A Rare Case of Mesothelioma Showing Micropapillary and Small Cell Differentiation with Aggressive Behavior

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Pleural mesothelioma is a rare neoplasm and roughly 70% are the epithelioid type, which is characterized by proliferation of epithelioid tumor cells with a tubulopapillary growth pattern. 1 There have been few reports of mesothelioma having a micropapillary pattern 2 and a small cell component. 3 To the best of our knowledge, there has been no previously reported case containing both micropapillary and small cell components. Herein, we report a rare case of mesothelioma with micropapil-lary component and small cell differentiation exhibiting aggressive behavior in a 59-year-old woman. A 59-year-old female presented with a 2-month history of pleuritic chest pain aggravated by exercise and breathing. She had hyperthyroidism and had been on medication for 10 years, but had no history of occupational or environmental exposure to asbestos. A computed tomography (CT) examination revealed band-like thickening along the right pleura and fissure, suggesting malignant mesothelioma. After video-assisted thoraco-scopic surgery of the right pleura, she diagnosed with malignant mesothelioma, epithelioid type. Despite of neoadjuvant chemotherapy, right pleural and fissural thickening was found to have progressed on CT (Fig. 1A). The patient underwent right extrapleural pneumonectomy. The right lung was entirely encased with diffusely thickened pleura (Fig. 1B). The tumor involved the visceral pleura, infiltrated the parietal pleura and lung parenchyma, and was accompanied by thick fibrosis. Discohesive and epithelioid tumor cells were arranged in a solid sheet and partly micropapillary patterns. Marked nuclear pleomorphism and frequent mitoses were found. The tumor cells frequently invaded lymphatic spaces and me-tastasized to 35 lymph nodes. There were two distinct subpop-ulations of tumor cells: micropapillary and small cell components (Fig. 1C-E). The micropapillary component at invasive fronts (Fig. 1C) had no fibrovascular core and was reminiscent of micropapillary adenocarcinoma of the lungs (Fig. 1D). The small cell component was found in multifocal areas of the tumor , admixed with larger epithelioid cells arranged in micro-papillary pattern (Fig. 1E). This component resembled small cell carcinoma of the lung, having a high nuclear-cytoplasmic ratio, and a finely dispersed " salt-and-pepper " chromatin pattern. Ultrastructural study of the tumor revealed long, slender microvilli on the cell surface, which supported the diagnosis of mesothelioma (Fig. 1F). Immunohistochemical staining for cal-retinin, D2-40, thyroid transcription …