IMPORT LOW

Clinical question: In selected low-risk early breast cancer after breast conservation, can reduced-volume or partial-breast irradiation preserve local control compared with whole-breast irradiation?

Registry / identifier: ISRCTN12852634 · One underlying study; aliases and follow-up publications consolidated.

Phase III · randomized · non-inferiorityEvidence cut-off: 8 November 2026

Design, population and eligibility

Design

UK multicentre, open-label, randomized phase III non-inferiority trial.

Sample size

2,018 recruited; 2,016 in the intention-to-treat analysis: 674 whole-breast, 673 reduced-dose, 669 partial-breast.

Eligibility

Women aged ≥50 years after breast-conserving surgery for unifocal invasive adenocarcinoma (excluding classical lobular carcinoma), pT1–2 ≤3 cm, pN0–1 (0–3 nodes), grades 1–3, margins ≥2 mm; no neoadjuvant or concurrent adjuvant chemotherapy.

Follow-up / maturity

Median follow-up approximately 120 months; prespecified 10-year outcome report published 2025.

Treatment and comparator

Intervention: Reduced-dose arm: 36 Gy/15 fractions to whole breast plus 40 Gy/15 fractions to partial breast. Partial-breast arm: 40 Gy/15 fractions to partial breast only.

Comparator: Whole-breast radiotherapy 40 Gy in 15 fractions.

Primary endpoint: Ipsilateral breast tumour recurrence (IBTR); five-year formal non-inferiority analysis.

Key efficacy outcomes

10-year IBTR: 2.8% whole breast; 1.9% reduced dose; 3.0% partial breast Cumulative incidence.

10-year absolute difference vs whole breast: −1.02% reduced dose (95% CI −1.98 to 0.99); +0.16% partial breast (95% CI −1.28 to 2.89) Ten-year estimation; not a new formal NI test.

10-year moderate/marked breast shrinkage: 9%, 9%, 7% Whole, reduced-dose, partial-breast groups.

Both experimental groups had a 10-year IBTR incidence around 2–3%. Formal non-inferiority was established in the original five-year analysis. The ten-year analysis supports persistence of low recurrence but should not be relabelled as a new formal NI test.

Absolute outcome visualization

Ten-year cumulative ipsilateral breast tumour recurrence.

Whole breast

2.8%

Reduced dose

1.9%

Partial breast

3.0%

Bars show the source-reported endpoint values; they are not adjusted or reconstructed from median survival. For single-arm studies, no comparative effect is implied.

NNT / NNH and absolute benefit–harm balance

No NNT is appropriate: the trial was designed for non-inferiority of local control, not superiority. The 10-year absolute differences are close to zero and confidence intervals include no difference.

No NNH is calculated. Ten-year clinician-reported moderate/marked breast shrinkage was 9%, 9% and 7%; other event-specific values should be taken from the corrected report.

Toxicity, grade 3–5 harms and treatment burden

At 10 years, clinician-reported moderate/marked breast shrinkage was 9% in whole-breast, 9% in reduced-dose and 7% in partial-breast groups. Other adverse effects were generally low and similar. The 2026 PubMed erratum is indexed; affected secondary table values are withheld pending full-text correction review.

All regimens used 15 daily fractions over 3 weeks. Partial-breast treatment reduces treated volume, not the number of fractions in this protocol.

Quality of life and patient-reported outcomes

Patient-reported outcomes were collected through five years but not at ten years; long-term patient-reported QoL cannot be inferred from clinician assessments.

Subgroups and applicability

Applicability is strongest in women ≥50 with unifocal invasive ductal disease, small tumours, clear margins and predominantly node-negative disease. Only 3% had node-positive disease.

Critical appraisal and limitations

Low-risk selected population; no chemotherapy; open-label; very low recurrence rates; 10-year absolute differences are estimates. A 2026 erratum exists and requires full correction review before secondary table extraction.

Clinical interpretation

IMPORT LOW supports partial-breast or reduced-dose irradiation for appropriately selected low-risk patients. Do not generalize to higher-risk, multifocal, lobular or chemotherapy-treated populations without separate evidence.

Contemporary-practice note (8 November 2026): This page separates the historical randomized or prospective study result from current treatment selection. A full guideline-level freshness review has not been completed for this batch; local decisions should be checked against current disease-specific guidance and multidisciplinary context.

Primary sources and related publications

All numerical claims above are linked to the primary publication or explicitly identified follow-up / meeting abstract. Missing data are not imputed.

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