A case of ALK-positive histiocytosis with multiple lesions in the unilateral breast

INTRODUCTION AND IMPORTANCE: Primary ALK-positive histiocytosis of the breast is rare. Here, we report a case of ALK-positive histiocytosis with multiple unilateral breast lesions. CASE PRESENTATION: Our patient was a 38-year-old female with primary ALK-positive histiocytosis of the breast with multiple lesions. There were no lesions in other organs, and the patient was considered surgically resectable and underwent a left total mastectomy and sentinel lymph node biopsy. Histopathologically, there were at least three lesions in the left breast in upper inner quadrant (UIQ), upper quadrant (UQ), and upper outer quadrant (UOQ). All lesions showed spindle-shaped tumor cells that were positive for CD163 and ALK and negative for AE1/AE3. Fluorescence in situ hybridization (FISH) showed ALK and KIF5B rearrangements, suggesting the presence of the KIF5B-ALK fusion gene. In conclusion, this case was confirmed to be ALK-positive histiocytosis with multiple lesions in the unilateral breast. The patient underwent surgery and was discharged without complications. CLINICAL DISCUSSION: Reports of ALK-positive histiocytosis are very rare, and reports of primary cases in the breast are even rarer. The basic treatment for ALK-positive histiocytosis is surgical resection; however, ALK inhibitors may be effective in unresectable or disseminated cases. Accurate diagnosis at the time of initial treatment is necessary to expand the treatment options. CONCLUSION: This is the first case of ALK-positive histiocytosis with multiple lesions in the unilateral breast.