Rosai-Dorfman disease of the breast: A case report of a rare extranodal presentation with imaging correlation

Imaging Findings

• Breast Ultrasound: A hypoechoic, wider-than-tall lesion with irregular margins, mild posterior acoustic enhancement, internal vascularity, and surrounding edema. Associated gynecomastia was noted in bilateral breasts. Color Doppler confirmed internal vascularity [Figure 1].

• Mammography: Circumscribed, non-calcified mass with mild architectural distortion but no nipple retraction or microcalcifications. The lesion was classified as BI-RADS 4 due to suspicious morphology [Figure 2].

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Figure 1:

A 45-year-old male with bilateral breast lumps (more prominent on the right). (a) Transverse B mode ultrasound image of the right breast demonstrates an oval hypoechoic mass, parallel to the chest wall (yellow arrow) in the subareolar location with lobulated margins. The surrounding breast parenchyma exhibits features of gynecomastia. (b) Transverse B mode ultrasound image of the right breast demonstrates a hypoechoic mass (white arrow) in the subareolar location showing internal vascularity on color Doppler images.

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Figure 2:

Mediolateral oblique view of the right breast on mammography reveals a retroareolar mass (white arrow) with no associated calcifications, closely mimicking malignancy.

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Histopathology

Core biopsy of the right breast lesion revealed dense inflammatory infiltrate with a predominance of lymphoplasmacytic cells and histiocytes. The histiocytes demonstrated emperipolesis, a characteristic feature of RDD, where histiocytes engulf other cells (such as lymphocytes or plasma cells) but retain them intact within the cytoplasm. Immunohistochemistry confirmed strong and diffuse S-100 positivity, with a subset of cells staining for CD68, supporting the diagnosis of RDD [Figure 3]. No evidence of malignancy was noted. The Ki-67 proliferation index was low (<5%), supporting a benign etiology.

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Figure 3:

Histopathology of the right breast lesion (Hematoxylin and Eosin stain, 200x magnification). The image demonstrates emperipolesis, a hallmark feature of Rosai-Dorfman disease, where intact lymphocytes are seen within the cytoplasm of large histiocytes. A dense inflammatory infiltrate composed predominantly of histiocytes and lymphoplasmacytic cells is also observed, along with preserved tissue architecture.

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Typical Imaging Findings in Breast RDD Include

• On ultrasound: A hypoechoic, irregular, or lobulated mass with internal vascularity on Doppler, occasionally with mild posterior enhancement.

• On mammography: Focal asymmetry or irregular, non-calcified masses without specific distinguishing features from carcinoma.

In our case, the imaging findings were largely consistent with prior reports, including a hypoechoic, wider-than-tall mass with irregular margins, internal vascularity, and mild surrounding edema on ultrasound, and a non-calcified mass with mild architectural distortion on mammography. However, our case presented with bilateral breast involvement, which is rarely reported in the literature, as most documented cases are unilateral. Additionally, the presence of associated gynecomastia in both breasts may represent a reactive or coincidental finding, but it adds an uncommon radiologic context not widely described in previous reports.^[3]

Given these radiological similarities to breast carcinoma, histopathology remains the gold standard for definitive diagnosis. The presence of emperipolesis and strong S-100 positivity differentiates RDD from other benign and malignant breast lesions. The differential diagnosis includes carcinoma, lymphoma, granulomatous mastitis, and idiopathic histiocytic disorders. Unlike malignancies, RDD lesions lack necrosis and demonstrate preserved architecture on imaging.

Laterality is often unilateral, though bilateral involvement (as in our case) is rare. Axillary lymphadenopathy is reported in only ~5% of mammary RDD cases.^[4]

Complications in RDD are generally uncommon but may include progressive local disease, recurrent mass formation, or compressive symptoms depending on lesion size and location. Systemic symptoms such as fever, weight loss, and lymphadenopathy may occur if other extranodal sites are involved. In rare cases, aggressive or disseminated forms of RDD can occur, particularly in association with immune dysfunction or systemic inflammatory diseases. However, no complications were observed in our patient, and clinical follow-up showed stability.

While surgical excision was historically performed for diagnosis, core biopsy is now preferred for early identification of RDD, preventing unnecessary surgical interventions. The preferred management is conservative observation, as spontaneous regression is common. However, in cases with progressive symptoms or systemic involvement, corticosteroids, immunosuppressants, or chemotherapy may be considered.^[5]