Early skin response to fractionated doses of pions for determining therapeutic gain factors.

This study concerns the biological effectiveness of fractionated doses of pions on early skin reactions near the implanted tumor, to evaluate the therapeutic gain factors of pions. The C3H mouse limbs bearing microscopical SCCVII tumors were irradiated with pions (9.6-38.4 Gy) or x-rays (14.4-50.4 Gy) in 2-7 fractions. Nicotinamide (500 mg/kg) and carbogen (a mixture of 95% O2 + 5% CO2), as hypoxic radiosensitizers, were administered prior to the x-rays, to confirm the presence of hypoxic cells in the skin. The mean skin scores and number of damaged nails assessed. The ratios of x-ray/pion doses needed for giving comparable skin reactions were 1.3-1.5. Nicotinamide and carbogen enhanced the skin reactions. When the ratios were compared with those of tumor cure, the pions showed no therapeutic gains. One of the possible causes was that the presence of hypoxic cells in the skin may have reduced the therapeutic gain.