Integration of pathological criteria and immunohistochemical evaluation for invasive lobular carcinoma diagnosis: recommendations from the European Lobular Breast Cancer Consortium.

Invasive lobular carcinoma (ILC) is the second most frequent type of breast cancer (BC) and its peculiar morphology is mainly driven by inactivation of CDH1, the gene coding for E-cadherin cell adhesion protein. ILC-specific therapeutic and disease-monitoring approaches are gaining momentum in the clinic, increasing the importance of accurate ILC diagnosis. Several essential and desirable morphological diagnostic criteria are currently defined by the World Health Organization, the routine use of immunohistochemistry (IHC) for E-cadherin is not recommended. Disagreement in the diagnosis of ILC has been repeatedly reported, but inter-pathologist agreement increases with the use of E-cadherin IHC. In this study, we aimed to harmonize the pathological diagnosis of ILC by comparing five commonly used E-cadherin antibody clones (NCH-38, EP700Y, Clone 36, NCL-L-E-cad [Clone 36B5], and ECH-6). We determined their biochemical specificity for the E-cadherin protein and IHC staining performance according to type and location of mutation on the CDH1 gene. Western blot analysis on mouse cell lines with conditional E-cadherin expression revealed a reduced specificity of EP700Y and NCL-L-E-cad for E-cadherin, with cross-reactivity of Clone 36 to P-cadherin. The use of IHC improved inter-pathologist agreement both for ILC as well as for lobular carcinoma in situ and atypical lobular hyperplasia. The E-cadherin IHC staining pattern was associated with variant allele frequency and likelihood of non-sense mediated RNA decay but not with the type or position of CDH1 mutations. Based on these results, we make recommendations for the indication for E-cadherin staining, choice of antibodies, and their interpretation in order to standardize ILC diagnosis in current pathology practice.