Soft tissue Rosai Dorfman disease mimicking inflammatory pseudotumor: a diagnostic pitfall.

Rosai Dorfman disease, or sinus histiocytosis with massive lymphadenopathy (SHML), may be a difficult diagnosis to make, especially in extranodal sites. With soft tissue involvement the characteristic diagnostic features of large histiocytic cells with emperipolesis may be overshadowed by a fibroinflammatory component. In these cases it is easy to confuse this lesion with reactive nodules and benign and malignant neoplasms. We report a case in which soft tissue SHML was confused with an inflammatory pseudotumor. Only after review, when other extranodal sites became apparent, was the correct diagnosis made. Pitfalls in the diagnosis of soft tissue SHML are discussed.