Squamous papillomas are common lesions of the oral mucosa with a predilection for the mucosa of the hard and soft palate, including the uvula and the vermilion of the lips. It is an innocuous lesion that is neither transmissible nor threatening. As an oral lesion, it raises concern because of its clinical appearance, which may mimic exophytic carcinomas, verrucous carcinomas, or condyloma acuminatum (a viral disease that is transmissible).
The squamous papilloma is also noteworthy for its uncertain pathogenesis. Many oral and maxillofacial specialists accept its pathogenesis as being from the human papillomavirus (HPV). This is based on its similar appearance to cutaneous warts and the identification of HPV subtypes 2, 6, 11, and 57 in some but not all oral squamous papillomas. However, despite extensive research, a definitive cause-and-effect relationship has not been established. If a DNA virus such as HPV were the stimulus, one would expect a direct contact transmission such as that seen with condyloma acuminatum and herpes. The squamous papilloma also shows no histopathologic signs of viral infection, such as intranuclear inclusion bodies and vacuolated nuclei. Furthermore, HPV is not identified in most squamous papillomas. However, HPV types 1, 2, 4, 6, 7, 11, 13, 16, 18, 30, 32, 40, and 57 have been identified in other lesions containing oral squamous cells, suggesting that HPV may be merely an incidental finding unrelated to the development of a squamous papilloma. This is further suggested by the failure of tests to show HPV DNA in the squamous cells or basal cells of most squamous papillomas.
Regardless of its pathogenesis, the squamous papilloma will usually present in one of the four sites of predilection, although it may occur on any oral mucosal surface. Usually appearing as asymptomatic single lesions without induration (clusters and multiple lesions occasionally develop), they generally have a sessile base but may sometimes have a stalk.
Single squamous papillomas may resemble verrucous carcinomas or even exophytic squamous cell carcinomas if they have a sessile base. Certainly the finding of induration or ulceration would lead the clinician to suspect these two concerning lesions more strongly than a squamous papilloma. In addition, clustered or multiple squamous papillomas would suggest focal epithelial hyperplasia (Heck disease). In addition, a verruciform xanthoma will clinically resemble squamous papilloma, but it is mostly seen on the gingiva or the edentulous alveolar ridge.
The papilloma is a benign proliferation of squamous epithelium. With the epithelium’s dependence for nutrition on the underlying fibrovascular tissue, the most efficient growth pattern is one of exophytic papillary projections, each with a fibrovascular core. The epithelium may show orthokeratosis, parakeratosis, and/or acanthosis. Mitoses may be numerous but are usually confined to the basal area.
The prickle cells may have a clear glycogen-filled cytoplasm, particularly in lesions of the soft palate. Koilocytic cells (epithelial cells with pyknotic nuclei surrounded by a clear halo), which are often associated with viral disease, also may be present, but they also can be found in nonvirally infected oral mucosa. Their presence is not sufficient to confirm a viral etiology for any particular papilloma. The lamina propria frequently contains a chronic inflammatory infiltrate.
Because of the varied lesions on the differential diagnosis, most of which have a more concerning prognosis than that of the squamous papilloma, all lesions resembling a squamous papilloma are recommended for excision at the base (1-mm margin) to the depth of the submucosa. This excision should be curative. Recurrence or new lesions should raise suspicions of a possible retransmission of a condyloma acuminatum or of carcinoma.