Sellar Metastatic Follicular Thyroid Carcinoma With Novel Mutations Clinically Mimicking Pituitary Macroadenoma: Intraoperative Frozen Section Diagnosis Challenges

Background/Aim: Intraoperative frozen section (FS) pathologic evaluation remains an important part of neurosurgical procedure to ensure proper resection. Follicular thyroid carcinoma (FTC) is a common type of thyroid carcinoma, but metastasis to sellar turcica/pituitary fossa is rare and can be misdiagnosed as pituitary adenoma on FS diagnosis. Case Report: A 72-year-old female with a history of FTC treated 11 years ago presented to critical care with a pituitary gland lesion. Magnetic resonance imaging (MRI) revealed a 2.8 cm pituitary macroadenoma with sellar expansion and upward displacement of the optic chiasm. Visualization of the lesion and the recent significant growth warranted tumor resection and the intraoperative FS diagnosis was pituitary adenoma. Given the history of FTC and pathological results with occasional follicles present on the permanent sections, immunohistochemistry was performed and the tumor cells were positive for paired box gen 8 (PAX-8), thyroid transcription factor 1 (TTF-1), and thyroglobulin. The final diagnosis was pituitary metastasis of FTC. Next-generation sequencing was performed and revealed novel gene pathogenic mutations of rapamycin-insensitive companion of mammalian target of rapamycin (RICTOR) and Fanconi anemia (FANCA) in the tumor cells. The patient completed radiation treatment without complications and was doing well six months after surgery, with no FTC recurrence or metastasis identified. Conclusion: This case is a rare example of FTC with novel mutations clinically and radiologically mimicking pituitary macroadenoma and illustrates the importance of rigorous histological analysis for accurate diagnosis, particularly during the intraoperative consultation, to ensure proper treatment.