Inflammatory Myofibroblastic Tumor of the Breast Mimicking Complex Fibrocystic Changes, With a Novel CSTF3::ALK Fusion.

Inflammatory myofibroblastic tumor (IMT) of the breast is an uncommon neoplasm that presents significant diagnostic challenges due to its overlap with more common breast lesions. Clinical and imaging features are often nonspecific, leading to potential misdiagnosis. We present a breast IMT in a 38-year-old woman, initially detected as a 7-mm lesion classified as BI-RADS 3 during routine screening and suspected to represent fibrocystic changes. Follow-up ultrasound demonstrated growth to 10 mm, prompting a core needle biopsy that revealed a spindle cell lesion with prominent inflammatory infiltrates. Immunohistochemistry showed positivity for ALK, CD34, and NTRK, and fluorescence in situ hybridization (FISH) confirmed an ALK rearrangement. Targeted RNA sequencing identified a novel CSTF3::ALK fusion. This report highlights the diagnostic complexity of IMT and underscores the importance of comprehensive histological and molecular evaluation. Accurate diagnosis is essential for appropriate management and adds to the limited body of literature on breast IMT, particularly regarding the significance of genetic testing in confirming the diagnosis.