FDG positron emission tomographic, radioiodine, and MIBI imaging in a patient with poorly differentiated insular thyroid carcinoma

Poorly differentiated insular thyroid carcinoma is now classified as a sepa rate entity among other tumors of the thyroid gland. Its histologic pattern and its clinical course are regarded as intermediate between well differen tiated and anaplastic thyroid cancer. Insular carcinoma accumulates I-131, but no data exist regarding its fluorodeoxyglucose (FDG) positron emission tomographic (PET) uptake, The authors report F-18 FDG PET, Tc-99m MIBI, and radioiodine imaging features in a 63-year-old patient with metastatic insu lar thyroid carcinoma. After total thyroidectomy (for poorly differentiated insular carcinoma pT3a), the patient was referred for radioiodine ablation , No signs of recurrence were present until 16 months later, when thyroglob ulin levels increased, An I-131 scan showed a single lesion in the right lu ng, and further radioiodine treatment was administered (cumulative dose [53 0 mCi], 19,610 MBq I-131). Three years after the initial diagnosis, FDG-PET and Tc-99m MIBI scans were performed within 5 days during thyroxine treatm ent, After that, thyroxine substitution was withdrawn; 6 weeks later, an I- 131 whole-body scan was performed, Bath radioiodine and MIBI images showed increased tracer uptake in the known lung lesion. However, FDG PET showed a normal tracer distribution. Magnetic resonance and computed tomographic im aging confirmed a 12-mm lesion in the right upper lobe, These findings supp ort the concept of the "flip-flop phenomenon" in insular thyroid carcinoma, an alternating pattern of metastases with either I-131 or FDG-uptake. Desp ite poorly differentiated histologic findings, glucose metabolism was not i ncreased in this patient with an insular tumor.