Abstract:
Rosai-Dorfman disease (RDD), also referred to as sinus histiocytosis with massive lymphadenopathy, is a rare benign proliferative disorder characterized by the excessive accumulation of histiocytes within various tissues, including lymph nodes and extranodal sites. The etiology of RDD remains unclear. Here, we present the case of a 37-year-old woman diagnosed with RDD, who presented with a slow-enlarging, 19 cm liposarcoma-like soft tissue tumor on her left flank, which had been progressively growing for four years. The tumor extended into her left paraspinalis muscle on her back. Histopathological examination revealed dense lymphoplasmacytic and histiocytic infiltration within the sclerotic stroma of the subcutaneous layer. Notably, histiocytes, identified by positive staining for S100 and CD68, exhibited emperipolesis and contained lymphocytes or plasma cells within their cytoplasm. In regions with prominent plasma cell infiltration, the IgG4/IgG ratio was approximately 25-30%. Remarkably, laboratory tests returned normal results.

Patient and Method:
Our patient, a 37-year-old woman, presented with a history of a slowly enlarging soft tissue mass on her left flank, which had been progressing over four years. On physical examination, the mass measured 19 cm in diameter and extended into her left paraspinalis muscle on her back.

Results:
A histopathological examination of the excised tumor revealed a dense lymphoplasmacytic and histiocytic infiltration within the sclerotic stroma of the subcutaneous layer. Remarkably, the histiocytes stained positively for S100 and CD68, exhibited emperipolesis (engulfment of lymphocytes or plasma cells within their cytoplasm), and displayed an IgG4/IgG ratio of approximately 25-30% in areas with prominent plasma cell infiltration. Notably, all laboratory tests yielded normal results.

Conclusion:
RDD, or Rosai-Dorfman disease, is a rare and enigmatic disorder characterized by histiocytic proliferation. Its diverse clinical presentations and histopathological variations pose diagnostic challenges, underscoring the importance of awareness and careful evaluation, especially when RDD presents as an extensive soft tissue mass. Further research is warranted to elucidate the underlying mechanisms of this intriguing condition and its potential associations with IgG4-related disease.

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