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Clinical trials have demonstrated conclusively the non-inferiority of breast-conserving surgery followed by breast radiation therapy (BCT) compared with mastectomy for the treatment of early-stage invasive breast cancer (BC). The definition of the required surgical margin to ensure adequate removal of the cancer by BCT to obtain an acceptable low local recurrence (LR) rate remains controversial. Meta-analyses published by Houssami et al. in 2010 and 2014 demonstrated significantly lower LR rates for patients with a negative margin compared with those with positive (ink on tumour) or close (defined as ≤1 mm or ≤2 mm) margins. Neither meta-analysis addressed whether 'no ink on tumour' was adequate to define a negative margin because of a lack of data. Nevertheless, in 2014, the Society of Surgical Oncology (SSO) and the American Society for Radiation Oncology (ASTRO) with advice from pathologists reviewed these data together and published guidelines recommending that a margin of 'no ink on tumour' was sufficient to define a clear margin in BCT. Subsequently, clinical practice has varied with some national and international bodies endorsing 'no ink on tumour', whilst others have recommended a ≥1 mm margin as acceptable margins for BCT. A more recent meta-analysis conducted by Bundred and colleagues in 2022 did have sufficient data to compare 'no ink on tumour' and 1 mm and concluded that 1 mm rather than 'no ink on tumour', should be used as a minimum negative margin, and recommended that international guidelines be revised. The current review presents a balanced assessment of the evidence relating margin width and local recurrence after BCT. This review concludes that guidelines should consider re-defining a negative margin as ≥1 mm rather than 'no ink on tumour' in the context of BCT, recognising there will be variation to tailor therapy for any individual patient situation to ensure optimal patient care. [ABSTRACT FROM AUTHOR]