TL-201 SCINTIGRAPHY TO PREDICT THERAPEUTIC OUTCOME OF I-131 THERAPY OF METASTATIC THYROID-CARCINOMA

We studied the relationship between Tl-201 uptake and the efficacy of radioiodine therapy in thyroid carcinoma. Methods: Forty-four patients with metastases of well-differentiated thyroid carcinoma received Tl- 201 scintigraphy within the 2 mo before their initial I-131 therapy. P atients were classified into two groups according to the tumor-to-back ground (T/B) ratio on the late Tl-201 scan: high Tl-201 uptake (T/B gr eater than or equal to 2.1) and low Tl-201 uptake (T/B < 2.1), The the rapeutic outcome was judged by the percent reduction in the tumor diam eter at 6 mo after the treatment. The treatment was defined as effecti ve when the tumor showed more than 50% reduction in the tumor diameter . The patients in whom radioiodine was ineffective were followed up to determine if the tumor showed further growth. Results: Of the 44 pati ents, 25 had high Tl-201 uptake and 19 had low Tl-201 uptake. The ther apy was effective in 15 patients and was ineffective in 29, All the pa tients in whom radioiodine was effective had low Tl-201 uptake. On the other hand, 25 of 29 patients in whom radioiodine was ineffective had high Tl-201 uptake. Eight patients, in whom radioiodine was ineffecti ve despite good I-131 uptake, had high Tl-201 uptake. There were no si gnificant differences in the positive predictive value and the negativ e predictive value for effective treatment between Tl-201 scintigraphy and therapeutic dose I-131 scintigraphy, Among the 25 patients in who m radioiodine was ineffective and who had high Tl-201 uptake, the tumo r diameter increased in 7 (28%). However, none of the tumors with low Tl-201 uptake increased in size during the follow-up period. Conclusio n: Thallium-201 scintigraphy has a high predictive value for the effic acy of radioiodine therapy in metastatic thyroid carcinoma. Thus, it i s helpful in determining the indication for radioiodine therapy and it seems to be an adjunct to tracer dose I-131 scintigraphy.