Cookies on this website

We use cookies to ensure that we give you the best experience on our website. If you click 'Accept all cookies' we'll assume that you are happy to receive all cookies and you won't see this message again. If you click 'Reject all non-essential cookies' only necessary cookies providing core functionality such as security, network management, and accessibility will be enabled. Click 'Find out more' for information on how to change your cookie settings.

EndoNET Trial logo

About EndoNET

Welcome to EndoNET, a clinical trial designed to understand if a period of treatment with hormone therapy prior to surgery reduces the requirement for mastectomy and improves quality of life in post-menopausal women. 

EndoNET was identified as a research priority by NICE for post-menopausal women with early breast cancer and the trial is driven by our patient advocates including Independent Cancer Patients’ Voice

Trial details

EndoNET is a Phase III, multi-centre trial that evaluates whether endocrine treatment that prevents the production of oestrogens, when taken for 6 months prior to surgery, will shrink the tumour allowing for a reduction in the extent of surgery. 

To be considered for the study, patients must be:

  • Post-menopausal
  • Strongly ER+
  • HER2-
  • Tumour ≥ 15 mm.

Participants will either be randomised to surgery at 2-4 weeks (up to 8 weeks permitted) or 6 months (+/- 1 month). Participants will complete quality of life questionnaires at intervals during their 15-month participation and rates of breast conservation surgery will be documented.


Eligible patients will either be randomised to either surgery at 2-4 weeks (up to 8 weeks permitted) or 6 months (+/- 1 month). Participants will start their endocrine treatment (aromatase inhibitors: letrozole, anastrozole or exemestane) after being randomised:

Endocrine treatment arm 1 and 2

The endocrine treatment offered in both arms is the same but the timing of surgery differs. This is to see if a longer duration of endocrine treatment before surgery will shrink the tumour to allow for less extensive surgery.

Trial Schema

EndoNET Trial Schema

Prospective Sites

If you are a UK hospital who would be interested in joining the 30+ centres taking part in EndoNET, please contact: to find out more information and request a feasibility form. We are part of the NIHR Associate PI scheme and are on the NIHR Portfolio.

Further Information

EndoNET is funded by the National Institute for Health Research HTA Programme. 

Visit us on the Association for Breast Surgery website