Distant Pulmonary Metastasis in Microsecretory Adenocarcinoma of the Salivary Gland: A Case Report Highlighting Histological Variation and Aggressive Potential.

Microsecretory adenocarcinoma (MSA) is a rare, recently defined, low-grade salivary gland neoplasm characterized by a recurrent MEF2C::SS18 gene fusion and a generally indolent clinical course. Most tumors arise in the oral cavity, particularly the hard palate and buccal mucosa, and display microcystic and tubular architecture with bland cytological features and a consistent immunoprofile, including SOX10, S100, and p63 positivity, while p40 is negative. To date, fewer than 50 patients have been reported, with only 2 demonstrating high-grade histological features or distant metastasis. Herein, we report a 34-year-old woman with a tumor of the retromolar trigone, initially classified as polymorphous adenocarcinoma. Fifteen years later, the patient developed pulmonary metastases with distinct histomorphological features, including solid-cribriform differentiation and comedo-type necrosis. Molecular analysis identified an MEF2C::SS18 gene fusion and an immunohistochemical profile consistent with MSA. This report highlights the histological variation and the potential for late metastatic progression in MSA, emphasizing the need for accurate molecular diagnosis and long-term clinical surveillance, even in tumors that appear histologically low-grade at initial presentation.