Rosai-Dorfman Disease, Presenting as a Right Atrial Mass with Involvement of the Tricuspid Valve in a 54-Year-Old Woman.

BACKGROUND Sinus histiocytosis with massive lymphadenopathy (SHML), Rosai-Dorfman disease, or Rosai-Dorfman-Destombes disease (RDD), is a rare non-Langerhans cell of unknown etiology. This report is of a case of isolated SHML, or Rosai-Dorfman disease, presenting as a right atrial mass with involvement of the tricuspid valve in a 54-year-old woman. This case shows the challenges of diagnosing this condition in the heart and the challenges of treating this rare disease with the limited information on the efficacy of the treatment modalities. CASE REPORT A 54-year-old Asian woman presented to the Emergency Department with chest and right upper quadrant pain. Transthoracic echocardiogram and computed tomography angiography showed a right atrium mass at the level of the tricuspid valve, causing moderate-severe regurgitation. Partial tumor debulking with biopsies later showed pink-yellow soft tissue, with histopathology showing histiocytes demonstrating emperipolesis. The tumor was positive for CD68 and S100 and negative for CD1a, consistent with Rosai-Dorfman disease. Subsequently the patient received targeted therapy with cobimetinib, without worsening cardiac function or disease progression. CONCLUSIONS This case highlights the challenging histopathological diagnosis of SHML, or Rosai-Dorfman disease, particularly in non-lymphoid tissue, such as the heart. Obtaining tissue for diagnosis can be challenging in this organ. Treatment is challenging when the mass cannot be extracted completely, like in our case, because other forms of therapies are not well studied and warrant further investigation, such as cobimetinib, which is a MEK pathway inhibitor approved in 2022 by the US Food and Drug Administration for histiocytic neoplasms.