A prospective study on tumour response assessment methods after neoadjuvant endocrine therapy in early oestrogen receptor positive breast cancer

Neoadjuvant endocrine therapy (NET) in oestrogen receptor-positive/HER2-negative breast cancer (ER+/HER2-BC) allows real-time evaluation of drug efficacy and biological changes upon estrogenic deprivation. Clinical and pathological evaluation after NET may be used to obtain prognostic and predictive information of tumour response. Scales developed to evaluate response after neoadjuvant chemotherapy are not useful and there are not many validated biomarkers to assess response to NET. In this prospective study, we analysed radiological and pathological tumour response of 104 postmenopausal ER+/HER2-BC patients, treated with NET for a mean of 7 months. Our results show that radiological evaluation underestimates pathological tumour size, although it can be used to assess tumour response. In addition, we propose that the tumour cellularity size (TCS), calculated as the product of the residual tumour cellularity in the surgical specimen and the tumour pathological size, could become a new tool to standardize response assessment to NET. It is simple, reproducible and correlates with the existing biomarkers. Our findings shed light on the dynamics of NET response, challenge the paradigm of the ability of NET to decrease surgical volume and point to the utility of the TCS to quantify the scattered tumour response usually produced by endocrine therapy.