Microscopically, the lesion demonstrates prominent papillarity that in some areas, appears to be centered around an excretory minor salivary gland duct. The opening of the duct is in direct continuity with the stratified acanthotic squamous epithelium covering the papillary areas of the tumor (Fig. 1a, b). Most of the ductal epithelium is bi-layered or stratified with tall columnar cells in the luminal layer and short, cuboidal cells in the basal layer. Scattered goblet cells and ciliated metaplastic cells are interspersed. Extensive squamous metaplasia is present focally. The papillary fronds of the tumor, which are supported by fibrovascular cores, protrude into the ductal space. Lymphocytic and plasmacytic inflammation is present in the cores. The above described lesion was interpreted as sialadenoma papilliferum.

The true origin and nature of sialadenoma papilliferum have been controversial. The authors who first described this lesion believed it arises from myoepithelial cells [1]; while others have proposed that it is a result of focal hyperplasia following blockage of a salivary gland duct [9]. The presence of ciliated cells in the lining epithelium of the duct in our case is consistent with the possibly metaplastic, reactive nature of sialadenoma papilliferum [10]. Most recently, it was reported that sialadenoma papilliferum is a neoplasm that originates from ductal surface cells [11].

Mucoepidermoid carcinoma sometimes is seen proliferating from a terminal salivary duct, supporting the concept that the tumor may originate from cells of the salivary gland excretory and intercalated ducts [12]. Although mucoepidermoid carcinoma can be associated with a large excretory salivary duct, a papillary, proliferating ductal lesion, as in this case, is not a feature. Chen [13] reported a case of intraductal low grade mucoepidermoid carcinoma occurring in minor salivary gland. The lesion was lobulated and composed of cystic ducts filled with tumor, which differs from the surface papillary configuration of a sialadenoma papilliferum.

Considering the papillary architecture of the lesion, our differential diagnosis consists of (1) a sialadenoma papilliferum with a papillary squamous cell carcinoma, (2) an adenosquamous carcinoma with prominent papillary features, (3) papillary squamous cell carcinoma, and (4) a papillary cystadenocarcinoma. Papillary squamous cell carcinoma is a distinct variant of squamous cell carcinoma characterized by an exophytic, papillary growth, which can resemble sialadenoma papilliferum in its gross appearance [14]. However, a glandular component with mucin-secreting cells is not present in this lesion. Adenosquamous carcinoma has distinctively separate components of adeno- and squamous differentiation with no papillary configuration [14]. Cystadenocarcinoma is a rare malignant tumor characterized by predominantly cystic growth and conceptually it is the malignant counterpart of the benign cystadenoma. It does not include squamous elements [14]. Distinctively squamous differentiation is not a feature of papillary cystadenocarcinoma. Cell types that comprise the lining epithelium of cysts in papillary cystadenocarcinoma include, most often, cuboidal and columnar cells. Other cells types such as mucous, clear and oncocytic cells occasionally are noted as well [14].

Thus far, three cases of malignant transformation of sialadenoma papilliferum have been documented in the literature [4–6] (Table 1). Solomon et al. reported a soft palate sialadenoma papilliferum with lymph node metastasis, which later was re-interpreted as a mucoepidermoid carcinoma by others [4, 8]. The case reported by Shimoda et al. [5] is a clearer example of malignancy associated with sialadenoma papilliferum. They described an epithelial–myoepithelial carcinoma containing a transition zone to adjacent sialadenoma papilliferum. Most recently, Ponniah [6] reported a case of sialadenoma papilliferum with carcinoma in situ in the exophytic component.