Microinvasion is the smallest morphologically identifiable stage of invasion. Its presence and distinction from in situ carcinoma may have therapeutic implications, and clinical staging also requires the recognition of this phenomenon. Microinvasion is established on the basis of several morphological criteria, which may be difficult and not perfectly reproducible among pathologists. The aim of this study was to assess the consistency of diagnosing microinvasion in the breast on traditional haematoxylin and eosin (HE) stained slides and to evaluate whether immunohistochemistry (IHC) for myoepithelial markers could improve this. Digital images were generated from representative areas of 50 cases stained with HE and IHC for myoepithelial markers. Cases were specifically selected from the spectrum of in situ to microinvasive cancers. Twenty-eight dedicated breast pathologists assessed these cases at different magnifications through a web-based platform in two rounds: first HE only and after a washout period by both HE and IHC. Consistency in the recognition of microinvasion significantly improved with the use of IHC. Concordance rates increased from 0.85 to 0.96, kappa from 0.5 to 0.85, the number of cases with 100% agreement rose from 9/50 to 25/50 with IHC and the certainty of diagnosis also increased. The use of IHC markedly improves the consistency of identifying microinvasion. This corroborates previous recommendations to use IHC for myoepithelial markers to clarify cases where uncertainty exists about the presence of microinvasion. Microinvasive carcinoma is a rare entity, and seeking a second opinion may avoid overdiagnosis.

2nd Department of Pathology Semmelweis University Budapest Üllői út 93 Budapest 1091 Hungary

Aix Marseille Université 13284 Marseille France

Anatomic Pathology Department of Biomedical and Neuromotor Sciences University of Bologna Bellaria Hospital Via Altura 3 40137 Bologna Italy

Bács Kiskun County Teaching Hospital Nyiri ut 38 Kecskemét 6000 Hungary

Centre National de la Recherche Scientifique UMR7258 13009 Marseille France

CMP Cerba European Labs 45A avenue Wybran 1070 Brussels Belgium

CRCM Molecular Oncology Equipe labellisée Ligue Contre le Cancer Inserm U1068 13009 Marseille France

Department of Histopathology St Vincent's University Hospital Dublin 4 Dublin Ireland

Department of Imaging and Pathology Laboratory of Translational Cell and Tissue Research KU Leuven University of Leuven Herestraat 49 3000 Leuven Belgium

Department of Medical Sciences University of Torino Italy Via Santena 7 10120 Torino Italy

Department of Pathology Complejo Hospitalario de Navarra Irunlarrea 3 31008 Pamplona Navarra Spain

Department of Pathology Corporació Sanitària Parc Taulí University Autònoma Barcelona 08202 Sabadell Spain

Department of Pathology Maria Skłodowska Curie Memorial Cancer Center and Institute of Oncology Gliwice Branch Gliwice Poland

Department of Pathology Mater Dei Hospital Tal Qroqq Msida MSD 2090 Malta

Department of Pathology Medical School University of Patras 26504 Patras Rion Greece

Department of Pathology Strasbourg University Hospitals Hôpital de Hautepierre 1 avenue Molière 67100 Strasbourg Cedex France

Department of Pathology University College London Level 2 Rockefeller Building 21 University Street London WC1E 6JJ UK

Department of Pathology University Hospitals Leuven Herestraat 49 3000 Leuven Belgium

Department of Pathology University Medical Center Utrecht Utrecht The Netherlands

Department of Pathology University of Szeged Allomás u 2 Szeged 6720 Hungary

Discipline of Pathology NUI Galway Clinical Sciences Institute Costello Road Galway Ireland

Division of Oncology and Pathology Department of Clinical Sciences Lund University Sölvegatan 25 221 85 Lund Sweden

Division of Pathological Anatomy Department of Surgery and Translational Medicine AOU Careggi Largo G A Brambilla 3 50134 Florence Italy

Divisions of Diagnostic Oncology and Molecular Pathology Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital Plesmanlaan 121 1066 CX Amsterdam the Netherlands

German Breast Screening Program Reference Centres Berlin and Muenster Department of Pathology Dietrich Bonhoeffer Medical Centre Allendestraße 30 17036 Neubrandenburg Germany

Institut Paoli Calmettes 13009 Marseille France

Institute of Pathology Breast Cancer Center Paderborn Husener Str 46a 33098 Paderborn Germany

Institute of Pathology Medical University Graz Auenbruggerplatz 25 8036 Graz Austria

Institute of Surgical Pathology University Hospital Zurich Schmelzbergstrasse 12 CH 8091 Zürich Switzerland

Laboratório de Anatomia Patológica Centro Hospitalar de São João e IPATIMUP 4440 563 Porto Portugal

Laboratório de Anatomia Patológica Instituto Portugues de Oncologia de Coimbra FG Av Bissaia Barreto 98 3000 075 Coimbra Portugal

Pathologisch Bakteriologisches Institut Donauspital am SMZO Langobardenstraße 122 Vienna Austria

Pathology Centre Riga East Clinical University Hospital Hipokrata St 2 Riga LV 1038 Latvia

School of Medicine Department of Pathology Marmara University Istanbul Turkey

The Fingerland Department of Pathology Charles University Medical Faculty and University Hospital Sokolska 581 50005 Hradec Kralove Czech Republic

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