AVATAR II: Testing whether adding radiation therapy to ongoing treatment can delay the need to switch to a new type of treatment for people with advanced ER+, HER2- breast cancer that has become oligoprogressiveStereotactic Ablative Body Radiotherapy (SABR) with Maintenance of Systemic Therapy Versus Physicians' Choice of Systemic Therapy for Oligoprogressive ER-positive, Her-2 Negative Breast Cancer II

Inclusion

Patients will be eligible for inclusion in this trial if all the following criteria apply:

1. Patient has signed the AVATAR-II Patient Information and Consent Form (PICF)
2. Male or female, ≥ 18 years of age at the time signing consent
3. Patients with histologically proven ER-positive, HER2-negative advanced breast cancer receiving an ET in combination with a CDK 4/6 inhibitor. Biopsy of metastatic disease if technically feasible but not mandatory
4. Patients must have evidence of extracranial metastatic disease, with no evidence of uncontrolled intracranial metastases. (Controlled intracranial metastases are defined as stable disease on repeat CT imaging performed at least one month apart.)

5. Patients must have evidence of radiological response to ET and CDK 4/6 inhibitor for a minimum of six months prior to randomisation (defined as either stable disease or partial response).

   Note: Patient must have ongoing stability/response in at least one lesion at the time of randomisation.

6. Evidence of new or existing OPD, as determined by the Investigator and defined according to RECIST1.1 (33), via CT on a per-lesion basis (between 1-5 metastases, including the primary) as follows:

   • At least a 20% increase in the diameter of a lesion, taking as reference the smallest diameter on a previous CT scan with an absolute increase of at least 5mm
   • Appearance of a new lesion(s) Note: A new lesion can be identified using various imaging modalities, such as PET-CT or WBBS, as long as the lesion is visible on serial CT scans.
7. For patients with liver or lung metastases, maximum of 3 oligoprogressive lesions in single organ
8. All OPD must be amenable to SABR, as per the radiotherapy guidelines in section 11.1 and Appendix 4 and 5
9. ECOG performance status 0-2
10. Life expectancy ≥ 6 months
11. Clinician and patient are willing to continue current line of therapy
12. Patient is able to complete QoL questionnaires, and other assessments required as part of the study