A 75-Year-Old Man With an Asymptomatic Pleural-Based Mass Discovered on Routine Chest Radiographs

A 75-year-old man was found to have a pleural-based mass on a routine preoperative chest radiograph for knee surgery. There was no mediastinal mass and no pleural effusion was seen. Mediastinoscopy and bronchoscopy were nondiagnostic. Thoracoscopy revealed multiple nodules on the visceral pleural and pericardial surfaces. The surgeon thought that the presentation was consistent with mesothelioma. Surgical resection of several pleural lesions was performed. The specimens received in the pathology laboratory consisted of multiple circumscribed unencapsulated nodules up to 2.5 cm in diameter. There was adjacent pleural thickening. The nodules had a lobulated, firm, gray-white cut surface. No areas of necrosis, hemorrhage, or cystic change were noted. Under low-power microscopy, the lesions showed dense lymphoid tissue with a lobular configuration, sharply separated by fibrous septa (Figure 1, A). With higher magnification, pale, vaguely epithelioid-appearing cells were visible, admixed with the lymphoid tissue (Figure 1, B). These pale cells were positive for pancytokeratin, cytokeratin 5/6, and p63 (Figure 2, A through C) by immunohistochemistry, although negative for calretinin, Wilms tumor 1 protein, and thyroid transcription factor 1. Lymphoid cells were positive for terminal deoxynucleotidyl transferase (Figure 2, D). An immunostain for CD30 was negative.What is your diagnosis?Primary pleural thymoma is a rare but well-recognized entity that occurs in adults, with a slight male predominance. Patients with primary pleural thymoma may be asymptomatic or present with chest pain and respiratory difficulty. The behavior of primary pleural thymoma is usually indolent; however, recurrence, distant metastasis, and even death caused by respiratory failure have been reported. Because of their peculiar location and variety of histologic patterns manifested, pleural thymomas may be confused with other neoplasms and may cause diagnostic problems clinically, radiologically, and morphologically. A combination of clinical information, histopathologic appearance of the tumor, and immunohistochemical studies will often help to distinguish a primary pleural thymoma from other neoplasms.Thymomas are epithelial neoplasms of the thymus. They typically arise in the anterosuperior mediastinum near the normal gland. Ectopic thymomas have been described in the pleura, pericardium, lung parenchyma, thyroid, and in the soft tissue of the neck. Because of their peculiar location, pleural thymomas may be confused with other neoplasms, and may cause diagnostic problems clinically, radiologically, and morphologically.1–5Primary pleural thymoma occurs in adults between the ages of 32 and 75 years (mean, 52.1 years). There is a slight male predominance. Patients with primary pleural thymoma may be asymptomatic or present with chest pain, respiratory difficulty, and nonspecific symptoms, such as fever and weight loss.1–3 The behavior of primary pleural thymoma is usually indolent; however, recurrence, distant metastasis, or both, or even death caused by respiratory failure have been reported.1–4 A clinical summary of 11 patients described in the literature is presented in the Table.The chest radiograph typically reveals pleural thickening with encasement of the lung or discrete pleural mass lesions. Pleural effusion may or may not be present.124 Computed tomographic studies show a pleural-based mass with encasement of the lung or diffuse circumferential nodular thickening with extension along the fissures.124Grossly, the pleura is thickened and replaced by a lobulated mass. There are fibrous septa subdividing the masses into angulated tissue compartments. The tumors have a fleshy pink-tan cut surface. Areas of necrosis, hemorrhage, and cyst formation may be present. Dystrophic calcification is described in some cases.2–4Microscopically, circumferential fibrous encapsulation of pleural thymomas is unusual. As a result, they may be misinterpreted as “invasive” lesions. In low-magnification microscopy, the tumor typically shows a dense lymphoid process with a lobular configuration sharply separated by fibrous septa. On higher magnification, the tumor shows a biphasic cell population with a net-like arrangement of neoplastic epithelial cells and nonneoplastic lymphoid cells. The lymphoid population is dominated by T lymphocytes. The cytology of the neoplastic epithelial cells varies in different tumors. They may be oval, polygonal, or spindle-shaped, with vague cellular borders, bland nuclei, fine chromatin, and indistinct nucleoli. In some tumors, they may show moderate nuclear irregularity and hyperchromasia with distinct nucleoli. Mitotic activity is rare. Hassall corpuscles are seen in only a small minority of cases. There is considerable variation in the number of neoplastic epithelial cells in different tumors. Based on the type of epithelial cell (spindle vs polygonal), the degree of epithelial atypia, and the relative proportion of epithelial cells and lymphocytes, pleural thymomas can be subclassified into lymphocyte-predominant, mixed lymphoepithelial, predominantly epithelial and polygonal cell, predominantly epithelial and spindle cell, and atypical epithelial-predominant types. As with their mediastinal counterparts, an “invasive” pleural thymoma will require additional treatment for the possibility of recurrence of metastasis.In immunohistochemistry examination, the neoplastic epithelial cells have a characteristic arborizing network of cytokeratin-positive cells, emphasizing the presence of interconnecting cytoplasmic processes.6 The epithelial component also shows immunoreactivity for cytokeratin subtype 5/6 and for p63 protein.78 The T lymphocytes in the tumors coexpress CD1a, nuclear terminal deoxynucleotidyl transferase and CD99.9 In contrast to thymic carcinoma, the neoplastic epithelial cells of a thymoma are uniformly not immunoreactive for CD5, although lymphocytes within tumors may have CD5 immunoreactivity.10Electron microscopy may be helpful to demonstrate tonofilaments, tight intercellular junctions, desmosomes, elongated cytoplasmic processes, and basal lamina of epithelial cells. There are no plasmalemmal microvilli. Lymphoid cells are usually intercalated between the epithelial cells.3Because of their ectopic site and variety of histologic patterns, as well as the occurrence of an “invasive” border, diagnosis of primary pleural thymomas by pleural biopsy can be very difficult, especially if the pathologist is not aware of this entity or if the more typical features of thymoma are lacking. Depending on the histopathologic appearance of the lesion being studied, the differential diagnosis of primary pleural thymoma includes (1) pleural invasion from a mediastinal thymoma, (2) malignant lymphoma, (3) metastatic carcinoma, (4) metastatic seminoma, (5) mesothelioma, and (6) pleural sarcoma.1–5 A combination of clinical information, histopathologic appearance of the tumor, and immunohistochemical studies will often help to distinguish a primary pleural thymoma from other neoplasms.

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