The role of molecular testing in the diagnosis of medullary thyroid cancer: A case report of oncocytic medullary thyroid carcinoma and review of the literature.

Background: Medullary thyroid carcinoma (MTC) is a somewhat rare, particularly aggressive form of thyroid cancer. The authors present what we believe to be the first case of MTC diagnosed solely on the basis of molecular testing, as well as a review of the literature concerning this topic and oncocytic variants of MTC.

Case Description: A 30-year-old female patient with a 1.1 cm thyroid nodule underwent a fine-needle aspiration biopsy showing a Bethesda IV Hurthle cell neoplasm. Molecular testing of the specimen identified a RET M918 T mutation. The patient underwent a total thyroidectomy and bilateral central neck dissection. Initial pathologic analysis yielded a diagnosis of Hurthle cell adenoma. Based on the patient's known RET mutation, immunohistochemistry for calcitonin was performed and yielded a positive result. The final diagnosis was amended to an oncocytic variant of medullary thyroid carcinoma.

Discussion: Had this patient undergone fine-needle aspiration (FNA) biopsy without molecular testing or serum calcitonin measurement, the patient's disease would have been diagnosed as a Hurthle cell adenoma. Despite the lack of characteristic features of malignancy and the rarity of oncocytic MTC, the diagnostic pitfall in this oncocytic lesion was avoided due to molecular testing at the time of FNA biopsy.

Conclusion: This case draws attention to the unique clinical value of molecular testing in the diagnosis of MTC. The authors believe this case supports the consideration for molecular testing to prevent missed diagnoses in cases of rare benign-appearing disease.