The effect of small radiation doses per fraction on mouse lip mucosa assessed using the concept of partial tolerance.

The acute effect of small radiation doses per fraction on mouse lip mucosa was investigated in the present study. In order to minimize the amount of additional sparing by regeneration during fractionated irradiations in this rapidly proliferating tissue, the overall treatment time had to be limited to at most 4 days, so that the number of irradiations that could be delivered was limited. Therefore, the concept of partial tolerance, established in the rat spinal cord model, was applied. The present experimental data confirm the validity of using this concept for assessing the effect of small radiation doses on tissues. The results of experiments covering a wide range of fraction sizes show that the isoeffective dose for a given mucosal reaction increases when the dose per fraction is progressively decreased to about 2 Gy per fraction. Further reduction of the size of dose per fraction, however, does not result in a detectable extra increase in the total dose to produce the same level of biological effect. It seems that the dose limit of sparing by fractionation in this rapidly proliferating normal tissue might be situated at larger fraction sizes than 0.6 Gy as estimated on basis of the mathematical linear-quadratic model, using an alpha/beta ratio of 6 Gy measured from data with doses per fraction in the range of 2 to 10 Gy.