The Oligopro-Breast trial is a Phase II study targeting women with ER+/HER2- metastatic breast cancer who have been on endocrine therapy and/or CDK4/6 inhibitors for at least 6 months, and show progressive disease at 1-3 extracranial metastases, which are treatable locally. The trial aims to investigate if treating these resistant metastases with SBRT (or other local treatments if SBRT is not possible) can extend the use of the current systemic therapy.

Patients will continue their existing systemic treatment while receiving SBRT on all progressive lesions. If new oligoprogression occurs, SBRT will be performed again. A new systemic treatment line will start if there is polyprogression (more than 3 lesions at once), progression of more than 6 lesions over 12 months, intracranial progression, or lesions that cannot be treated locally.

The scientific question is whether local treatment of resistant metastases can prolong the effectiveness of ongoing systemic therapy, which is particularly beneficial if the treatment is well-tolerated. The primary objective is to measure the proportion of patients surviving without changing their systemic treatment line at 6 months after SBRT.

This trial is significant for patients as it explores a method to potentially extend the duration of effective and well-tolerated treatments, offering hope for better management of metastatic breast cancer.

Inclusion Criteria:

• ECOG performance status 0-2.
• Histologically confirmed ER+/HER2- MBC.
• History of polymetastatic disease. Patients with genuine oligometastatic disease and 1-3 oligoprogressive lesions are only allowed if ablative therapy of all metastases is deemed impossible.
• Under 1st to 2nd systemic treatment line with hormonotherapy and/or CDK4/6 inhibitors for at least 6 months before progression.
• Progressive disease at 1-3 extracranial sites.
• Ability to treat all progressive lesions locally according to the treating radiation oncologist.

Exclusion Criteria:

• Second malignancy if it is not in complete remission.
• Previous local treatment for oligoprogression under the current systemic treatment line
• Current progression in a lesion that has been treated with SBRT before and is not amendable for surgery or radiofrequency ablation (RFA).
• Progressive or newly diagnosed brain metastases. Known brain metastases that have been nonprogressive for at least 6 months, are not an exclusion criterion.
• Inability to continue the same ST line after local therapy (for example because of toxicity or patient refusal).
• Pregnancy.
• Inability to sign the informed consent.