The basic histologic structure of salivary glands consists of acinar cells and ductal components, the latter being divided into intercalated, striated, and excretory ducts. The intercalated duct, which is hypothesized to contain stem cells that give rise to several salivary gland neoplasms [1
], lies closest to the acinus and receives its secretions. Simple cuboidal epithelium comprises its luminal border, with a continuous abluminal layer of myoepithelial cells at the periphery [2
Intercalated duct lesions (IDLs) are abnormal proliferations of the intercalated duct cells, and range along the morphologic spectrum from hyperplasia (IDH) to adenoma (IDA) [3
]. Although proliferation of the intercalated ducts can occur as a reaction to many conditions, including chronic sialadenitis and after radiation therapy, it is particularly relevant for otolaryngologists because it is also seen in conjunction with several low-grade salivary gland tumors [3
]. It has been suggested that some of these tumors, including epithelial-myoepithelial carcinomas (EMCs) and basal cell adenomas, may be derived from hyperplasia of the intercalated ducts [3
]. A recent pathologic review of 34 IDLs by Weinreb and colleagues concurs, noting IDL as a probable precursor to these neoplasms [3
]. However, this has not been shown in all studies, and it therefore remains unclear if the link between hyperplasia and true neoplasia is causal or simply associative [3
Clinically, IDLs typically present as a unifocal palpable salivary gland mass, most often in the parotid. IDLs are usually small, ranging in size from 1 to 8 mm, and in some cases, they can be incidental findings in the setting of other salivary gland pathology. On histopathology, IDLs show a spectrum of architectural appearances, including intercalated duct hyperplasia (IDH), a non-encapsulated intercalated duct proliferation that blends somewhat with the surrounding salivary gland parenchyma. In contrast, intercalated duct adenomas (IDA) have a well-defined fibrous capsule separating them from the surrounding normal salivary gland parenchyma. Hybrids of these two types have also been described [3
]. Given the lack of encapsulation and presence of occasional acinar cells, our patient’s lesion was compatible with IDH.
The pathologic diagnosis of IDLs can be challenging given the overlap with other low-grade salivary gland tumors including striated duct adenoma, basal cell adenoma and adenocarcinoma, acinic cell carcinoma, adenocarcinoma NOS, oncocytoma and canalicular adenoma [7
]. They can often be differentiated from reactive lesions (e.g., tissue response to radiation, starvation, infection, or sialadenitis) by the lack of fibrosis, edema, inflammation, or atrophy in the adjacent salivary gland [3
]. Additionally, staining for myoepithelial markers can be positive in IDLs versus other similar lesions such as striated duct adenoma (SDA) or canalicular adenoma [7
]. IDLs typically resemble intercalated ducts on immunohistochemistry (positive for S100, CK7, and lysozyme) [3
] and they lack mitotic activity, necrosis, atypia, and features of parenchymal, perineural, and lymphovascular invasion [6
]. The differential diagnosis of IDLs with basal cell neoplasms (basal cell adenoma and basal cell adenocarcinoma) is probably the most challenging because of significant histomorphologic overlap. Hyperplastic forms of IDLs are distinguished, in part, by their lack of encapsulation, lack of basal lamina-like deposits, and incorporation of acinic cells in some cases. For IDLs with features of adenoma, several additional findings that aid in distinguishing them from basal cell neoplasms are documented by Weinreb et al. [3
] and mostly relate to the histologic and immunohistochemical pattern of the myoepithelial component, which is less conspicuous in IDLs than in basal cell neoplasms.
There are no specific criteria for the management of IDLs, since they are rare and often go undetected. However, given the likelihood that IDLs act as precursor lesions to malignant neoplasms, accurate diagnosis and surgical consideration are crucial for proper management. In our patient, a superficial parotidectomy was performed after a non-diagnostic FNA, without any further treatment due to the benign histologic appearance of the lesion. In cases where IDL is associated with neoplasia that is located deep in the parotid, associated with nodal disease, or found to have positive margins, post-operative radiation therapy may be warranted. The histopathologic diagnosis and analysis of IDL, therefore, can have a significant impact on the therapeutic course of these salivary gland tumors.