Differentiating squamous cell carcinoma from keratoacanthoma using histopathological criteria. Is it possible? A study of 296 cases.

BACKGROUND: Squamous cell carcinoma (SCC) and keratoacanthoma (KA) are sometimes difficult to distinguish by histopathological examination, since cytological features are similar in both tumors. Distinctive criteria - mainly architectural - have therefore been proposed as an aid in diagnosis.
OBJECTIVE: The purpose of this study was to evaluate the reliability of some of the criteria used to make a distinction between SCC and KA.
METHODS: 296 fully excised tumors previously classified as SCC or KA were randomized and examined independently by two examiners. Fourteen criteria, mainly based on the architecture of the tumors, were determined on the 262 slides for which a consensual diagnosis was made.
RESULTS: No single criterion was sufficiently sensitive and specific to allow a clear-cut differential diagnosis. The 5 most relevant criteria were epithelial lip, sharp outline between tumor and stroma in favor of KA and ulceration, numerous mitoses and marked pleomorphism/ anaplasia in favor of SCC. Intraepithelial polymorphonuclear abscesses, intraepithelial elastic fibers, parakeratosis and dyskeratosis and extension more lateral than downward were not distinctive criteria, although they are considered as classic distinctive features.
CONCLUSION: Many of the criteria commonly used for the differential diagnosis of SCC and KA are not reliable. The combination of the 5 most useful criteria does not significantly increase the specificity or sensitivity of the histological diagnosis in difficult cases. Atypical or difficult cases should therefore be considered and treated as SCC, since a clear-cut distinction is not possible even with the aid of the most relevant criteria.