Perplexing Case of IgG4-Related Mastitis: Hitherto Undescribed Contrast-Enhanced Mammography Features.

Immunoglobulin G4-related mastitis (IgG4-RM) represents localized form of IgG4-related disease (IgG4-RD), an immune-driven condition marked by abnormal infiltration of plasma cells and raised IgG4 levels. Breast involvement is exceedingly rare and often misinterpreted as malignancy. We report a case of a 40-year-old woman presenting with right axillary lymphadenopathy and clinical suspicion of occult breast malignancy. Contrast-enhanced mammography (CEM) revealed multiple small, equal-density, microlobulated masses in both breasts, exhibiting moderate heterogeneous enhancement. The findings and pattern of involvement favored a secondary breast involvement rather than primary malignancy. 18F-fluorodeoxyglucose (FDG) positron emission tomography-computed tomography demonstrated FDG-avid lesions in the breasts, liver, and spleen, pointing toward systemic immune-mediated pathology. Core biopsy from the breast confirmed IgG4-RM. The patient responded extremely well to oral corticosteroids. This case highlights the diagnostic challenges of IgG4-RM and underscores the critical role of radiologic-pathologic correlation. To the best of our knowledge, this case is the first report in the literature to illustrate the CEM features of IgG4-RM, offering novel imaging insights into this underrecognized entity.