The radiobiology of thyroid epithelium

Radiogenic thyroid neoplasia is an important human problem, particularly among those exposed before puberty. The acute and late neoplastic effects of ionizing radiation on hormone responsive normal cell population were examined in order a) to measure cell survival and repair capacity following radiation exposure, b) to measure neoplasis in terms of both initial and surviving cell numbers, and c) to investigate the action of hormones and physical factors. Each rat received inocula in the cephalad and caudal ends of each inguinal fat pad and in interscapular white fat pad. All 5 sites in a rat received the same number of viable epithelium cells. Glandular follicular units (FU), which developed in the thyroid-removed recipients maintained on normal diets and tap water, concentrated radio-iodine, promoted body growth, and were morphologically identical to highly stimulated thyroid follicles in situ. In order to examine radiation dose-thyroid cell survival relationship, the cells were irradiated with X-ray in vitro after enzymatic dispersion, and were grafted immediately. In other experiments, it was found that the survival parameters of the cells irradiated in situ in donor rats and removed immediately after the exposure differed insignificantly from those of cells irradiated in vitro. The significant increase in post-irradiation repair was noted in the rat mammary cells irradiated and left in situ in donor rats for 24 hours before the removal for survival assays as compared to the repair in the glands removed immediately after the exposure. Thyroid cells also have the capability for in situ repair. (Yamashita, S.)