Cytogenetic study of radiation burden in thyroid disease patients treated with external irradiation or radioiodine.

Where clinically permitted, either external irradiation or radioiodine therapy is usually recommended for the treatment of differentiated thyroid cancer patients. The choice depends on the treatment philosophy of the responsible physician. This paper describes an attempt to clarify the radiation burden on the lymphocytes in consequence of these two therapeutic modalities. An analysis was made of the extent to which exposure to local neck irradiation (25 x 2 Gy) or radioiodine therapy (1734-2600 MBq) causes chromosomal aberrations in the lymphocytes of thyroid disease patients after total or subtotal thyroidectomy. External irradiation caused many more chromosomal aberrations than 131I therapy did, but analysis of the distribution of the aberrations suggested a homogeneous dose distribution only in 131I-treated and thyroidectomized cancer patients. In thyrotoxic patients with intact thyroid glands, the lower therapy doses (185-595 MBq) caused a significantly higher frequency of aberrations than that observed in thyroid cancer patients, and the dose distribution in the lymphocytes was inhomogeneous. Thus, in the modelling of accidental environmental radioiodine exposure, thyroid patients with small if any residual thyroids are not a suitable group.