ALK-Positive Histiocytosis With Unilateral Breast Involvement: A Case Report.

APH is a rare disorder characterized by the proliferation of ALK-expressing histiocytes with variable anatomical involvement; however, mammary involvement is exceptionally rare. A 32-year-old woman presented with a painless right breast mass. Ultrasound identified a 9 × 8 mm hypoechoic nodule, categorized as BI-RADS 4A. Microscopy showed spindle cells in whorled patterns with characteristic nuclear folding. Immunohistochemistry demonstrated positivity for CD68, CD163, and ALK, and negativity for S-100, CD1a, and CD207. The Ki-67 proliferation index was approximately 10%. Fluorescence in situ hybridization (FISH) confirmed ALK gene rearrangement (The partner gene not identified), establishing the diagnosis of APH. The patient underwent complete resection without adjuvant therapy, and no recurrence was observed at the 15-month ultrasound follow-up. Accurate diagnosis of APH requires integration of histopathology, immunohistochemistry, and molecular testing to distinguish from other histiocytic disorders, with this case highlighting the rare presentation of APH in the breast.