Thymoma

Thymoma is the most common primary mediastinal neoplasm in adults and the most frequent tumor of the anterior mediastinum. It usually affects adults older than 40 years, with no gender predilection. Approximately 30% of patients with thymoma have thoracic symptoms of cough, dyspnea, and/or chest pain; 40% to 70% have symptoms related to one or more of the parathymic syndromes, typically myasthenia gravis (MG), but hypogammaglobulinemia, pure red cell aplasia, and nonthymic malignancies may also occur. Some affected patients are asymptomatic and are discovered incidentally because of abnormal chest radiographs. Although the association of thymoma and MG is well recognized, approximately 85% of patients with MG have thymic lymphoid hyperplasia, and only 15% are found to have thymoma. In contrast, 30% to 50% of patients with thymoma may develop MG. Hypogammaglobulinemia and pure red cell aplasia occur in 10% and 5% of patients who have thymoma, respectively. Nonthymic malignancies occur in 12% to 20% of patients with thymoma and include thyroid carcinoma, lung cancer, and lymphoma.

Thymomas manifest on radiography as rounded, well-circumscribed, unilateral, anterior mediastinal masses (Figure 71-2). They are typically located anterior to the aortic root but may be found anywhere from the thoracic inlet to the diaphragm and rarely in the neck. On cross-sectional imaging, thymomas are often homogeneous, soft tissue masses but may exhibit heterogeneity because of cystic change, hemorrhage, or necrosis, especially in large neoplasms. Drop metastases to the ipsilateral pleura, pericardium, or upper abdomen through a diaphragmatic hiatus are well documented and represent invasive disease (Figure 71-3). Pleural drop metastases may encase the lung and mimic diffuse malignant mesothelioma, although associated pleural effusion is infrequent. Lymph node and hematogenous metastases are rare. Mediastinal invasion may be detected with CT and may manifest as an irregular tumoral surface, contralateral extension of thymoma across the midline, and obvious invasion of mediastinal fat and structures. Imaging evidence of local invasion must be correlated with microscopic evidence of capsular transgression and tissue invasion, because encapsulated thymomas may produce fibrous adhesions to adjacent structures. MRI is more sensitive than CT in the detection of vascular invasion. When evaluating a thymic mass, PET-CT may be useful for depicting tumoral invasion, differentiating subgroups of thymic epithelial neoplasms, and staging extent of disease. The maximum standard uptake values (SUVs) of thymoma are typically lower than those of thymic carcinoma. Guided needle biopsy, mediastinotomy, mediastinoscopy, and video-assisted thoracoscopy may establish the diagnosis. However, histologic diagnosis before excisional surgery may not be required in classic cases of thymoma.

Figure 71-2 Elderly woman with thymoma with microscopic capsular invasion. A, Posteroanterior (PA) chest radiograph; B, lateral chest radiograph; C, contrast-enhanced chest computed tomography (CT) scan (mediastinal window). A 6 × 5 cm discrete homogeneous thymoma (arrows) extends inferiorly in the mediastinum without invasion. Thymomas may occur in any location between the thoracic inlet and the diaphragm.

Figure 71-3 Invasive thymoma. A, Posteroanterior chest radiograph; B, unenhanced chest CT (mediastinal window). Thymoma manifests as a lobular right anterior mediastinal mass (solid arrows) with multiple drop metastases to ipsilateral pleura (open arrows).

Thymomas represent neoplastic proliferation of thymic epithelial cells, intermixed with mature lymphocytes. The histologic typing has been complex and challenging. The Revised 2004 World Health Organization (WHO) classification divides thymic epithelial neoplasms into A, B, and C types on the basis of epithelial cell morphology, the ratio of lymphocytes to epithelial cells, and prognosis. Types A and B correlate with the traditional major histologic cell types of thymoma—epithelial, lymphocytic, mixed lymphoepithelial, and spindle cell. Type C refers to thymic carcinoma. The multidisciplinary International Thymic Malignancy Group (ITMG) assesses diagnostic and therapeutic options of thymoma. Anatomic staging is based on the presence or absence of capsular invasion, determined macroscopically during surgery and confirmed microscopically. Thymomas are now considered malignant neoplasms, and all stages have the potential to metastasize. Most thymomas are completely encapsulated; however, approximately 30% are invasive and grow through the capsule into surrounding adipose tissue, pleura, pericardium, great vessels, and/or heart. Encapsulated and invasive thymomas are microscopically identical, and “invasive” is used to denote capsular invasion.

The treatment of choice for encapsulated and invasive thymomas confined to the mediastinum is surgical resection. Postoperative radiation therapy is included in the treatment of invasive thymoma, but the role of preoperative irradiation is controversial. Chemotherapy with cisplatin-based regimens is generally recommended for metastatic or unresectable recurrent disease, with an overall response rate of 50% to 70% in small studies.

The prognosis of patients with thymoma varies with the stage and extent of surgical resection. Patients with completely resected, encapsulated lesions have the best prognosis. Overall 5-year and 10-year survival rates in patients who have encapsulated thymomas are 75% and 63%, respectively, whereas patients with invasive thymoma have survival rates of 50% and 30%, respectively. Delayed recurrence of thymomas may occur, even in patients with completely resected encapsulated lesions, which emphasizes the importance of long-term follow-up.

The role of thymectomy in the treatment of parathymic syndromes is controversial. Thymectomy is more effective in patients with MG in the absence of a thymoma, with a clinical remission rate of approximately 35% and improvement in another 50%. Neurologic improvement is less likely when MG is associated with thymoma. Thymectomy in patients with pure red cell aplasia results in a 40% to 50% remission rate, in contrast to those with hypogammaglobulinemia, who do not benefit from thymic resection.

Thymic Carcinoma

Thymic carcinomas are a heterogeneous group of aggressive epithelial malignancies with a tendency for early local invasion and distant metastases. Men are more often affected (mean age, 46 years). The Revised 2004 WHO classification of thymic epithelial neoplasms describes thymic carcinomas as “nonorganotypic” malignancies (type C) that resemble carcinomas of extrathymic origin, such as lung carcinomas. By traditional histopathologic classification, the two most common cell types are squamous cell and lymphoepithelial-like carcinoma. Unlike thymoma, thymic carcinoma has malignant histologic features and typically manifests as a large, poorly defined, infiltrative, anterior mediastinal mass, frequently with regional lymph node and pulmonary metastases, and pleural and/or pericardial effusions (Figure 71-4).

Figure 71-4 Unenhanced chest CT image (mediastinal window) of patient with thymic carcinoma. The heterogeneous left anterior thymic carcinoma extends across the midline of the chest and exhibits multi-cystic change (arrow) and an eccentric mural soft tissue component (open arrow).

The histologic grade and neoplastic staging determine the therapy and prognosis for the patient with thymic carcinoma. Surgical resection, when feasible, is the preferred treatment. Adjuvant cisplatin-based chemotherapy and concurrent radiotherapy may be used. Response rates are generally poor, with 5-year survival rates of approximately 30%.

Thymic Carcinoid

Thymic carcinoid is a rare, aggressive neuroendocrine neoplasm that typically affects men in the fourth and fifth decades of life. These are large, symptomatic, locally invasive neoplasms with frequent distant metastases. Approximately 35% of affected patients develop endocrine abnormalities caused by ectopic hormone production, most often Cushing syndrome. Multiple endocrine neoplasia syndrome type I (MEN-I) may also occur. However, the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and the carcinoid syndrome are rare. Radiologically, thymic carcinoid is a large, lobular, heterogeneous, and usually invasive anterior mediastinal mass indistinguishable from thymic carcinoma or thymoma (Figure 71-5). There may be associated intrathoracic metastatic lymphadenopathy. Surgical resection, when feasible, is the treatment of choice, and response to chemotherapy and radiotherapy is poor.

Figure 71-5 Thymic carcinoid. A, Posteroanterior chest radiograph; B, contrast-enhanced chest CT (mediastinal window). The 6-cm, discrete, rounded left anterior mediastinal thymic carcinoid (arrow) exhibits central low attenuation and circumferential mural enhancement. C, Fluorodeoxyglucose positron emission tomography (FDG-PET) scan. Multiple FDG-avid foci within the cyst wall (arrows) correspond with malignancy.

Mediastinal Lymphoma

Lymphoma represents 10% to 20% of all mediastinal neoplasms in adults. Approximately two thirds of all lymphomas are non-Hodgkin lymphoma (NHL). Mediastinal involvement typically denotes systemic lymphoma but may also represent primary disease. Although both Hodgkin disease (HD) and NHL may affect the mediastinum, mediastinal involvement from systemic lymphoma is much more common in HD than in NHL. Nodular sclerosis is the most common subtype of HD to affect the mediastinum. Diffuse, large B-cell lymphoma and lymphoblastic lymphoma represent the most common primary mediastinal subtypes of NHL. Patients affected by NHL have a higher incidence of systemic symptoms and concomitant involvement of extrathoracic and extranodal lymphoid tissue. PET-CT is superior to CT alone in depicting the extent of disease involvement and plays an important role in assessing disease response to therapy and residual viable neoplasm.

Hodgkin Disease

Hodgkin disease is most often seen in adults age 20 to 30 and in those older than 50. Even though the nodular sclerosis subtype is more common in women, HD in general does not exhibit a gender predilection in young patients. Patients who have mediastinal lymphoma tend to be younger. The usual presenting finding is cervical and supraclavicular lymphadenopathy. In less than 25% of patients, HD is limited to the thorax. Approximately one third of patients have systemic symptoms. Patients with mediastinal involvement are generally asymptomatic, although bulky lymphadenopathy may induce symptoms related to mediastinal compression.

Radiologically, most patients with HD have bilateral, asymmetric, mediastinal lymphadenopathy, which frequently involves the prevascular and paratracheal lymph nodes and rarely the posterior mediastinal or juxtacardiac lymph nodes. Nodular sclerosis HD can manifest as a discrete, unilateral anterior mass (Figure 71-6) or bilateral mediastinal mass (Figure 71-7) or as large, lobular, coalescent lymphadenopathy in the anterior mediastinum (Figure 71-8). Invasion, compression, and displacement of mediastinal structures, lung, pleura, and chest wall may occur. Affected lymph nodes usually exhibit homogeneous attenuation but may be heterogeneous because of hemorrhage, necrosis, or cystic change. De novo lymph node calcification rarely occurs but may develop 1 to 5 years after radiation therapy. Direct invasion of the lung occurs in 8% to 14% of untreated patients and is usually associated with hilar lymphadenopathy. The diagnosis is established with either core biopsy or surgical excision of affected palpable lymph nodes or masses found on imaging.

Figure 71-6 Primary mediastinal Hodgkin lymphoma in 45-year-old man. A, Posteroanterior chest radiograph; B, contrast-enhanced chest CT (mediastinal window). Hodgkin lymphoma manifests as a smooth, homogeneous, right anterior mediastinal mass (arrow), without lymphadenopathy.

Figure 71-7 Mediastinal mass in Hodgkin disease (HD). A, Posteroanterior chest radiograph; B, contrast-enhanced chest CT (mediastinal window). Homogeneous central anterior mediastinal mass (arrows) extends across the midline bilaterally in woman with HD.

Figure 71-8 Contrast-enhanced chest CT (mediastinal window) of patient with mediastinal Hodgkin disease. A dominant right anterior mediastinal nodal mass (arrow) has associated enlarged bilateral contiguous mediastinal nodal groups (open arrows).