Pulmonary histiocytic sarcoma mimicking pulmonary Langerhans cell histiocytosis in a young adult presenting with spontaneous pneumothorax: a potential diagnostic pitfall.

We present a case of a histiocytic sarcoma incidentally detected in peripheral lung tissue resected for a spontaneous pneumothorax. Furthermore, we discuss the practical approach to pulmonary Langerhans cell histiocytosis, the main differential diagnosis of this lesion in the lung, based on morphological and immunohistochemical features. A 23-year-old male patient presented with recurrent pneumothoraces. The pulmonary tissue showed a single round granuloma-like lesion measuring 4 mm in diameter in close neighbourhood to a bronchial wall. The granuloma consisted of histiocytic cells with enlarged pale nuclei, plasma cells, lymphocytes and scanty eosinophilic granulocytes giving the impression of a granuloma of pulmonary Langerhans cell histiocytosis on haematoxylin and eosin (H&E) stains. Immunohistochemically, the histiocytic cells were negative for CD1a and S-100. They were positive for CD68, HLA-DR, CD14, CD4, CD11c, CD45LCA and lysozyme. MIB1 (Ki67) showed a nuclear staining of approximately 10% of the histiocytic cells. In summary, these findings were in keeping with a histiocytic sarcoma, a rare haematopoetic neoplasm. By demonstrating this particular case, we emphasise the importance of proving the diagnosis of pulmonary Langerhans cell histiocytosis by means of immunohistochemistry. In case of a negative CD1a reaction in a histiocytic lesion, further immunohistochemical studies have to be performed in order not to misdiagnose a malignant haematopoetic lesion.