Glycophorin A as a Biological Dosimeter for Radiation Dose to the Bone Marrow from Iodine-131

The frequency of peripheral blood erythrocyte variants exhibiting allelic loss of glycophorin A (N/M antigen) has been used previously as a biological dosimeter to assess somatic mutations in bone marrow cells from external whole-body irradiation. The aim of the present study was to determine whether this marker could be used as a measure of bone marrow genotoxicity induced by 131 I in the treatment of thyroid cancer. Flow cytometry of immunolabeled erythrocytes was performed to enumerate glycophorin A variants before and after eight therapy doses of 131 I administered to five patients with differentiated thyroid carcinoma. Bone marrow radiation exposure from each dose was calculated from the integrated retention of 131 I in the whole body and in the blood. In addition, the accumulated dose to the bone marrow received from earlier 131 I therapy was calculated for each patient. Regression analysis was performed on the frequency of two glycophorin A variant cell types (N/Ø and N/N) as a function of accumulated dose to the bone marrow. Frequency of N/Ø variant cells showed a significant dose-related increase with a slope of $10.9\times 10^{-6}$ per sievert. This dose effect is about one-half that previously observed after whole-body external irradiation at high dose rate. This decreased response could be explained by the low dose rate of the radiation to the bone marrow from 131 I.