AVATAR: This trial is trying to understand if radiotherapy could work as an effective treatment for patients with breast cancers that have had partial response to prior treatmentStereotactic Radiotherapy for Oligoprogressive ER-positive Breast Cancer

Inclusion

1. Male or female, 18 years of age or older
2. Patients with histologically proven ER-positive, HER2-negative advanced breast cancer receiving an AI in combination with a CDK 4/6 inhibitor. Biopsy of metastatic disease if technically feasible but not mandatory.
3. Patients must have evidence of extracranial metastatic disease with or without intracranial metastases.
4. Radiological evidence of stable or responding disease to an AI in combination with a CDK 4/6 inhibitor for a period of at least 6 months prior to study entry. (Patient must have ongoing stability/response in at least one lesion at the time of registration).
5. Evidence of new or existing OPD in 1-5 lesions. With reference to RECIST 1.132 and or PERCIST 1.033, OPD is defined as follow:
a. Using CT for intracranial or extracranial OPD:
i. greater than 5 mm increase in the diameter of an existing lesion OR
ii. more than 20% increase in the diameter of an existing lesion on 2 consecutive imaging studies at least 2 months apart OR
iii. The appearance of one or more new soft tissue lesions, measuring more than 5 mm.
b. Using Positron Emission Tomography for extracranial OPD:
i. greater than 30% increase in 18F-FDG SUV peak, with more than 0.8 SUV units increase in tumour SUV from the baseline scan in pattern typical of tumour and not of infection/treatment effect OR
ii. New 18F-FDG avid lesions typical of cancer (including new bone lesion) and not related to treatment effect and/or infection.
6. For patients with liver or lung metastases, maximum of 3 oligoprogressive lesions in single organ.
7. All OPD must be amenable to SRT.
a. Patients with risk of bone fracture are not candidate for SRT
8. For patients with intracranial metastases, SRT is the preferred treatment option, however, if clinical assessment indicated that upfront surgery is better, then post-operative SRT to the surgical cavity is recommended.
9. ECOG performance status 0-2.
10. Life expectancy of greater than 6 months.
11. Provision of written informed consent.
12. Clinician and participant are willing to continue current line of therapy.