Erdheim–Chester Disease with Breast and Axillary Involvement Diagnosed by Ultrasound-Guided Biopsy: A Case Report and Literature Review

Background/Objectives: Erdheim–Chester disease (ECD) is a rare non-Langerhans cell histiocytosis characterized by multisystem infiltration of foamy histiocytes, leading to chronic inflammation, fibrosis, and organ dysfunction. Breast involvement in ECD is extremely uncommon, and the sonographic features of ECD involving the breast remain poorly described. Case Presentation: We report the case of a 59-year-old woman with chronic bone pain and multisystem disease who experienced an extended diagnostic course despite undergoing renal biopsy, biopsy of a right elbow lesion, bone marrow examination, and multidisciplinary evaluation. Breast ultrasound revealed bilateral infiltrative hypoechoic lesions involving the breasts and axillae. These were classified as BI-RADS 4C and were highly suspicious for breast malignancy. Subsequently, an ultrasound-guided core needle biopsy was performed on the breast and axillary lesions. Results: Histopathology showed fibroadipose tissue infiltrated by numerous foamy histiocytes, scattered epithelioid cells, and occasional Touton giant cells. Immunohistochemistry showed positivity for CD68, CD163, CD4, and Cyclin D1, partial positivity for OCT2 and CD30, and negativity for S100, CD1a, Langerin, ALK, CK (Pan), and GATA3. The Ki-67 index was approximately 3%. Molecular testing detected the BRAF V600E mutation, supporting the diagnosis of ECD. A review of reported cases showed that breast involvement in ECD lacks specific ultrasound findings and may closely mimic primary breast malignancy. Conclusions: Breast involvement in ECD is rare and may present as bilateral infiltrative hypoechoic lesions with axillary involvement on ultrasound. In patients with chronic bone pain, symmetric osteosclerosis, or multisystem disease, ECD should be considered in the differential diagnosis. Ultrasound-detected superficial lesions may provide accessible biopsy targets, helping to establish a timely diagnosis and reduce diagnostic delay.