Aims Idiopathic granulomatous mastitis (IGM) is a rare, benign, inflammatory breast disorder of unknown aetiology usually affecting women of reproductive age. It classically presents as a unilateral painful breast mass. It is frequently mistaken for carcinoma or other inflammatory breast diseases. Diagnostic investigations include clinical examination, appropriate imaging and tissue sampling. A link between IGM and infection with the Corynebacterium species in particular Corynebacterium kroppenstedtii has been described.

Methods A retrospective single-centre cohort study was conducted over a 5-year period (2017–2022); all cases of IGM were identified.

Results Forty-one patients were diagnosed with IGM. Breast lump was the most common presenting complaint (n=29). The average age was 45 years. Eighteen patients had samples sent for culture and sensitivity, 11 of which had positive microbiology results indicative of Corynebacterium spp infection.

An 82% resolution rate (27 of 33) was recorded in those who received either a short-antibiotic course or none at all. Eight patients reported persistent disease at 3 months, five of which had evidence of Corynebacterium spp.

Discussion This 5-year review highlights the impact of IGM in a tertiary centre in Dublin, Ireland. Although no treatment guidelines exist, options include antibiotics, immunomodulators and surgery. Due to risk of fistulae and unfavourable cosmetic outcomes, surgery should be reserved for refractory IGM. We suspect that there may be a subset of patients where prolonged antibiotic therapy should be considered. Defining this subgroup requires further study, but likely includes those with cystic neutrophilic granulomatous mastitis, relapsing disease and in whom Corynebacterium spp is recovered.