Langerhans' Cell Histiocytosis Masquerading as Metastatic Papillary Thyroid Cancer on F-18 FDG PET/CT: Diagnostic Dilemma Solved by PET/CT-Guided Biopsy.

We present a case of papillary thyroid cancer (post-thyroidectomy status) on regular treatment with suppressive Levothyroxine therapy. On follow-up at 6 months after radioactive iodine ablation for remnant thyroid tissue, her thyroglobulin, and anti-thyroglobulin levels were 0.06 ng/ml and 670 IU/ml, respectively. Low-dose whole-body I-131 scan was negative. To look for the cause of isolated increased anti-thyroglobulin level, a whole-body 18F-FDG PET/CT was done which revealed multiple FDG-avid lytic skeletal lesions suggestive of metastases. For confirmation of diagnosis, 18F-FDG PET/CT-guided metabolic biopsy was done, which revealed Langerhans' cell histiocytosis on histopathological examination.