What are the indications for a boost in breast radiotherapy?

Indications for Boost in Breast Radiotherapy

A radiation boost to the tumor bed after whole breast irradiation should be strongly considered for patients with high-risk features including age <50 years, positive or close margins (<2mm), lymphovascular invasion, positive axillary nodes, or high-grade disease, as these factors significantly increase local recurrence risk despite whole breast radiation. 1

Mandatory Boost Indications (High-Risk Group)

The boost is considered mandatory in the following situations:

• Age ≤40 years with close margins, extensive intraductal component, or triple-negative phenotype 2
• Positive resection margins regardless of patient age 1, 2
• Patients with 4% local recurrence rate with boost vs 13% without boost when margins are positive 1

For these high-risk patients, dose escalation above 16 Gy EQD2 should be considered, with typical boost doses of 10-16 Gy in 4-8 fractions. 1, 2

Strong Consideration for Boost (Intermediate-Risk Group)

Boost (10-16 Gy) is strongly recommended for:

• Age 40-50 years regardless of other risk factors 2
• Age >50 years with any of the following risk factors: 1, 2
  • Close margins (<2mm)
  • Tumor size >3 cm
  • Extensive intraductal component
  • Lymphovascular invasion
  • Positive lymph nodes
  • Multicentric or multifocal tumors
  • Triple-negative phenotype
  • High-grade disease (Grade III)
  • Residual tumor after neoadjuvant chemotherapy

The absolute benefit of boost is highest in younger patients, though the relative reduction in local recurrence is similar across all age groups. 1

Optional Boost (Low-Risk Group)

Boost may be omitted in highly selected low-risk patients:

• Age ≥50-60 years with all of the following: 2, 3
  • Unicentric, unifocal disease
  • Clear surgical margins ≥2mm
  • No axillary lymph node involvement
  • No lymphovascular invasion
  • No high-grade features

In this low-risk group, the 20-year cumulative incidence of local recurrence was 16.4% without boost versus 12.0% with boost, representing a modest absolute benefit. 3

Critical Caveats

Important considerations when deciding on boost:

• Young age (<45 years) remains a powerful predictor of local recurrence even with aggressive dose escalation, with relative risk increases of 11.1-17.4 fold compared to older patients. 4
• Boost increases the risk of moderate to severe fibrosis (5.2% at 20 years with boost vs 1.8% without boost), which must be balanced against local control benefits. 3
• Patients with close/positive margins plus age <45 years, Grade III, lymphovascular invasion, or ≥4 positive nodes have >10% 5-year local recurrence despite whole breast plus boost radiation and should be considered for more definitive surgery rather than relying on radiation alone. 5
• The boost does NOT improve overall survival, only local control, so the decision must weigh cosmetic outcomes and quality of life against the risk of local recurrence requiring salvage mastectomy. 3

Boost Delivery Techniques

Boost can be delivered using: 1

• Enface electrons
• Photons
• Brachytherapy

All three modalities are acceptable, with choice based on institutional expertise and tumor bed characteristics. 1