Invasive carcinoma ex intraductal carcinoma of the salivary gland showing extensive infiltrative growth despite deceptively low-grade cribriform morphology.

Abstract

Intraductal carcinoma (IDC) of the salivary gland is an uncommon neoplasm frequently associated with RET rearrangements, particularly in the intercalated duct subtype. Although invasive carcinoma ex IDC has increasingly been recognized, most reported cases have demonstrated overtly invasive or high-grade morphology distinct from the associated intraductal component. Herein, we report a case of NCOA4::RET fusion-positive IDC of the parotid gland showing extensive infiltrative growth into adjacent soft tissue despite deceptively low-grade cribriform histomorphology. Tumor cells showed diffuse SOX10 and S100 expression, supporting intercalated duct differentiation, while cribriform nests demonstrated heterogeneous myoepithelial cell preservation, ranging from an intact peripheral layer to its complete absence. Targeted next-generation sequencing identified an in-frame NCOA4::RET fusion. This case expands the morphologic spectrum of invasive carcinoma ex IDC and highlights that unequivocal infiltrative growth may occur despite retention of a deceptively low-grade feature.