BACKGROUND: Histopathological classification of basal cell carcinoma (BCC) has important prognostic and therapeutic implications, but reproducibility of BCC subtyping among dermatopathologists is poor. OBJECTIVES: To obtain a consensus paper on BCC classification and subtype definitions. METHODS: A panel of 12 recognized dermatopathologists (G12) from nine European countries used a modified Delphi method and evaluated 100 BCC cases uploaded to a website. The strategy involved five steps: (I) agreement on definitions for WHO 2018 BCC subtypes; (II) classification of 100 BCCs using the agreed definitions; (III) discussion on the weak points of the WHO classification and proposal of a new classification with clinical insights; (IV) re-evaluation of the 100 BCCs using the new classification; and (V) external independent evaluation by 10 experienced dermatopathologists (G10). RESULTS: A simplified classification unifying infiltrating, sclerosing, and micronodular BCCs into a single "infiltrative BCC" subtype improved reproducibility and was practical from a clinical standpoint. Fleiss' κ values increased for all subtypes, and the level of agreement improved from fair to moderate for the nodular and the unified infiltrative BCC groups, respectively. The agreement for basosquamous cell carcinoma remained fair, but κ values increased from 0.276 to 0.342. The results were similar for the G10 group. Delphi consensus was not achieved for the concept of trichoblastic carcinoma. In histopathological reports of BCC displaying multiple subtypes, only the most aggressive subtype should be mentioned, except superficial BCC involving margins. CONCLUSIONS: The three BCC subtypes with infiltrative growth pattern, characteristically associated with higher risk of deep involvement (infiltrating, sclerosing, and micronodular), should be unified in a single group. The concise and encompassing term "infiltrative BCCs" can be used for these tumors. A binary classification of BCC into low-risk and high-risk subtypes on histopathological grounds alone is questionable; correlation with clinical factors is necessary to determine BCC risk and therapeutic approach.

Department of Dermatology Centre Hospitalier Universitaire Saint Pierre Université Libre de Bruxelles Brussels Belgium

Department of Dermatology Fundación Jiménez Díaz Universidad Autónoma Madrid Spain

Department of Dermatology Hospital Clínic de Barcelona University of Barcelona IDIBAPS Barcelona and CIBERER Barcelona Spain

Department of Dermatology Medical University of Vienna Vienna Austria

Department of Dermatology University Hospital Zurich Zürich Switzerland

Department of Dermatology University Hospital Zürich Zürich Switzerland

Department of Pathology Hospital Universitari General de Catalunya Grupo Quironsalud and Universitat Internacional de Catalunya Sant Cugat del Vallés Spain

Dermatology Clinic IRCCS San Raffaele Hospital Vita Salute University Milan Italy

Dermatology Department University Hospital Strasbourg France

Dermatopathology Practice Friedrichshafen Lake Constance Friedrichshafen Germany

Kempf Pfaltz Histologische Diagnostik Zurich Switzerland

Sikl's Department of Pathology Medical Faculty in Pilsen Charles University Prague Pilsen Czech Republic

St John's Institute of Dermatology St Thomas Hospital London UK

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