Correlation of tumour subtype with long-term outcome in small breast carcinomas: a Swedish population-based retrospective cohort study

PURPOSE: To investigate if molecular subtype is associated with outcome in stage 1 breast cancer (BC). METHODS: Tissue samples from 445 women with node-negative BC ≤ 15 mm, treated in 1986–2004, were classified into surrogate molecular subtypes [Luminal A-like, Luminal B-like (HER2−), HER2-positive, and triple negative breast cancer (TNBC)]. Information on treatment, recurrences, and survival were gathered from medical records. RESULTS: Tumour subtype was not associated with overall survival (OS). Luminal B-like (HER2−) and TNBC were associated with higher incidence of distant metastasis at 20 years (Hazard ratio (HR) 2.26; 95% CI 1.08–4.75 and HR 3.24; 95% CI 1.17–9.00, respectively). Luminal B-like (HER2−) and TNBC patients also had worse breast cancer-specific survival (BCSS), although not statistically significant (HR 1.53; 95% CI 0.70–3.33 and HR 1.89; 95% CI 0.60–5.93, respectively). HER2-positive BC was not associated with poor outcome despite no patient receiving HER2-targeted therapy, with most of these tumours being ER+. CONCLUSIONS: Stage 1 TNBC or Luminal B-like (HER2−) tumours behave more aggressively. Women with HER2+/ER+ tumours do not have an increased risk of distant metastasis or death, absent targeted treatment. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10549-022-06691-4.