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Thymoma: Current Concepts

Thymoma: Current Concepts

Thymomas are unusual tumors, representing no more than 1% of all malignancies. However, thymomas are the most common epithelial tumors of the anterior mediastinum. Unfortunately, there is no general agreement regarding the best parameters to use to predict clinical behavior in these tumors. This review considers the status of the different histological classifications thus far presented for thymomas and offers an analysis of the association between histology and clinical behavior. It also emphasizes the importance of proper staging of thymomas, delineating the benefits and shortcomings of different proposed staging systems and offering thoughts on a better and more accurate staging stratification for patients with these tumors. All of the different parameters are presented in relation to survival rates. Based on current information, staging with proper stratification remains the most important parameter for predicting prognosis. For tumors limited to the mediastinal compartment, surgical resection is the most effective treatment, while induction therapy is a good alternative for patients in whom surgical resection is not possible.

Introduction

Thymomas are among the most common tumors of the anterior mediastinum. However, considered in the context of the entire spectrum of malignancies, thymomas are unusual neoplasms. It is important to note that thymoma does not represent the benign counterpart of thymic carcinoma, nor is thymic carcinoma the malignant counterpart of thymoma. These two neoplasms, thymoma and thymic carcinoma, represent two different clinicopathological entities, which have been shown to be associated with different clinical outcomes. It is also important to point out that these two conditions may be treated differently depending on the clinical circumstances at the time of diagnosis.

Over the last two decades, controversy has arisen over several views that have been presented on the histological classification, staging, treatment, and clinical behavior of thymomas. As more knowledge is gathered from the different specialties involved in the treatment of patients with thymomas, some views on this subject are likely to be upheld while others will not be, with the result that patients with this neoplasm will receive better care. However, it is important to note that due to the rarity of thymomas, a concentrated experience with a large number of cases is difficult to assemble.

Clinical Aspects

Although the symptomatology that patients with thymoma exhibit on presentation is varied, it is worthwhile highlighting the association of thymomas with paraneoplastic syndromes. Perhaps the most important of these syndromes, or at least one that appears to be among those most frequently associated with thymoma, is myasthenia gravis. Approximately 10% to 15% of patients with myasthenia gravis have thymoma; conversely, about 40% of patients with thymoma have an associated paraneoplastic syndrome, of which myasthenia gravis is among the most common. Other paraneoplastic syndromes seen in patients with thymoma include limbic encephalitis, neuromyotonia, stiff person syndrome, myotonic dystrophy, and polymyositis.[1] Thymoma patients may also have a wide spectrum of associated non-neurologic conditions, including myocarditis, graft-versus-host disease, hypogammaglobulinemia, pure red cell aplasia, intestinal pseudo-obstruction, mixed connective tissue disease, thyroiditis, and many more.[1]The prognosis of patients with thymoma is closely linked to the underlying condition.

Patients affected with thymoma may also present with signs and symptoms unrelated to any underlying condition, including such nonspecific symptoms as chest pain, cough, dyspnea, and other symptoms of mediastinal compression.When symptoms such as these are seen, the initial study to use in evaluation of the patient is chest radiography. Chest imaging will disclose the presence of an anterior mediastinal mass, which can be further analyzed using more advanced studies, such as chest CT, a procedure that provides more specific information regarding the circumscription or invasive nature of the neoplasm.[2]

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