DOSIMETRIC CONSIDERATIONS IN THE RADIOIODINE TREATMENT OF MACROMETASTASES AND MICROMETASTASES FROM DIFFERENTIATED THYROID-CANCER

When macrometastases are delineated clearly using current radiographic techniques and/or physical examination and can be shown to concentrat e I-131, the therapeutic activity to be administered may be determined quantitatively. Administrations of I-131 that will deliver 30,000 rad to residual thyroid tissue or 10,000 +/- 2000 rad to lymph node metas tases will ablate them successfully 80% of the time, and bone marrow d epression that is severe enough to require specialized treatment will be avoided if the whole blood dose from a single administration does n ot exceed 200 rad. When micrometastases are detected only by diagnosti c radioiodine imaging and/or elevations of serum thyroglobulin levels, and when a clinical decision is made to treat them with radioiodine, then I-131 may not be the isotope of choice. With small lesions <0.05 mm in diameter, the lower energy emissions of I-125 therapy may be mor e suitable. With the advent of alternative methods of patient preparat ion for radioiodine therapy, empiric approaches that were derived from experience with endogenously hypothyroid patients will require full r e-evaluation. Approaches based on quantitative radiodosimetric calcula tions will continue to be valid because they already consider individu al differences in radioiodine kinetics.