesthetised, and the fur over the tumour shaven and carefully to ensure good electrical contact between electrodes and skin. The electrical impedance dispersion of tissue modelled with an RC-equivalent circuit from which collective impedance parameters corresponding the cell membranes, R_m C_m intra -cellular resistance, R_i, and extra-cellular resistance R0.Impedance measurements performed over a tumour before irradiation to 5 Gy and every minute after the irradiation up to 8 minutes. A slight increase of impedance, and with a time constant of 10 minutes. The growth might be due to dry skin after irradiation or a decrease of tumour vascularity during the treatment. The capacitance of the cell membrane related to the characteristic frequency fc does not change significantly before and after radiation exposure. A special parameter, the "Loss Change Index" (LCI) which defined to vary between zero if there is no change in the phase angle and one if the phase angle after exposure approach zero. LCI reach an extreme at the characteristic frequency. The LCI value recorded at the characteristic frequency fc varied with the accumulated absorbed dose and fitted to a sigmoidal dose/response relationship.