Multilocular thymic cyst with follicular lymphoid hyperplasia in a male infected with HIV. A case report with fine needle aspiration cytology

BACKGROUND: Multilocular thymic cyst with follicular lymphoid hyperplasia is a rare complication in HIV-infected patients, causing pseudotumorous enlargement of the anterior mediastinum. There have been six reported cases, all with only histologic findings. This paper reports another such case and includes perhaps the first cytologic findings on this rare entity. CASE: A 35-year-old, HIV-infected male intravenous drug abuser, who complained of worsening central chest discomfort and pain on deep inspiration, was found to have a large, septated anterior mediastinal mass. Computed tomography-guided fine needle aspiration biopsy was performed. The cytologic presentation mimicked that of thymoma, with cystic degeneration and a dual population of epithelial cells and lymphocytes as well as large aggregates of "epithelial" cells intermixed with lymphocytes in a background of macrophages and cyst fluid. Histologic examination of the resected mass revealed a multilocular thymic cyst with follicular lymphoid hyperplasia. HIV-1 core protein p24 was localized immunohistochemically in the dendritic follicular cells of the germinal centers. In retrospect, the quantity of epithelium derived from the cyst lining was too scanty for thymoma, and the presence of plasma cells and lymphohistiocytic aggregates suggested follicular lymphoid hyperplasia. CONCLUSION: Multilocular thymic cyst with follicular lymphoid hyperplasia should be considered in the differential diagnosis of an anterior mediastinal mass in HIV-infected patients after lymphoma and tuberculosis.