Impact of patient age on the iodine/FDG 'flip-flop' phenomenon in lung metastasis from thyroid cancer

Radioiodine therapy is an effective treatment for lung metastasis from thyroid cancer. However, cases of lung metastasis without iodine uptake are often encountered. In such cases, FDG accumulation in lung lesions is often observed. There is a reverse relationship between iodine and FDG accumulation in thyroid cancer lesions, the so-called “flip-flop” phenomenon. The aim of this study was to assess the relationship between patient age and the occurrence of the flip-flop phenomenon. Eighty-six patients who underwent radioiodine therapy for lung metastasis were studied retrospectively (age 17–73 years; median 60 years; males:females 22:64). We compared the clinical data and imaging findings (size and FDG uptake of lung nodules) between patients with (n = 44) and without (n = 42) iodine uptake in lung metastasis. Significantly more young patients showed iodine accumulation in lung metastasis than old patients (p = 0.0025). Lung metastases with larger size or greater FDG uptake showed no iodine uptake more frequently with significant difference (p = 0.015 and