Spontaneous Regression of Rosai-Dorfman Disease Presenting as a Thigh Mass With Vascular Involvement: A Case Report.

Rosai-Dorfman disease (RDD) is a rare non-Langerhansian histiocytosis, classically manifesting as massive cervical lymphadenopathy. Isolated extra-nodal forms, particularly in soft tissues, are exceptional and may simulate a malignant tumor. We report the case of a 56-year-old patient presenting with a painless mass of the right thigh that had been evolving for several weeks. Clinical examination revealed a deep mass measuring approximately 10 cm. Magnetic resonance imaging showed a tissue formation encompassing the femoral vessels, responsible for thrombosis with vascular stenosis. A surgical biopsy was performed. Histological analysis, coupled with immunohistochemistry, confirmed the diagnosis of RDD. Anticoagulant therapy with apixaban was instituted for the venous thrombosis. The evolution was marked by spontaneous regression of the mass without recourse to additional surgery. This case illustrates an atypical presentation of RDD through isolated involvement of the soft tissues of the thigh, complicated by deep venous thrombosis. Diagnosis is based on histopathological examination. Treatment is individualized and may be limited to monitoring in nonprogressive forms. RDD must be included in the differential diagnosis of deep soft tissue masses. Early recognition can avoid invasive treatment.