Secretory Carcinoma of the Thyroid: A Case Report and Update of Literature

Primary secretory carcinoma (SC) of the thyroid gland is a rare neoplasm, characterized by the presence of oncogenic ETV6 :: NTRK3 fusions, which are amenable to tropomyosin receptor kinase (TRK) inhibitor therapy. Despite its morphologic, immunophenotypic, and genetic similarities to SC of the salivary and mammary glands, diagnostic pitfalls may arise in differentiating from papillary thyroid carcinoma due to overlapping features such as papillary growth, nuclear irregularity, and variable expression of PAX8. Tumor misclassification may lead to delayed consideration of molecular testing and targeted therapy. A total of 13 cases of thyroid SC have been documented in the literature, indicating a tendency for advanced clinical presentation followed by a protracted clinical course, with most patients surviving until the end of the study period despite some experiencing recurrences. However, tumor-related mortality occurred in around 30% of cases, with the overall survival ranging from days to years, underscoring the variability in tumor behavior and the need for further research efforts. Among documented cases of thyroid SC, prognostic factors established for salivary SC have shown broad distributions, including a mitotic activity ranging from