ASSESSMENT OF COMPLETENESS OF THYROID-ABLATION BY ESTIMATION OF NECK UPTAKE OF I-131 ON WHOLE-BODY SCANS - COMPARISON OF QUANTIFICATION ANDVISUAL ASSESSMENT OF THYROID BED UPTAKE

Thyroid cancer is treated by thyroidectomy followed by radioiodine abl ation of the residual active tissue in the thyroid bed. Completeness o f ablation can be assessed from neck images of whole-body I-131 scans by visual estimation or quantitative analysis. By visual assessment, a blation can be considered complete if there is no uptake in the neck o r the uptake is empirically considered too small. By quantification, a blation is considered complete if neck uptake is <1%. Further radioiod ine therapy is considered neces sary only if neck uptake exceeds 1% of the administered dose. Both visual assessment and quantification of t hyroid bed uptake were applied to 46 scans after diagnostic or therape utic doses of I-131 had been administered to 25 patients who were bein g followed up for follicular or papillary carcinoma of the thyroid. Th e results were compared to assess the effect of either method on deter mining the need for a further ablative dose of I-131. Visual assessmen t overestimated thyroid bed uptake in 10 of 46 (22%) of the scans. Bea ring in mind the unpleasantness of radioiodine ablation and the potent ial for bone marrow toxicity, it is recommended that quantification of neck uptake should be routinely performed as a guide to completeness of ablation and to determine the need for a therapeutic dose of the is otope. This should help to avoid unnecessary radioiodine treatment in patients with thyroid cancer.