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Thymic Lesion

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Overview Thymic tumors occupy the anterior mediastinum, which is immediately posterior to the sternum and the anterior surface of the pericardium and great vessels. Tumors of thymic, lymphatic, or germ cell origin most commonly occur in this compartment, although aberrant parathyroid or thyroid tissue masses are sometimes found, along with vascular and mesenchymal tissue masses. (See the images below.) Fairly large thymus in an infant.  View Media Gallery Gross appearance of a thymoma showing distinct multinodularity and scattered, focal cystic changes.  View Media Gallery Thymolymphoma. Chest radiograph shows a large, lobulated mediastinal mass.  View Media Gallery About one half of all thymic tumors are malignant in individuals aged 20-40 years, and one third are malignant in persons younger than 20 years and those older than 40 years. (A malignant thymic tumor is seen in the image below.) Malignant thymoma. Chest CT scan in a 61-year-old man with myasthenia gr
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Normal Thymus

The thymus arises from the third and fourth branchial pouches and contains elements from all three germ cell layers [15]. The gland consists of two lateral lobes placed in close contact along the midline of the upper chest [14]. The thymus increases from birth to between 4 and 8 months of age and then decreases [14]. It normally weighs about 15 gm at birth and 35 gm at puberty [14]. It is most active and largest during puberty after which it shrinks in size and activity in most people and is replaced with fat [14]. Total fatty involution of the thymus occurs around the age of 40 years when only about 5% of residual thymic tissue is retained [19]. However, the normal thymus can still be identified in up to 100% of patients under the age of 30 years, in up to 73% of patients between the ages of 30-49 years, and in up to 17% of patients older than 49 years [13].  On CXR in infants and young children the thymus is strikingly large- it usually has smooth borders and remains visible on
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Laboratory Test

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Anti–acetylcholine receptor antibody The anti–acetylcholine receptor (AChR) antibody (Ab) test is reliable for diagnosing autoimmune myasthenia gravis (MG). It is highly specific (as high as 100%, according to Padua et al).  [ 4 ]  Results are positive in as many as 90% of patients who have generalized MG but in only 50-70% of those who have only ocular MG; thus false negatives are common in cases of purely ocular MG. False-positive anti-AChR Ab test results have been reported in cases of thymoma without MG and in patients with  Lambert-Eaton myasthenic syndrome , small cell lung cancer, or rheumatoid arthritis treated with penicillamine, as well as in 1-3% of the population older than 70 years. Tindall reported AChR Ab results and mean Ab titers in a group of patients with MG (see the Table below).  [ 25 ]  Table. Prevalence and Titers of Antibody to Acetylcholine Receptor in Patients with Myasthenia Gravis  (Open Table in a new window) Osserman MG Class* Mean Anti-AC
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