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The morphological heterogeneity of anal carcinomas is well recognised, although the prognostic significance of variable squamous, basaloid and ductal differentiation, by the carcinoma cells is uncertain. The clinical classification is inconsistent due to lack of standard definitions of anal canal and margin. In this study, statistical methods were used to investigate whether discrete groups of tumours can be identified on the basis of shared morphological characteristics. The findings were correlated with the tumour site and prognosis. Principal component analysis on 100 resected cases of anal carcinoma showed that three sets of features were co-expressed by tumour cells: duct formation and diastase Periodic Acid Schiff (dPAS) positivity; keratinization, stratification and the presence of intercellular bridge formation (prickles); basaloid cells, a palisade arrangement, retraction artefact and eosinophilic necrosis. 98% of tumours showed keratinization and two thirds showed at least one basaloid feature. No tumours showed a full set of basaloid characteristics without squamous features. Ductal differentiation (16% tumours) showed a significant association with dPAS positivity and the combination was associated with poor prognosis. A correlation was found between clinical and histological assessment of site, with anal margin carcinoma more likely to show pure squamous morphology than canal tumours. Anal carcinoma should therefore be regarded as keratinizing squamous cell carcinoma. The existence of basaloid carcinoma as a separate entity is not supported by these results. Basaloid and ductal changes represent further pathways of differentiation superimposed on an underlying keratinizing squamous carcinomatous pedigree in these tumours. © 1995 Pearson Professional Ltd.

Original publication




Journal article


Current Diagnostic Pathology

Publication Date





32 - 37