8.1 Introduction

More than 90% of malignancies that occur in the oral cavity are squamous cell carcinoma (SCC). While this statistic suggests a certain monotony to the pathologic examination of these lesions, it actually belies the rich variety of tumor morphology, grade, and variants included in this category as well as the challenging constellation of staging and prognostic variables that must be evaluated for each tumor. Correct and comprehensive pathological evaluation of these lesions forms the foundation for appropriate patient management and follow-up. This chapter provides an overview of the most relevant features and findings in the pathologic diagnosis and evaluation of oral cavity SCC and its precursors with an emphasis on new developments and priorities in the eighth edition of the AJCC Cancer Staging Manual ¹ and the 2017 WHO Classification of Head and Neck Tumors. ²

8.2 Precursor Lesions

Squamous dysplasia is well established as the main precursor of invasive SCC in the oral cavity. Pathologic diagnosis of dysplasia hinges upon assessment of both architectural and cytological atypia in squamous mucosa. As defined in the 2017 WHO Classification of Head and Neck Tumors, architectural changes that support a diagnosis of dysplasia include abnormalities in cellular stratification, loss of basal cell polarity, drop-shaped rete ridges, increased mitotic rate, the presence of superficial mitotic figures, single-cell keratinization, keratin pearl formation within rete ridges, and loss of epithelial cohesion. ³ Cytological changes that are seen in epithelial dysplasia include variation in nuclear size, variation in nuclear shape, variation in cell size, variation in cell shape, increased nuclear: cytoplasmic ratio, atypical mitotic figures, increased number and size of nucleoli, and nuclear hyperchromasia. ³ Unfortunately, there is no consistent threshold for determining how well developed or extensive these changes must be to differentiate dysplasia from hyperplastic and reactive processes.

The prognosis and management of squamous dysplasia is contingent upon the histologic grade that pathologists assign. Oral cavity dysplasia traditionally has been graded on a three-tiered system, with the atypia in mild dysplasia limited to the basal third of the epithelium, moderate dysplasia extending into the middle third of the epithelium, and severe dysplasia reaching the upper third of the epithelium (▶ Fig. 8.1). ³ Nevertheless, there is some flexibility in grading such that markedly increased cytological or architectural atypia, regardless of the epithelial stratum involved, supports an increased grade. The term carcinoma in situ can be used for full-thickness epithelial atypia but is essentially considered synonymous with severe dysplasia. Although a two-tiered system of just low-grade or high-grade dysplasia is now recommended for use in the larynx, ⁴ such a system has not yet been validated in the oral cavity. Given these flexible criteria, there is unsurprisingly poor interobserver agreement in the diagnosis and grading of squamous dysplasia. ^{5 – 8} Consequently, the prognostic implications of squamous dysplasia can vary widely between studies, with an overall rate of malignant transformation estimated at 12% in a large meta-analysis. ⁹ The likelihood of developing SCC is closely tied to histologic grade of dysplasia, being approximately 6% for mild dysplasia, 18% for moderate dysplasia, and 39% for severe dysplasia. ⁶ Nevertheless, pathologists must remember that SCC can arise even in the setting of mild or moderate dysplasia, in a pattern known as “drop” invasion.

A rarer precursor lesion to SCC that does not fit neatly into the category of squamous dysplasia is proliferative verrucous leukoplakia (PVL). PVL is a progressive, multifocal condition that can be very difficult to diagnose pathologically because of its variable and sometimes subtle histologic findings. At its earliest stage, biopsy sampling of PVL may demonstrate only hyperkeratosis and verrucous hyperplasia. These lesions are frequently accompanied by interface mucositis, and may be mistaken for lichen planus or another reactive process. ³ As PVL progresses, however, lesions tend to develop more overtly dysplastic changes. Nevertheless, architectural atypia may be more marked than cytological atypia throughout the course of the disease. Pathologists must put these histologic changes in context of a clinical picture of recurrent and multifocal disease to help suggest this diagnosis. In contrast to conventional dysplasia, PVL has an approximately 50% risk of malignant transformation and can be associated with the development of both verrucous and conventional SCC. ^{10 , 11}

Several benign and reactive changes can pose a pitfall for pathologists when evaluating for squamous dysplasia. Broadly speaking, reactive changes in squamous epithelium comprise many features that overlap with dysplastic changes, including nuclear enlargement, mild nuclear membrane irregularity, prominent nucleoli, and increased mitotic figures; however, the presence of prominent inflammation and associated ulceration or granulation tissue can provide an important reminder to consider a reactive process. A wide range of insults can induce these reactive changes in squamous epithelium, including local trauma, inflammatory diseases such as lichen planus, and even unsampled underlying tumor. Candida infections can be especially treacherous in mimicking dysplasia because they can demonstrate marked architectural distortion, most notably elongation of the rete ridges, to accompany these reactive cytological alterations. Additionally, normal structures can also be mistaken for dysplasia in the oral cavity, particularly if a pathologist is not cognizant of anatomic site. Squamous epithelium surrounding the taste buds in fungiform papillae can demonstrate basal stratification that mimics dysplasia; recognition of sensory and sustentacular cells and underlying nerve fibers is essential to avoid this pitfall. Likewise, if approaching a pharyngeal site that is lined by respiratory epithelium, immature squamous metaplasia can be suggestive of disordered maturation. Seeing transition to adjacent respiratory mucosa and intermixed goblet cells can be helpful for detecting this benign process.

8.3 Squamous Cell Carcinoma

The pathologic diagnosis of SCC in the oral cavity hinges primarily on two features: the identification of invasive growth and the demonstration of squamous differentiation. ² Histologically, oral cavity SCC is characterized by nests, cords, islands, and sheets of tumor cells that invade through the basement membrane to involve the subepithelial tissues and beyond. Invasion can be subtle in small and superficial tumors, and pathologists must meticulously evaluate the epithelial-stromal interface for small nests of infiltrative cells. However, infiltrative growth is not uniformly seen; invasion can also come in the form of broad, pushing-downward extension into stroma. Tumor cells can show varying evidence of squamous differentiation, manifesting as cytoplasmic keratinization with keratin pearl formation and intracellular bridges. In the highest-grade tumors, histologic hallmarks of squamous differentiation can be entirely absent, with sheets, nests, and cords of undifferentiated malignant cells. Immunohistochemical markers of squamous differentiation such as p40, p63, and CK5/6 may be necessary to establish the diagnosis in these cases. ¹² Nonetheless, pathologists must remember that these markers are not entirely specific for squamous differentiation, being frequently seen in salivary tumors among others. ^{13 , 14}

Also analogous to squamous dysplasia, oral cavity SCC has traditionally been graded using a three-tiered system, ² loosely based on the degree to which they resemble normal keratinizing squamous epithelium. In addition to the growth pattern and degree of keratinization, cytological atypia plays an important role in this classification. Well-differentiated SCC is characterized by obvious squamous differentiation, with large cells, prominent keratinization, and discernable intracellular bridges (▶ Fig. 8.2). These tumors frequently show minimal cytological atypia and some degree of maturation from basal-type peripheral cells to central keratinized cells, and can be difficult to distinguish from normal epithelium in small or tangentially embedded samples. Moderately differentiated SCC generally still show overt evidence of squamous differentiation, although keratin pearl formation and intracellular bridges might be focal compared to well-differentiated tumors. These tumors have more pronounced cytological atypia, including nuclear enlargement, nuclear pleomorphism, increased nuclear-to-cytoplasmic ratio, atypical mitotic figures, prominent nucleoli, and nuclear hyperchromasia; it is rarely difficult to discern that these tumors represent carcinoma (▶ Fig. 8.3). Finally, poorly differentiated SCC may have no overt histologic evidence of squamous differentiation, with nondescript to basaloid tumor cells that demonstrate minimal if any keratinization. These tumors also tend to have diffuse and well-developed cytological atypia (▶ Fig. 8.4). Although histologic grading does not always accurately reflect tumor prognosis, ¹⁵ it remains a key part of pathologic diagnosis.

Several benign processes can create findings that mimic well to moderately differentiated SCC. Necrotizing sialometaplasia poses a particular pitfall because it may present as an ulcerated lesion that also raises clinical suspicion for carcinoma. Histologically, this process is characterized by ischemic necrosis of minor salivary glands with resultant squamous metaplasia. Pathologists can distinguish necrotizing sialometaplasia from SCC due to its lobulated nature and contiguity with other salivary glands and ducts; however, marked reactive changes may pose a difficult differential diagnosis. Another important consideration is pseudoepitheliomatous hyperplasia, which can be induced by a wide range of reactive processes or tumors such as granular cell tumors or salivary gland neoplasms. In pseudoepitheliomatous hyperplasia, irregular tongues of squamous epithelium penetrate deeply into the subepithelium. These tongues of epithelium should lack the cytological atypia seen in SCC as well as significant abnormalities in cell cohesion and mitotic rate; however, abnormal keratinization is occasionally seen. Finally, normal structures can mimic SCC to the unwary observer. If specimen margins extend into the oropharynx, the reticulated epithelium of tonsillar tissue demonstrates an irregular stromal contour and numerous infiltrating lymphocytes that can be mistaken for invasive growth. Similarly, benign odontogenic remnants, including the juxtaoral organ of Chievitz in the retromolar area and rests of Serres in the gingival mucosa, can mimic invasive growth.

The histologic features of oral cavity SCC are mirrored in cytological sampling of these tumors. Cytopathological evaluation of oral cavity tumors occurs almost exclusively through fine-needle aspiration of cervical lymph node metastases; although epithelial brushings can be performed for primary diagnosis, this technique has inferior sensitivity compared to surgical biopsy. ¹⁶ As with histologic samples, fine-needle aspirates must be evaluated for squamous differentiation; however, without the benefit of tissue architecture, identification of cytological atypia takes precedence over invasive growth. ² Squamous differentiation is generally easy to establish in common cytological preparations, with a characteristic robin egg blue color on Romanowsky-based stains and distinctive orangeophilic staining on Papanicolaou stains. Tumor cells can show varying degrees of cytological atypia, including nuclear hyperchromasia, nuclear enlargement, nuclear membrane irregularity, and atypical mitotic figures (▶ Fig. 8.5). Aspirates from lymph node metastases with cystic degeneration or treatment effect sometimes show only a mix of necrotic debris and rare parakeratinized cells, precluding a definitive diagnosis. Another important pitfall in cytological diagnosis of SCC is benign squamous-lined cysts such as branchial cleft cysts, which can raise misplaced concern for metastatic disease. However, these lesions should lack cytological atypia and have abundant foamy macrophages, indicative of their cystic nature.

8.4 Squamous Cell Carcinoma Variants

Several subtypes of SCC can rarely be seen in the oral cavity. In this site, the three variants that have equivalent or worse prognosis compared to conventional SCC are basaloid SCC, spindle cell (sarcomatoid) SCC, and adenosquamous carcinoma. Basaloid SCC is characterized by hyperchromatic cells with a high nuclear-to-cytoplasmic ratio, an adenoidal growth pattern, comedo-type necrosis, and prominent extracellular basement membrane-type material; it frequently raises diagnostic consideration of high-grade adenoid cystic carcinoma or neuroendocrine tumors. ¹⁷ It has overall similar prognosis to conventional SCC in the oral cavity but a higher rate of metastases. ^{18 , 19} Spindle cell (sarcomatoid) SCC frequently presents as a polypoid ulcerated lesion with overtly malignant spindled cells and variable morphologic and immunohistochemical evidence of squamous differentiation. ²⁰ These tumors most commonly occur postradiation or as a second primary in the oral cavity and have a worse prognosis than conventional SCC. ^{21 , 22} Finally, adenosquamous carcinoma is characterized by biphasic architecture that encompasses a superficial squamous component and a deep glandular component. ²³ These aggressive tumors also tend to have worse prognosis than conventional SCC. ^{24 – 26}

In contrast, verrucous carcinoma, papillary SCC, and carcinoma cuniculatum tend to have a better prognosis than conventional SCC in the oral cavity. Verrucous carcinoma is characterized by exophytic growth with prominent spires of parakeratin, minimal cytologic atypia, and a pushing pattern of invasion into underlying stroma. ²⁷ Although verrucous carcinoma has the capacity for destructive local growth, it lacks true metastatic potential. ^{28 , 29} Papillary carcinoma also demonstrates exophytic growth, but its delicate fibrovascular cores are surrounded by overtly malignant squamous epithelium with notable cytological atypia. ³⁰ Papillary carcinoma does have the potential for metastatic growth even in the absence of conventional invasion. ^{28 , 29 , 31} Finally, carcinoma cuniculatum is characterized by deeply invasive growth in a burrowing pattern with prominent tunnels of cytologically bland epithelium and prominent keratin production. Carcinoma cuniculatum has a tendency for local recurrence but only rarely metastasizes. ³²