P070 Unmasking an autoimmune breast disease: rheumatology’s evolving role in idiopathic granulomatous mastitis

Background/Aims Idiopathic granulomatous mastitis (IGM) is a rare, benign but clinically aggressive inflammatory breast condition that often mimics malignancy on imaging and examination. Its exact cause remains uncertain, but growing evidence supports an autoimmune or autoinflammatory basis rather than a purely infectious one. In the UK, IGM is often under-recognised, leading to diagnostic delays and unnecessary surgery. The overlap between IGM and immune-mediated inflammatory disease places rheumatology in a key position to support accurate diagnosis and medical management. This study aimed to review all IGM cases seen in our regional breast referral centre, serving multiple NHS trusts, and to evaluate outcomes following the introduction of a rheumatology-led multidisciplinary approach. Methods A retrospective review was conducted of ten consecutive patients with biopsy-proven IGM diagnosed between April 2016 and August 2024. All underwent triple assessment in the breast clinic (clinical examination, imaging, and core biopsy). Microbiological cultures were extended using lipid-enriched media, and where indicated, 16S PCR was performed for Corynebacterium kroppenstedtii. Rheumatology review included detailed autoimmune and vasculitis screening (ANA, ENA, dsDNA, ANCA, complement, ACE, prolactin, and QuantiFERON-TB). Immunosuppressive treatment was based on disease severity, with prednisolone (20-40 mg daily) as induction therapy and methotrexate (15-25 mg weekly) as a steroid-sparing agent where required. Clinical outcomes included remission, relapse, treatment tolerance, and diagnostic delay before rheumatology involvement. Results All ten patients were female (mean age 37 years, range 26-48) with a mean BMI of 29.8 kg/m². Seven were Caucasian and three were from Syria, Afghanistan, and Ethiopia. All presented with unilateral painful breast masses; four had imaging suspicious for malignancy (BI-RADS 3-5). Corynebacterium kroppenstedtii was isolated in four patients. Two patients had positive autoimmune serology (ANA or anti-smooth muscle antibody). Five patients achieved remission with steroids alone, and six required methotrexate, including one switched to intramuscular methylprednisolone acetate due to steroid intolerance. Complete remission was achieved in nine of ten patients, sustained for up to nine years; one patient remains under treatment. No serious adverse effects were reported. The median delay before rheumatology referral was six months (range 2 weeks-18months), underscoring the importance of early multidisciplinary involvement. Conclusion Our experience supports IGM as an immune-mediated granulomatous disorder that responds well to immunosuppressive therapy within a multidisciplinary framework. Early rheumatology input allows prompt exclusion of systemic autoimmune disease, targeted immunosuppressive therapy, and avoidance of unnecessary surgical procedures. Collaboration between rheumatology, breast surgery, radiology, pathology, and microbiology is vital for diagnostic accuracy and durable remission. As a regional referral centre, our model demonstrates excellent long-term outcomes and aligns with the 2022 International GLM Consensus and EUBREAST GRAMAREG recommendations. Incorporating rheumatology leadership into IGM management across UK breast networks could standardise care, improve patient experience, and contribute prospectively to the GRAMAREG registry. Disclosure F.A. Khan: Honoraria; UCB. Other; Congress Sponsorship EULAR (UCB) and BSR 2025 (Novartis). T. Fasih: None. M. Khan: None.