# ALK‐Positive Histiocytosis With Unilateral Breast Involvement: A Case Report

## TL;DR

A rare case of ALK-positive histiocytosis in the breast was diagnosed and successfully treated with surgery alone.

## Contribution

This case report highlights the rare occurrence of ALK-positive histiocytosis in the breast and its successful management with surgical resection.

## Key findings

- ALK-positive histiocytosis can present as a painless breast mass and is diagnosed using histopathology and immunohistochemistry.
- Complete surgical resection without adjuvant therapy resulted in no recurrence after 15 months of follow-up.
- Accurate diagnosis requires a combination of histopathology, immunohistochemistry, and molecular testing to differentiate from other histiocytic disorders.

## Abstract

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.

ALK‐positive histiocytosis (APH) is rare in the breast. Diagnosis requires histopathology showing spindle cells, CD68, CD163, and ALK‐positive immunohistochemistry excluding CD1a and CD207, and confirmed ALK rearrangement. No recurrence was achieved with complete surgical resection alone, pointing to an indolent clinical course in this case of localized mammary APH, but calls for further validation.