What are the American Society for Radiation Oncology (ASTRO) guidelines for Accelerated Partial Breast Irradiation (APBI) in early-stage breast cancer?

ASTRO Guidelines for Accelerated Partial Breast Irradiation in Early-Stage Breast Cancer

According to the updated 2016 ASTRO consensus statement, APBI is an acceptable treatment option for patients with low risk for local recurrence, specifically those who are at least 50 years old with invasive ductal carcinoma measuring ≤2 cm (T1 disease) with negative margins by ≥2 mm, no lymphovascular invasion, hormone receptor-positive, and BRCA-negative, or those with low to intermediate grade screen-detected ductal carcinoma in situ measuring ≤2.5 cm with negative margins by ≥3 mm. 1

Patient Selection Criteria for APBI

The ASTRO guidelines categorize patients into groups based on their suitability for APBI:

Suitable Candidates (APBI acceptable outside clinical trials):

• Age ≥50 years
• Invasive ductal carcinoma ≤2 cm (T1 disease)
• Negative margins ≥2 mm
• No lymphovascular space invasion (LVSI)
• Hormone receptor-positive
• BRCA-negative
• OR low to intermediate grade screen-detected DCIS ≤2.5 cm with negative margins ≥3 mm

Additional Low-Risk Features (from ESTRO consensus):

• Luminal-like subtypes
• Non-lobular invasive carcinoma
• Tumor grade 1-2
• Unicentric or unifocal lesion
• Node negative (including isolated tumor cells)
• No preoperative systemic therapy 1

APBI Techniques and Dosing

Several techniques can be used for APBI delivery:

• External beam radiation therapy
• Brachytherapy (most commonly used in early studies)
• Endocavitary techniques
• Intraoperative radiation therapy

Recommended Dose and Schedule:

• 34 Gy in 10 fractions delivered twice per day with brachytherapy
• 38.5 Gy in 10 fractions delivered twice per day with external-beam RT 1
• Other fractionation schemes are under investigation

Efficacy and Outcomes

Studies have shown that APBI provides excellent results with low local recurrence rates equivalent to whole breast radiation therapy (WBRT) in appropriately selected patients 1. The rationale for APBI is that the majority of local failures occur in the vicinity of the primary tumor site, while "elsewhere" in-breast failures may represent new primary tumors 1.

However, it's important to note that intraoperative RT techniques (ELIOT and TARGIT trials) showed significantly higher ipsilateral breast cancer recurrence rates compared to WBRT 1.

Cautions and Contraindications

APBI is generally not recommended for:

• Patients younger than 50 years
• Tumors >3 cm
• Positive margins
• Presence of lymphovascular invasion
• Lobular histology
• BRCA mutation carriers
• Multifocal/multicentric disease
• Node-positive disease
• Patients who received preoperative systemic therapy

Clinical Implementation

When implementing APBI, clinicians should:

1. Carefully evaluate patient characteristics against the ASTRO suitability criteria
2. Consider technical factors when planning treatment:
   • Ensure full coverage of the target volume
   • Limit breast V20 to <45% to improve tolerability 2
   • Monitor for skin toxicity, which is generally mild but can occur

Comparison to Whole Breast Irradiation

Patients who choose APBI should be informed that:

• WBRT is an established treatment with a longer track record
• APBI offers the advantage of shorter overall treatment time
• APBI may result in fewer late skin side effects in appropriately selected patients 1
• APBI appears to have comparable efficacy to WBRT in suitable patients, though long-term data continue to accumulate

APBI represents an important option in the trend toward treatment de-escalation for early-stage breast cancer, allowing for targeted therapy with potentially fewer side effects and greater convenience for patients who meet the appropriate selection criteria.