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

Abstract

Abstract Background Neoadjuvant endocrine therapy (NET) in oestrogen receptor positive (ER+) HER2 negative (HER2-) breast cancer allows real-time evaluation of drug efficacy as well as investigation of the biological and molecular changes that occur after estrogenic deprivation. Clinical and pathological evaluation after neoadjuvant endocrine therapy may be used to obtain prognostic and predictive information of tumour response to decide adjuvant treatment. In this setting, clinical scales developed to evaluate response after neoadjuvant chemotherapy are not useful and there are not many validated biomarkers to assess response to neoadjuvant endocrine therapy beyond Ki67 levels and preoperative endocrine prognostic index score. Methods In this prospective study, we extensively analysed radiological (by ultrasound (USS) and magnetic resonance imaging (MRI) and pathological tumour response of 104 postmenopausal patients with ER+/HER2- resectable breast cancer, treated with neoadjuvant endocrine treatment for a mean of 7 months prior to surgery. We defined a new score, tumour cellularity size (TCS), calculated as the product of the residual tumour cellularity in the surgical specimen and the tumour pathological size. Results Our results show that radiological evaluation by both USS and MRI underestimates pathological tumour size, although they support the use of MRI over USS to clinically assess tumour response. In addition, we propose that the tumour cellularity size could become a new tool to standardize response assessment to NET given its simplicity, reproducibility and its good correlation with existing biomarkers and potential added value. Conclusion Our findings shed light on the dynamics of tumour response to neoadjuvant endocrine therapy, challenge the paradigm of the ability of NET to decrease surgical volume and point to the utility of the tumour cellularity size to quantify the scattered tumour response usually produced by endocrine therapy.