Abstract
AIMS: Metaplastic breast carcinoma (MBC) is a heterogeneous group of invasive breast carcinoma with squamous, spindle cell, or mesenchymal elements. It may be monophasic or biphasic, and often coexists with invasive breast carcinoma of no special type (IBC-NST). Currently, there are no standardized guidelines for reporting MBC, and a diagnostic threshold for the metaplastic component is not established by the WHO classification. METHODS AND RESULTS: A survey conducted from April-July 2023 gathered responses from 44 pathologists worldwide. Most respondents were academic breast pathologists, and attended weekly breast tumour boards. The criteria for diagnosing MBC were highly varied, with cutoffs ranging from any/<10% to >90% metaplastic component. Although 90% was the most common threshold used, only 25% of respondents applied it. Pathologists generally preferred diagnosing invasive carcinoma with mixed features when the metaplastic component was ≤50% and MBC when the metaplastic component >50%. Most pathologists reported both the type and percentage of metaplastic component. In all, 43% reported core biopsy and resection specimen with different approaches. Diagnostic guidelines reportedly used (if any) were highly varied. CONCLUSION: The study underscores the need for standardized guidelines for MBC. Without clear and established diagnostic criteria, current data on MBC remains inconsistent and hinders further research into the clinical significance and prognostic implications of the metaplastic component.