Telomerase Reverse Transcriptase-mutated Follicular Variant Papillary Thyroid Cancer With Tibial Metastasis

Follicular variant of papillary thyroid carcinoma (FV-PTC) generally has a favorable prognosis but can behave aggressively, notably in the presence of TERT promoter mutations. We present a case of a 58-year-old woman with a remote history of thyroid cancer treated by total thyroidectomy without radioactive iodine, who presented with acute right leg pain. Imaging revealed a destructive lesion in the right proximal tibia with biopsy-proven metastatic FV-PTC. Her thyroglobulin was >500 ng/mL (> 500 µg/L) (reference range: < 35.00 ng/mL; < 35.00 µg/L) and was above the assay detection despite dilutional efforts. She had undergone surgical resection of the metastatic lesion with a pathology consistent with FV-PTC. Molecular testing identified TERT promoter mutation c.124C > T. She had received radioactive iodine treatment, and subsequent posttreatment whole-body iodine scan revealed iodine-avid lesions in the right tibia and posterior second rib. Appendicular skeletal metastasis of papillary thyroid carcinoma is not well established in large cohort studies but in single institution studies reporting incidences of approximately 1% of thyroid carcinomas. This case highlights the potential of late-onset and aggressive metastatic spread of TERT promoter mutation FV-PTC and the necessity of dynamic risk assessment to ensure early identification of recurrence to guide treatment decisions.