Ten-Year Outcomes of Partial-Breast Radiotherapy After Breast Conservation Surgery: Insights from the UK IMPORT LOW Trial

Highlights

• Ten-year data from the UK IMPORT LOW trial demonstrate non-inferiority of partial-breast and reduced-dose radiotherapy compared to whole-breast radiotherapy for women with low-risk early breast cancer.
• The rates of ipsilateral breast tumour recurrence (IBTR) at 10 years are low and comparable across all treatment arms.
• Long-term adverse effects, including breast shrinkage, are similar or lower for partial-breast and reduced-dose groups compared to whole-breast irradiation.
• Partial-breast radiotherapy with intensity-modulated techniques is reaffirmed as a standard of care for selected patients.

Study Background and Disease Burden

Breast cancer remains the most common malignancy among women worldwide, with early detection and improved treatments resulting in high rates of breast conservation surgery (BCS). Postoperative radiotherapy is a mainstay after BCS to minimize the risk of ipsilateral breast tumour recurrence (IBTR). Conventional whole-breast radiotherapy, while effective, carries risks of radiation-induced toxicity, cosmetic changes, and inconvenience due to prolonged treatment schedules. For women with early-stage, low-risk breast cancer, there is a growing impetus to de-escalate treatment without compromising oncologic outcomes. Partial-breast radiotherapy, especially when delivered with modern intensity-modulated radiotherapy (IMRT), targets only the tumour bed, potentially reducing toxicity and improving patient quality of life. The IMPORT LOW trial addresses a critical clinical question: Can partial-breast or reduced-dose radiotherapy safely replace whole-breast irradiation in this population?

Study Design

The IMPORT LOW trial was a multicentre, open-label, randomised, controlled, phase 3, non-inferiority trial conducted across 30 UK radiotherapy centres. Eligible participants were women aged 50 years or older with unifocal invasive ductal carcinoma (tumour size ≤3 cm, pT1-2, N0-1, grades 1–3) who had undergone breast conservation surgery with margins of ≥2 mm. Patients with previous malignancies (other than non-melanoma skin cancer), prior mastectomy, or prior/concurrent chemotherapy were excluded. A total of 2018 women were randomised (1:1:1) to one of three radiotherapy regimens:

• Whole-breast group: 40 Gy in 15 fractions to the whole breast
• Reduced-dose group: 36 Gy in 15 fractions to the whole breast plus 40 Gy in 15 fractions to the partial breast
• Partial-breast group: 40 Gy in 15 fractions to the partial breast only

Stratification was by treatment centre, and the primary endpoint was the incidence of ipsilateral breast tumour recurrence (IBTR) at 10 years. Secondary outcomes included late adverse effects, particularly breast shrinkage, induration, and cosmetic results.

Key Findings

A total of 2016 participants (after exclusions) were analysed with a median follow-up of 10 years. The median age was 63 years, with the majority having grade 1 or 2 tumours and node-negative disease.

• At 10 years, IBTR occurred in 17 of 674 (2.8%) in the whole-breast group, 11 of 673 (1.9%) in the reduced-dose group, and 17 of 669 (3.0%) in the partial-breast group.
• The cumulative incidence of IBTR was low across all arms, with no statistically significant differences. The absolute difference in 10-year IBTR incidence was -1.02% (95% CI -1.98 to 0.99) for the reduced-dose group and 0.16% (-1.28 to 2.89) for the partial-breast group versus whole-breast group, confirming non-inferiority.
• Adverse effects were infrequent and similar across all groups. The most common moderate or marked late effect was breast shrinkage (whole-breast: 9%, reduced-dose: 9%, partial-breast: 7%). Rates of induration, telangiectasia, and other late toxicities were low and did not significantly differ by group.
• These findings were consistent across subgroups, including tumour grade and nodal status, supporting the generalizability within the defined low-risk cohort.

Expert Commentary

The IMPORT LOW trial provides robust, long-term evidence supporting the safety and efficacy of partial-breast and reduced-dose radiotherapy in appropriately selected women with early-stage, low-risk breast cancer. The trial’s rigorous design, large sample size, and decade-long follow-up strengthen the validity of its conclusions. Notably, the non-inferiority margin for IBTR was met, and late adverse effects were not increased with de-escalated regimens. This aligns with evolving guidelines from the UK, US, and international bodies, which increasingly endorse partial-breast irradiation for suitable patients.

However, it is essential to recognize that these results may not be applicable to higher-risk patients (e.g., younger age, high-grade or multifocal tumours, extensive nodal involvement) who were not included in the study. The open-label design and clinician-reported adverse effect assessments, while pragmatic, introduce some risk of bias, though this is mitigated by objective recurrence endpoints and large sample size. In the context of global efforts to tailor cancer therapy, IMPORT LOW offers a compelling case for less intensive yet equally effective treatments in low-risk populations.

Conclusion

Ten-year results from the UK IMPORT LOW trial conclusively demonstrate that partial-breast and reduced-dose radiotherapy are non-inferior to standard whole-breast radiotherapy for women with early-stage, low-risk breast cancer following breast conservation surgery. These approaches yield similarly low recurrence rates and favorable long-term toxicity profiles, validating partial-breast radiotherapy with IMRT as a standard of care in this setting. Future research may refine patient selection criteria further and explore integration with novel systemic therapies, but for eligible women, treatment can now be more targeted and convenient without sacrificing oncologic safety.

References

Kirby AM, Finneran L, Griffin CL, Brunt AM, Cafferty FH, Alhasso A, Chan C, Haviland JS, Jefford ML, Sawyer EJ, Sydenham MA, Syndikus I, Tsang YM, Wheatley D, Yarnold JR, Coles CE, Bliss JM. Partial-breast radiotherapy after breast conservation surgery for women with early breast cancer (UK IMPORT LOW): 10-year outcomes from a multicentre, open-label, randomised, controlled, phase 3, non-inferiority trial. Lancet Oncol. 2025 Jul;26(7):898-910. doi: 10.1016/S1470-2045(25)00194-9 IF: 35.9 Q1 B1

Additional references:
– Moran MS, et al. Society of Surgical Oncology–American Society for Radiation Oncology Consensus Guideline on Accelerated Partial Breast Irradiation. Ann Surg Oncol. 2017;24(10):2803-2814.
– National Comprehensive Cancer Network (NCCN) Clinical Practice Guidelines in Oncology: Breast Cancer. Version 1.2024.