Practical Dosimetry of 131 I in Patients with Thyroid Carcinoma

Abstract

Radioiodine treatments of patients with well-differentiated thyroid carcinoma have generally been safe and beneficial. Safety can be ensured while efficacy is increased through practical methods of dosimetry that measure body retention of ¹³¹I. Prescriptions for therapeutic ¹³¹I can be decreased when the retention level is high and increased when the level is low. Assays of serum free T4 will alert the physician to possible increased radiation to blood and bone marrow, and appreciable concentrations of free T4 are indications to reduce the therapeutic ¹³¹I. Carcinomas ≥1 cm in diameter that are not visible on diagnostic scintigraphy are unlikely to respond to the commonly prescribed mCi of ¹³¹I. Biologic responses to commonly prescribed levels of therapeutic¹³¹I, as seen in toxic changes of normal tissues and in indices of tumor size, will be the final dosimeters. With lower levels of prescribed diagnostic ¹³¹I, stunning should not impair dosimetry. Thus, readily obtained measurements make dosimetry a practical method for improving carcinoma therapy with ¹³¹I.