Heterotopic salivary duct inclusions in Warthin tumor - A cryptic histological finding involved in its pathogenesis

Warthin tumor (WT) is the second most common benign parotid gland tumor after pleomorphic adenoma. WT is characterized by cystic and papillary proliferation of a two-layered oncocytic epithelium supported by lymphoid tissue. Heterotopic salivary duct inclusions (SDIs) are frequently observed in lymph nodes (LNs) of WT (SDI/LNs), and are thought to be the origin of WT. If this is true, SDIs should also persist in the lymphoid tissue of WT itself (SDI/WT), as a missing link between SDIs and WTs, but studies of this issue are limited. From 2008-2023, 138 WT cases were surgically excised at our hospital. SDI/LNs and SDI/WTs were histologically examined. Of 100 WT cases with LNs, SDI/LNs were observed in 67 cases (67 %). SDI/WTs were detected in 114 of 138 cases (82.6 %), including 107 of 127 smokers (84.3 %) and 7 of 11 never-smokers (63.6 %). SDI/WTs were located mainly in the subcapsular lymphoid tissue and often surrounded by a fibrous coat resembling salivary excretory ducts. This study revealed a high incidence of SDIs in WT itself, strongly supporting the theory that WT develops from heterotopic salivary ducts.