Pr 1e2

The study was approved by the Institutional Review Board. A retrospective search of the Washington University in St. Louis Department of Pathology and Immunology archive was searched from 1993 to 2013 for cases of MSH. Ten cases were identified that were diagnosed as MSH (a term used by one of the authors, HMM) and showed characteristic histopathologic features. Clinical and radiologic features were gathered from the electronic medical record. Hematoxylin and eosin slides were reviewed, and formalin fixed paraffin embedded tissue blocks were retrieved for immunohistochemical studies (IHC). Pankeratin (AE1/AE3/PCK26, Ventana, Tucson AZ), keratin (34βE12, Ventana, Tucson AZ), cytokeratin 5/6 [(CK5/6), D5/16B4, Ventana, Tucson AZ], beta-catenin (14, Cell Marque, Rocklin CA), p63 (4A3, Ventana, Tucson AZ) estrogen receptor [(ER), SP1, Ventana, Tucson AZ], progesterone receptor [(PR), 1E2, Ventana, Tucson AZ], smooth muscle actin [(SMA), 1A4, Cell Marque, Rocklin CA], and CD34 (QBEnd/10, Ventana, Tucson AZ) (prediluted per standard protocol, single antibody stain procedure with adequate controls) were performed on eight cases at the Washington University AMP Core Laboratory. In addition, the pathology departmental archive was searched for all in-house breast core biopsies from 2014 to 2017 to aid in an approximate determination of the incidence of MSH.

Patients diagnosed with synchronous DCIS and adjacent IBC and treated at the Erasmus Medical Center -Cancer Institute in the period between 2010 and 2015 were included (n = 73). Patients with neoadjuvant treatment, previous breast irradiation or a history of breast cancer were excluded. The tumor cell percentage was estimated beforehand by a pathologist and only samples with an estimated tumor cell percentage ≥50% were included. DCIS grade was assessed primarily based on cytonuclear differentiation and IBC grade was assessed according to the modified Bloom Richardson score (Elston & Ellis 1991) (link). Immunohistochemistry (IHC) was performed on each IBC, using ER (ER SP1; Ventana Medical Systems, Inc.), PR (PR 1E2; Ventana Medical Systems, Inc.) and HER2 (HER2 4B5; Ventana Medical Systems, Inc.) antibodies. ER and PR status were scored positive when ≥10% of the tumor cells were positive, according to the Dutch Breast Cancer Guideline (NABON 2017). HER2 status was scored according to the international guidelines (Wolff et al. 2014) (link). For this study, coded leftover patient material was used in accordance with the Code of Conduct of the Federation of Medical Scientific Societies in The Netherlands (FEDERA 2011). Therefore, there was no need for an informed consent or study approval by an ethical committee. Figure 1 provides an overview of the workflow.