The patient in this case was eventually diagnosed with thymoma. Symptoms from a thymoma or thymic carcinoma may be due to the presence of a tumor in the mediastinum or they may be a manifestation of a paraneoplastic syndrome. As many as one half of thymomas are diagnosed incidentally on the basis of a radiographic abnormality in an asymptomatic patient. Clinical signs and symptoms are related to both the size of the tumor and its effects on adjacent organs (eg, chest pain, shortness of breath, cough, phrenic nerve palsy, and superior vena cava obstruction). Less commonly, systemic ("B") symptoms, including fever, weight loss, and/or night sweats, may be present. Pleural or pericardial effusions are the most common manifestation of metastatic involvement. Extrathoracic metastases occur in less than 7% of patients, most commonly to the kidney, extrathoracic lymph nodes, liver, brain, adrenals, thyroid, and bone. Metastases to the ipsilateral lung are unusual.
For patients who present with mediastinal masses, an extensive differential should come to mind. Anterior mediastinal masses include germ cell tumors, lymphoma, thymic carcinoma, and masses arising from the thyroid. Tumors that can arise directly from the thymus include thymoma, lymphoma, carcinoid tumors, thymolipoma, and thymic carcinoma.[1,2] Thymomas account for about 20% of mediastinal neoplasms. Most patients are between age 40 and 80 years; thymomas are more common in males, Asians, and Pacific Islanders. No risk factors have been conclusively identified. Thymic carcinomas account for less than 1% of thymic malignancies. Malignant thymoma is exceptionally rare, with an overall incidence of 0.15 per 100,000 person-years.[4,5]
Thymoma is an uncommon tumor that is best known for its association with the neuromuscular disorder myasthenia gravis. Indeed, approximately 70% of symptomatic patients with thymoma have an associated systemic syndrome, such as myasthenia gravis (30-50%), hypogammaglobulinemia, red cell aplasia, dermatomyositis, systemic lupus erythematosus, Cushing syndrome, or syndrome of inappropriate antidiuretic hormone secretion.[6,7]
Differentiating thymoma from the more aggressive thymic carcinoma is extremely important. Thymic carcinoma exhibits aggressive cytologic features with evidence of mediastinal invasion in the majority of patients. Histologically, thymic carcinomas exhibit cellular atypia, increased proliferative capacity, and anaplastic features. Radiographically, thymomas are more likely to exhibit smooth contours and a round shape on CT scans (Figure 1) than thymic carcinomas, which more commonly exhibit irregular contours.
The histologic diagnosis of thymic neoplasms can be difficult. A continuum of differentiation from thymoma to thymic carcinoma is apparent, and primary thymic epithelial neoplasms can have features of both. Carcinoma and thymoma can occur synchronously or carcinoma can develop within a preexisting thymoma after an interval of years. The World Health Organization (WHO) system is widely used to classify thymomas into 6 types, on the basis of histologic differences. Two main systems for staging thymomas are the Masaoka staging system and the French Groupe d'Etudes des Tumeurs Thymiques (GETT) system. Staging by the Masaoka system has correlated well with overall 5-year survival rates in several large series.
Paraneoplastic autoimmune syndromes associated with thymoma include myasthenia gravis, polymyositis, systemic lupus erythematosus, rheumatoid arthritis, thyroiditis, and Sjögren syndrome, among others. Several case reports have described a syndrome of thymoma-associated multiorgan autoimmunity that is similar to graft-vs-host disease. Patients present with variable combinations of a morbilliform skin eruption, chronic diarrhea, and liver enzyme abnormalities. Histopathology of the skin or bowel mucosa is similar to that seen with graft-vs-host disease. Autoimmune pure red cell aplasia and hypogammaglobulinemia affect approximately 5% and 5%-10% of patients with thymoma, respectively. Thymoma-associated autoimmune disease involves an alteration in circulating T-cell subsets.
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