Relationship between ER expression by IHC or mRNA with Ki67 response to aromatase inhibition: a POETIC study

BACKGROUND: In clinical practice, oestrogen receptor (ER) analysis is almost entirely by immunohistochemistry (IHC). ASCO/CAP recommends cut-offs of < 1% (negative) and 1–10% (low) cells positive. There is uncertainty whether patients with ER low tumours benefit from endocrine therapy. We aimed to assess IHC and mRNA cut-points for ER versus biological response of primary breast cancer to 2 weeks’ aromatase inhibitor treatment as measured by change in Ki67. METHODS: Cases were selected from the aromatase inhibitor treatment group of POETIC. We selected the 15% with the poorest Ki67 response (PR, < 40% Ki67 suppression, n = 230) and a random 30% of the remainder categorised as intermediate (IR, 40–79% Ki67 suppression, n = 150) and good-responders (GR, ≥ 80% Ki67 suppression, n = 230) from HER2 − group. All HER2 + cases available were selected irrespective of their response category (n = 317). ER expression was measured by IHC and qPCR. RESULTS: ER IHC was available from 515 HER2 − and 186 HER2 + tumours and ER qPCR from 367 HER2 − and 171 HER2 + tumours. Ninety-one percentage of patients with ER IHC < 10% were PRs with similar rates in HER2 − and HER2 + cases. At or above ER IHC 10% substantial numbers of patients showed IR or GR. Similar proportions of patients were defined by cut-points of ER IHC < 10% and ER mRNA < 5 units. In addition, loss of PgR expression altered ER anti-proliferation response with 92% of PgR − cases with ER IHC < 40% being PRs. CONCLUSIONS: There was little responsiveness at IHC < 10% and no distinction between < 1% and 1–10% cells positive. Similar separation of PRs from IR/GRs was achieved by IHC and mRNA. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13058-022-01556-6.