HER2-positive grade 1 invasive carcinomas of the breast

Aims: The American Society of Clinical Oncology and College of American Pathologists HER2-guidelines recommend repeat testing for most grade 1 mammary carcinomas that are HER2-positive in the core biopsy. This study aimed to assess the value of repeat HER2-testing and the histological features of HER2-positive grade 1 carcinomas. Methods and results: A case-series of HER2-results of grade 1 carcinomas was conducted of patients with no pre-operative systemic treatment over a 5-year period. HER2-positive carcinomas had histological review. Twelve HER2-positive carcinomas were initially reported as grade 1. On review, two were reclassified as grade 2. The remaining 10 carcinomas represented 2% of the 508 grade 1 carcinomas. Eight HER2-positive grade 1 carcinomas from other years were also studied. HER2-positive carcinomas more often had marked nuclear pleomorphism (50 versus 6%) and were more often oestrogen receptor-negative (17 versus 0.8%) and progesterone receptor-negative (28 versus 8%) compared with HER2-negative grade 1 carcinomas. Six carcinomas that were HER2 3+ in the core biopsy were also 3+ on repeat assessment. Five of seven carcinomas that were 2+ amplified in the core biopsy were also HER2-positive in the excision. Conclusions:<bold> </bold>HER2-positive grade 1 carcinomas are uncommon, and more often have marked nuclear pleomorphism and lack oestrogen receptor and progesterone receptor expression compared with HER2-negative grade 1 carcinomas. A HER2-poitive result in the core biopsy was confirmed in 11 of 13 tumours that had repeat testing.