Diverse histomorphology of HER2-positive breast carcinomas based on differential ER expression

Aims HER2-positive (HER2+) breast carcinoma (BC) cases are often treated similarly; however, they can be classified as either luminal B (LH) or non-luminal type (NLH) BC, which have different prognoses. In this study, we investigated the clinicohistomorphological features of each HER2+ BC subgroup. Methods and results We classified 166 patients with HER2+ invasive BC into LH (n = 110, 66.3%) and NLH groups (n = 56, 33.7%). We further subclassified LH into patients with carcinomas expressing high levels of hormone receptors [LH-high; Allred score, oestrogen receptor (ER) and/or progesterone receptor (PgR) 4-8, n = 89, 53.6%] or low levels (LH-low; Allred score, ER and/or PgR 2 or 3, n = 21, 12.7%) for clinicohistomorphological characterisation. Morphological review showed that NLH included a percentage of patients with comedo necrosis, while LH patients had significantly more central scarring. In terms of immune responsiveness, NLH showed significantly higher rates of tumour-infiltrating lymphocytes and healing. The LH-high and NLH groups showed distinct characteristics (by both models, P < 0.05) and the LH-low group appeared to demonstrate intermediate characteristics according to multinomial analyses using covariates reflecting tumour morphology and immune response outcomes. Conclusions These results support the classification of HER2+ BC into two major subgroups, LH-high and NLH, based on tumour morphology and immune response; LH-high proliferates via scirrhous and/or spiculated growth with a central scar, while the primary proliferation pattern of NLH is based on in-situ carcinomas containing comedo necrosis with noticeable TILs and healing.