Occurrence of Mammary Tumors in Rats after Exposure to Tritium Beta Rays and 200-kVp X Rays

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Abstract
The RBE [relative biological effectiveness] for tritium was estimated in reference to 200 kVp X rays, using acceleration of breast tumor appearance in the female Sprague-Dawley rat as the end-point. Chronic X-ray of 0.3-2.0 Gy [grays] were delivered over 10 days. I.p. injections of tritiated water ranging in concentrations from 45 to 370 MBq[mega becquerels]/100 g body wt were administered, followed by 4 additional injections at 2 day intervals and half of the initial concentrations. Of the total tritium dose, 75% was delivered to the mammary gland within the first 10 days and 95% within the first 20 days after the start of the tritium exposure. RBE estimations were based on various criteria including the tumor incidence per Gy at 450 days postirradiation and the time required to induce tumors in 50% of the animals at risk. Tritium .beta. rays evidently are about 1.1-1.3 times more effective than chronic 200 kVp X-rays for acceleration of the appearance of rat mammary tumors. However, the uncertainties involved in these calculations are such that the effects of tritium .beta. rays could not be reliably distinguished from those of chronic 200 kVp X-rays. Measured differences in RBE values were slightly larger for the comparison between acute and chronic X-rays than for the comparison between chronic tritum .beta. rays and chronic X-rays.