Thymoma is a neoplasm of the thymus gland, which normally resides immediately behind the upper sternum, but may slip to either the left and/or the right, as far down as resting along the cardiac border along the diaphragm. Disorders of the thymus may be benign or malignant, and, where malignant may involve the epithelial cells of the thymus or any one of any other cell type normally found in the thymus gland. Malignant degeneration ranges from malignant degeneration of the cells, encroaching to and through the capsule of the gland, to possibly involving adjacent organs, or even spreading through "drop metastases" to rest elsewhere along the pleural cavities and/or diaphragm and/or adjacent lung and heart tissues. Certainly, of thymic malignancies, the disorders characterized as thymic carcinomas and sarcomas are among the most aggressive neoplasms seen.
Thymomas originate from the epithelial cell population in the thymus, historically characterized by Bernatz and others from the more differentiated cell types to the more bizarre and dedifferentiated histology, along a classification ranging from "lymphocyte predominant, to mixed lymphocytic and epithelial cells, to epithelial cell predominance, and the most dedifferentiated (more likely to be associated with malignancy) of a spindle cell predominance." While it is not fully understood by any means why patients develop thymomas, the association of thymomas with dysregulation of the thymus gland and association with autoimmune and immunodeficiency disorders is well understood. Patients with thymomas may present with myasthenia - as many as fifty percent of patients with thymoma have myasthenia gravis. Of course, it is well known that only fifteen percent of patients with myasthenia gravis harbor thymoma. Other immunologic associations are well known, including thymoma’s association with Lambert-Eaton syndrome, an autoimmune disease characterized by muscle strength reduced at rest and paradoxically improving with repetitive exercise. Additional autoimmune disorders include: subacute sensory neuronopathy, a rare disorder of painful lower extremities, red cell aplasia and other immunodeficiency disorders, such as hypogammaglobulinemia or agammaglobulinemia which may significantly improve or disappear with resection of the underlying thymoma.
The staging of thymomas is probably best delineated by the Masaoka staging classification, which is a staging system based on the extent of the malignancy within the gland capsule and relating to the surrounding structures. Stage I is a completely encapsulated tumor with no microscopic capsular invasion or involvement. Stage II tumors invade into the capsule or the surrounding fatty tissue, mediastinal pleura, or both. Stage III thymomas invade neighboring organs, such as pericardium, lung, or great vessels. Stage IV tumors show significant dissemination either by "drop metastases" - stage IV-A - where there is pleural or pericardial dissemination - or (stage IV-B) where lymphogenous or hematogenous (lymph or blood born) metastases are seen.
The clinical presentation of thymomas may range from asymptomatic masses seen on a routine chest x-ray to patients complaining of local discomfort, such as may be associated with the growth of the mass beneath the sternum, where neighboring organs may be compressed. Or Alternately, if the patient has autoimmune symptomatology, as above described above (such as myasthenia gravis), the symptoms referable to the autoimmune disorder may bring about recognition of the substernal mass.
The critical feature regarding diagnosis is that not all that resides behind the sternum arises within the thymus gland. Lymphomas and germ cell tumors, as well as infectious and inflammatory disorders, may present as anterior mediastinal masses, and as they may be better treated with chemotherapy or other medical measures first, before surgery needs be resorted to, appropriate biopsies should be performed to discriminate thymomas from other disorders that are better treated non-surgically. Of course, thymomas ultimately are best treated with surgical resection, whether primarily occurring or recurring down the road after initial resection and/or supplementary therapy.
As delineated under a separate Link - Myasthenia Gravis - thymectomy may offer significant improvement in symptoms, such that patients with myasthenia gravis may over time be able to be completely removed from medical treatment, or their medical treatment needs may be significantly downgraded in terms medication of frequency and dosages. Thymectomy, therefore, is an excellent treatment that may be recommended to patients with myasthenia gravis, with surgery the mainstay of their treatment. Surgical excision of thymoma is recommended in almost every case, barring significant extenuating circumstances, such as extreme debility and/or advanced age. Finally, medical and radiation oncologic therapies may be indicated in certain cases, depending upon the cell type involved, the extent of the tumor, and the performance status of patients.
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