Primary Inflammatory Myofibroblastic Tumour of the Breast: A Potential Diagnostic Pitfall on Aspiration Cytology

Inflammatory myofibroblastic tumour (IMT) is an uncommon mesenchymal neoplasm with intermediate malignant potential, typically arising in the lungs, abdomen, or retroperitoneum. Breast involvement is extremely rare, with limited reported cases. To the best of our knowledge, less than 50 cases of this lesion in the breast have been reported to date and cytological descriptions are very rare. A 47-year-old female presented with a painless mass in the right breast. Imaging revealed a well-defined lobulated soft tissue mass categorised as BIRADS 4A. The cytomorphologic features on fine needle aspiration smears showed spindle cell fragments with inflammatory cells, suggestive of a chronic inflammatory lesion. A subsequent histopathological examination of the excised specimen revealed proliferating spindle cells with a dense lymphoplasmacytic infiltrate consistent with IMT. Immunohistochemistry (IHC) was crucial in ruling out differential diagnoses such as myofibroblastoma, fibromatosis and IgG4-related sclerosing disease. The patient remained recurrence-free post lumpectomy until 24 months of follow-up. Primary breast IMT remains a diagnostic challenge on aspiration cytology due to its rarity and overlap with other spindle cell lesions. Comprehensive evaluation and ancillary testing, including ALK immunohistochemistry and molecular assays, may be essential for accurate diagnosis.