Background
Phase III, randomized non-inferiority trial. 2018 patients with low-risk early breast cancer (T1-3a pN0 M0, age ≥50) who had undergone BCS with clear margins. Three-arm design testing partial breast irradiation (APBI) or reduced-dose whole-breast irradiation vs standard whole-breast irradiation, all using 15-fraction schedules. IMPORT LOW addressed whether accelerated partial breast irradiation to the tumor bed alone could replace whole-breast irradiation.
Interventions and follow up
Arm A: Standard WBI 40Gy in 15 fractions (control)
Arm B: Reduced-dose WBI 36Gy in 15 fractions + tumor bed boost 40Gy/15f
Arm C: APBI 40Gy in 15 fractions to partial breast (tumor bed + 1cm margin) only
Primary endpoint: Ipsilateral breast tumor relapse (IBTR) at 5 years (non-inferiority vs WBI)
mFollow up: 72.2 months
Arm B: Reduced-dose WBI 36Gy in 15 fractions + tumor bed boost 40Gy/15f
Arm C: APBI 40Gy in 15 fractions to partial breast (tumor bed + 1cm margin) only
Primary endpoint: Ipsilateral breast tumor relapse (IBTR) at 5 years (non-inferiority vs WBI)
mFollow up: 72.2 months
Results
IBTR (5-yr, APBI vs WBI): 0.9% vs 2.1% — non-inferior (absolute difference -1.2%, 90% CI -2.3 to -0.2%)
IBTR (5-yr, reduced-dose WBI vs WBI): 1.1% vs 2.1% — non-inferior
Moderate/marked induration (5-yr): 7.4% (APBI) vs 8.0% (WBI) — similar
IBTR (5-yr, reduced-dose WBI vs WBI): 1.1% vs 2.1% — non-inferior
Moderate/marked induration (5-yr): 7.4% (APBI) vs 8.0% (WBI) — similar
Adverse events
Breast tissue effects: Physician-assessed moderate/marked breast induration similar across all arms (~7–8%) at 5 years; breast edema and telangiectasia similar between arms.
Cosmesis (patient-reported): APBI arm had slightly better breast appearance scores than WBI.
Other: No increase in contralateral breast cancer or distant relapse with APBI.
Cosmesis (patient-reported): APBI arm had slightly better breast appearance scores than WBI.
Other: No increase in contralateral breast cancer or distant relapse with APBI.
Conclusions
APBI (40Gy/15f to partial breast) was non-inferior to whole-breast irradiation for IBTR in low-risk early BC patients after BCS, with similar or better normal tissue outcomes. IMPORT LOW established partial breast irradiation as a safe alternative to WBI in appropriately selected low-risk patients.
Key Limitations
Low overall recurrence rates in the WBI control arm (2.1% at 5 years) mean the non-inferiority margin operates at absolute risk levels of <2% — very small absolute differences. Limited to low-risk patients (pN0, T1-3a) — results cannot be extrapolated to node-positive, large-tumor, or high-risk disease. 5-year follow-up may underestimate late recurrence (HR+ tumors can relapse beyond 10 years). The 15-fraction once-daily APBI schedule differs from BID schedules used in RAPID, limiting direct comparison.
Clinical Context
IMPORT LOW supports partial breast irradiation as an option in low-risk early BC, complementing RAPID and other APBI trials. ASTRO and ESMO guidelines endorse APBI in selected low-risk patients (e.g., age ≥50, invasive ductal carcinoma, pT1 pN0, ER+, no extensive intraductal component, clear margins). Not all patients need whole-breast RT — this trial is pivotal for individualized RT decisions. ESMO-MCBS: not applicable (RT trial).
References