What are the American Society of Clinical Oncology (ASCO) guidelines, updated ASCO guidelines, and Gynecologic European Cancer Study Group (GEC-ESTRO) guidelines for Accelerated Partial Breast Irradiation (APBI) in patients with early-stage breast cancer, and what were the findings of the RAPID trial?

Accelerated Partial Breast Irradiation (APBI) Guidelines

ASCO/ASTRO Guidelines for APBI

APBI is an acceptable treatment option for carefully selected low-risk early-stage breast cancer patients, but should not be considered equivalent to whole-breast irradiation for all patients. The most recent high-quality evidence from the NSABP B-39/RTOG 0413 trial failed to demonstrate equivalence between APBI and whole-breast irradiation, though the absolute difference in local recurrence was small 1.

Patient Selection Criteria

The strictest and most appropriate candidates for APBI include 2, 3:

• Age ≥60 years (NCCN/ASTRO criteria) or ≥50 years (broader criteria)
• Unifocal, unicentric disease only
• Stage I disease (tumor ≤3 cm)
• Node-negative (including isolated tumor cells)
• ER-positive tumors
• Infiltrating ductal carcinoma or favorable histology (non-lobular)
• No extensive intraductal component
• No lymphovascular space invasion
• Negative surgical margins (≥2 mm)
• Not BRCA1/2 mutation carriers
• No prior neoadjuvant systemic therapy

ASCO/ASTRO Recommended Dosing Regimens

Two specific regimens are endorsed 2, 3:

Brachytherapy:

• 34 Gy in 10 fractions delivered twice daily (minimum 6 hours apart)

External Beam Radiation Therapy:

• 38.5 Gy in 10 fractions delivered twice daily (minimum 6 hours apart)

Updated ASCO/ESMO Guidelines (2024)

The most recent ESMO 2024 guidelines state that APBI produces excellent results equivalent to whole-breast irradiation for well-selected patients with low-risk disease according to ESTRO consensus recommendations 2.

Expanded Low-Risk Criteria (ESMO 2024)

The 2024 ESMO guidelines provide broader selection criteria 2:

• Age ≥50 years (expanded from ≥60)
• Luminal-like subtypes
• Tumor ≤3 cm
• Grade 1-2 tumors
• No lymphovascular space invasion
• Non-lobular invasive carcinoma
• Clear surgical margins >2 mm
• Node-negative (including isolated tumor cells)
• Unicentric or unifocal lesion
• No use of primary systemic therapy

For DCIS: low-to-intermediate grade, sized ≤2.5 cm with clear surgical margins ≥3 mm 2.

Acceptable APBI Techniques (ESMO 2024)

Any technique allowing full coverage of the entire target volume is suitable, including 2:

• External beam radiation therapy
• Brachytherapy
• Endocavitary techniques
• Intraoperative radiation therapy with electron techniques

GEC-ESTRO Guidelines

The GEC-ESTRO (Gynecologic European Cancer Study Group - European Society for Radiotherapy and Oncology) consensus recommendations form the basis for the ESMO 2024 APBI criteria 2. The ESTRO consensus emphasizes that APBI should only be used in carefully selected low-risk patients meeting the criteria outlined above, with particular attention to tumor biology (luminal-like subtypes) and technical delivery ensuring complete target volume coverage.

RAPID Trial Findings

The OCOG-RAPID trial demonstrated non-inferiority of APBI compared to whole-breast irradiation for local control, but with worse cosmetic outcomes and increased late toxicity 4.

Key RAPID Trial Results

• 8-year ipsilateral breast tumor recurrence: 3.0% (APBI) vs 2.8% (whole-breast irradiation), HR 1.27 (90% CI: 0.84-1.91) 4
• APBI regimen used: 38.5 Gy in 10 fractions delivered twice daily (minimum 6 hours apart) using external beam radiation therapy 4
• Acute toxicity: Reduced with APBI 4
• Late toxicity: Worse with APBI 4
• Cosmetic outcomes: Worse with APBI 4

Critical Caveat from NSABP B-39/RTOG 0413

The larger NSABP B-39/RTOG 0413 trial with broader eligibility criteria failed to demonstrate equivalence between APBI and whole-breast irradiation 1. At 10-year follow-up:

• 10-year cumulative IBTR: 4.6% (APBI) vs 3.9% (whole-breast irradiation), HR 1.22 (90% CI: 0.94-1.58) 1
• Did not meet pre-specified equivalence criteria 1
• Absolute difference <1%, suggesting APBI may be acceptable for some women despite not meeting statistical equivalence 1

Important Clinical Caveats

APBI remains considered investigational by some guidelines, and patients should be encouraged to participate in prospective clinical trials when possible 2, 3.

Intraoperative radiation therapy techniques have shown significantly higher ipsilateral breast cancer recurrence rates compared to whole-breast radiation therapy and should not be offered outside clinical trials 5.

The ASTRO stratification guidelines may not adequately predict ipsilateral breast tumor recurrences following APBI 2.

The strongest evidence supports interstitial brachytherapy and intensity-modulated radiation therapy APBI, with moderate evidence for applicator brachytherapy or three-dimensional conformal radiotherapy APBI 5.

Follow-up data remains limited for some APBI techniques, and adherence to strict patient selection criteria is essential to achieve outcomes comparable to whole-breast irradiation 2, 3, 4.