The histological features of SCC have been discussed in Chapter 3 on tumours of the hypopharynx, larynx and trachea. The findings in the oral cavity and oropharynx do not differ significantly from those of the larynx and hypopharynx. A minority of oral and oropharyngeal cancers show different histological subtypes that can be associated with differences in prognosis. These are discussed below. It is clearly important that pseudo epithe-liomatous hyperplasia (PEH) is distinguished from SCC. PEH can occur in mucosa overlying a granular cell tumour, in necrotising sialometaplasia and in papillary hyperplasia of palate. PEH occurring with mucositis, particulary after irradiation, may be difficult to distinguish from squamous cell carcinoma. The majority of cases of SCC present no difficulty in diagnosis for the experienced pathologist. However, the recognition of the earliest stages of invasion can be problematic. No consistent guidelines for this exist. The deepest layers of the epithelium and the interface between the epithelium and the lamina propria need to be examined in detail. This is frequently made more difficult where there is a prominent inflammatory infiltration.

Relevant features include the loss of a his-tologically well-defined interface, described previously as loss of basement membrane and disturbed architecture of the basal layers of the epithelium, particularly the replacement of basal cells by larger irregular cells with cytoplasmic processes extending into connective tissue. In some cases the degree of cyto-logical atypia and mitotic feature may suggest malignancy, but these are not always present. To an extent the judgement about early invasion is subjective and it can be important for the pathologist to communicate the difficulty in interpretation to the clinician. Some pathologists will indicate that while no unequivocal evidence of invasion is demonstrated, they nevertheless feel that the lesion should be regarded as early invasive carcioma.

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