Papillary carcinoma (PC), which is a rare type of breast cancer, comprises a heterogeneous group of tumours. The diagnostic categorization of PC as in‐situ and invasive disease remains a matter of debate with respect to interpretation of its overlapping histological features, and with respect to the uncertainty in clinical behaviour that this dilemma raises. The aim of this study was to assess the diagnostic agreement regarding PC among reporting breast pathologists.
Methods and results
Six cases of PC included in the UK National Health Service Breast Screening Programme breast pathology interpretive external quality assurance scheme in the last 10 years were reviewed. In this scheme, one representative haematoxylin and eosin‐stained slide from each case is circulated to an average of 600 participants. Data on diagnostic categories were collected and slides were reviewed according to the World Health Organization (WHO) diagnostic criteria. The number of final diagnoses of malignancy (in situ or invasive) was highest for invasive PC (99% of the participants diagnosed it as malignant), followed by solid PC (94% and 95%, respectively), encapsulated PC (92% and 92%, respectively), and papillary ductal carcinoma in situ (DCIS) (88%). Most cases of papillary DCIS were correctly classified as in‐situ disease (77%), but 28% of the participants classified invasive PC cases as in‐situ disease. Of the participants, 24% reported encapsulated PC as invasive disease. Of the two solid PC cases, one showed some features consistent with the WHO description of invasive solid PC, whereas the other showed features of classic (non‐invasive) solid PC. Both cases were reported as invasive by 75% and 77% of participants, respectively. Breast specialists more frequently classified PC as an in‐situ carcinoma than did non‐specialist participants, and the difference was significant (P = 0.013).
Recognition of PC as a malignant entity (in situ or invasive) is high, but concordance of its classification into in‐situ and invasive disease is low. Histological features that can define invasion in PC should be better defined. These rare lesions require additional diagnostic work‐up, and difficult cases should trigger consensus opinion or expert referral.