HER2‐positive grade 1 invasive carcinomas of the breast

AimsThe 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 resultsA 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.ConclusionsHER2‐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.