What is the role of Stereotactic Body Radiation Therapy (SBRT) in treating oligometastatic breast cancer?

SBRT in Oligometastatic Breast Cancer

SBRT may be offered to patients with oligometastatic breast cancer (≤5 metastases) following systemic therapy, particularly for those with hormone receptor-positive disease, non-visceral metastases, and controlled primary tumors, though the evidence shows more modest benefits compared to other cancer types. 1

Definition and Patient Selection Criteria

The consensus definition of oligometastatic disease includes:

• One to five metastatic lesions 1
• Preferably a controlled primary tumor 1
• All metastatic sites must be safely treatable 1

Evidence for SBRT in Oligometastatic Breast Cancer

Efficacy Data

The most recent randomized controlled trial (CURB trial, 2024) showed no significant progression-free survival benefit for SBRT in oligoprogressive breast cancer, with median PFS of 4.4 months in the SBRT group versus 4.2 months in standard-of-care alone (HR 0.78, p=0.43). 2 This contrasts sharply with the dramatic benefit seen in NSCLC patients in the same trial (10.0 vs 2.2 months, HR 0.41, p=0.0039). 2

However, more recent real-world data (2025) from patients with oligoprogressive disease shows more promising results:

• Median post-radiotherapy PFS of 11.3 months (95% CI, 9.1-13.5) 3
• Median time to next systemic treatment of 13.6 months (95% CI, 11.5-15.2) 3
• Only 15% experienced subsequent progression in SBRT-treated sites 3

Early prospective data (2008) in oligometastatic disease demonstrated:

• 2-year overall survival of 50% and 4-year OS of 28% 4
• Patients with breast cancer had significantly better outcomes for OS, PFS, local control, and distant control compared to other primary sites 4
• 29 of 45 patients alive at last follow-up had no evidence of disease, including 23 with ≥2 years follow-up 4

Optimal Patient Characteristics

Patients most likely to benefit from SBRT have:

• Hormone receptor-positive/HER2-negative disease (77% of successful cases) 3
• ≤2 oligometastatic lesions (90% of successful cases) 3
• Non-visceral metastases involving bones or lymph nodes (91% of successful cases) 3
• Durable pre-oligoprogression PFS ≥11 months on systemic therapy 3
• Lower net tumor volume (significantly predicts better outcomes) 4

Safety Profile

Toxicity Rates

Grade 2 or worse adverse events occurred in 62% of patients receiving SBRT plus standard of care versus 41% with standard of care alone. 2

SBRT-related grade 2 or worse toxicities occurred in 16% of patients, including:

• Gastrointestinal reflux disease 2
• Pain exacerbation 2
• Radiation pneumonitis 2
• Brachial plexopathy 2
• Low blood counts 2

Critical safety concern: 37.5% of patients receiving SBRT concurrently with CDK4/6 inhibitors experienced grade 3-5 toxicities, suggesting this combination requires careful consideration. 5

Most toxicities across studies were grade 1-2 (62.5% of 40 reported events). 5

Treatment Approach Algorithm

Step 1: Confirm Oligometastatic Status

• Document ≤5 metastatic lesions on PET-CT or CT 2
• Verify controlled primary tumor 1
• Ensure all sites are safely treatable 1

Step 2: Assess Favorable Prognostic Features

Proceed with SBRT if patient has:

• HR+/HER2- disease 3
• Non-visceral metastases (bone/lymph nodes) 3
• ≤2 metastatic sites 3
• Prior PFS on systemic therapy ≥11 months 3
• Low net tumor volume 4

Exercise caution if patient has:

• Triple-negative or HER2+ disease (less evidence) 2
• Visceral metastases 3

• 2 metastatic sites 3

• Rapid progression on prior systemic therapy 3

Step 3: Multidisciplinary Team Review

Required expertise includes:

• Medical oncology 1
• Radiation oncology 1
• Surgical oncology (if applicable) 1
• Radiology 1

Step 4: Treatment Delivery

SBRT dosing varies by location:

• Most patients treated with 10 fractions of 5 Gy 4
• Doses ranging from 24-60 Gy in 1-10 fractions based on lesion location/size 5
• Avoid concurrent CDK4/6 inhibitors due to high grade 3-5 toxicity rates 5

Step 5: Continue Systemic Therapy

Maintain the same systemic treatment that was controlling disease prior to oligoprogression, as this strategy achieved median time to next systemic treatment of 13.6 months. 3

Critical Pitfalls to Avoid

Do not offer SBRT to patients with:

• Widespread polymetastatic disease (>5 lesions) 1
• Uncontrolled primary tumor 1
• Sites that cannot be safely treated 1
• Rapidly progressive disease on current systemic therapy 3

Do not combine SBRT with CDK4/6 inhibitors without careful risk-benefit discussion, given 37.5% grade 3-5 toxicity rate. 5

Do not expect the same dramatic benefits seen in oligometastatic NSCLC, as the CURB trial showed no PFS benefit in the oligoprogressive breast cancer cohort. 2

Nuances in the Evidence

The divergence between the CURB trial (2024) showing no benefit 2 and the 2025 real-world cohort showing substantial benefit 3 likely reflects different patient populations: the CURB trial included more aggressive disease subtypes (34% triple-negative), while the successful 2025 cohort was 77% HR+/HER2- with predominantly non-visceral disease. This underscores the critical importance of patient selection.