Radiation fractionation sensitivity of epiphyseal cartilage in a weanling rat model.

Tibial growth at 100 days of age was measured in Sprague-Dawley rats treated at 21 days to the proximal tibia with various courses of fractionated radiation. In split-dose and multiple-fraction experiments, a minimum interval of 5-6 hr was required to achieve maximal sparing of growth arrest. Total doses required to reduce growth to 80% of untreated controls were computed from dose-response curves for fractionated radiation (dose/fraction 1.0-10 Gy). When fitted to a linear-quadratic model of radiation response the data described an estimated alpha/beta of 4.47 (95% C.I. (3.71, 5.23) Gy). This value suggests that the fractionation sensitivity of the epiphyseal plate is substantially greater than that of most neoplasms, predicting a favorable therapeutic gain with the use of hyperfractionated radiation therapy.