¹³¹I / ¹⁸FDG flip-flop phenomenon in the patient's with bone metastasis from thyroid cancer

Objectives: Radioiodine therapy is used to treat distant metastases from thyroid cancer. A reverse relationship between iodine and FDG accumulation is found in thyroid cancer lesions, the so-called “flip-flop” phenomenon. The aim of this study was to assess the relationship between iodine and FDG uptake in bone metastasis lesions from thyroid cancer.

Methods: The cases of 20 patients who underwent radioiodine therapy for bone metastasis were studied retrospectively (age, 27-74 yrs; median, 62 yrs; males : females, 6 : 14). Iodine uptake was evaluated visually and the lesions showing iodine uptake more than background were determined as positive. FDG uptake was assessed using SUVmax. We compared FDG uptake between the lesions with (n=19) and without (n=88) iodine uptake in bone metastasis.

Results: The bone metastasis lesions without iodine showed significantly higher FDG uptake than those with iodine uptake (p < 0.0001). However, the degree of FDG uptake in bone metastasis lesions was also relatively high (Median of SUVmax: 4.64). Thirty-nine out of 88 iodine-positive bone metastasis lesions (44.3 %) showed more than 5 of SUVmax. On the other hand, only one iodine-positive lesion (1.1 %) showed more than 10 of SUVmax, while 9 out of 19 (47.4 %) iodine-negative lesions did.

Conclusions: Bone metastasis lesion from thyroid cancer not showing iodine uptake showed higher FDG uptake compared to those with iodine uptake. On the other hand, more than 40 % of bone metastasis lesions with iodine uptake showed relatively high FDG uptake (SUVmax>5). Enormously high degree of FDG uptake (SUVmax>10) was rare in the lesions with iodine uptake, while it was common in those without iodine uptake.