Types Of Hyperplasia
There are two forms of thymic hyperplasia:
1- True thymic hyperplasia (or rebound hyperplasia), in which there are normal proportions of glandular/ lymphoid elements in an enlarged gland secondary to rebound following stress-related atrophy, illness, or immune suppression (following chemotherapy or discontinuation of corticosteroid administration) [15,19]. When the body is exposed to stress the thymus may shrink to as little as 40% of its original volume (depending on the severity and duration of the stress) [20]. Once the body recovers, the thymus can usually grows back to its original size within 9 months, but it can grow to be as much as 50% larger [20]. Approximately 10-25% of patient who undergo chemotherapy may develop rebound hyperplasia [20]. Most cases of thymic hyperplasia secondary to chemotherapy occur within a year or two and the gland typically returns to normal size [8,20]. Because of bolstered cell-mediated immunity, thymic hyperplasia has been associated with a good prognosis in post-chemotherapy patients [19]. Thymic hyperplasia has also been described in patients with endocrine abnormalities such as Graves' disease, acromegaly, and thyrotoxicosis [8], as well as during recovery from burn injuries, in patients with infections, and after cardiovascular surgery [19]. On CT the gland may be normal sized or there may be symmetric enlargement of the gland with preservation of its normal shape and no discrete mass. The borders of the enlarged thymus are most often concave, but occasionally the gland may have a convex margin and appear oval. Streaks of fat should be identified and the gland should maintain a bilobed appearance.
2- Thymic lymphoid follicular hyperplasia:
Thymic lymphoid follicular hyperplasia is characterized by the presence of an increased number of hyperplastic lymphoid follicles and germinal centers in the thymic medulla that is associated with lymphocytic and plasma cell infiltration [20,22]. It is more common than true hyperplasia and is associated with myasthenia gravis [20]. In fact, lymphoid hyperplasia is seen in 65-85% of patients with myasthenia. About 10-15% of myasthenia patients are discovered to have a thymoma, and about 20% have a normal or involuted thymus. Following thymectomy, a clinical remission occurs in 35% of myasthenia patients with lymphoid follicular hyperplasia, and 30-50% will demonstrate clinical improvement [8]. However, even myasthenia patients with a normal thymus may improve clinically following thymectomy (up to two-thirds of such patients). Thymic lymphoid hyperplasia is also associated with collagen vascular disorders such as scleroderma, rheumatoid arthritis, and SLE [15]; as well as endocrinopathies such as Graves disease [15], Addisons disease, acromegaly, and the early stages of human immunodeficiency virus infection [20].
On CT scan, thymic lymphoid hyperplasia can appear as an anterior mediastinal mass (20%), as a diffusely enlarged thymic gland (35%), or as a normal thymus (45%) [20]. It has been suggested that the non-contrast attenuation of the thymus is significantly higher in patients with lymphoid hyperplasia than in true hyperplasia (HU greater than 41.2) [22]. Note: The use of iodinated contrast agents in patients with myasthenia gravis has been reported to cause increased weakness. Although not contraindicated, intravenous contrast agents should not be used routinely in the assessment of these patients. [1]
Thallium may be beneficial in differentiating thymic hyperplasia from other thymic lesions. Normally, no thallium uptake should be seen in the mediastinum which appears photon deficient against the normal faint pulmonary activity. Therefore- the normal thymus is usually gallium avid, but will be negative on the thallium scan.
MR chemical shift imaging can also be used to differentiate thymic hyperplasia from other thymic lesions [18,24]. Signal loss on out-of-phase images compare to in-phase images suggests the presence of fat and thymic hyperplasia [18,22,24]. However, it should be noted that the amount of fat tissue within the thymus increases with age [24].
FDG PET imaging is not useful for evaluation of thymic hyperplasia as the condition can be associated with FDG accumulation and there is considerable overlap with other thymic lesions [18].
REFERENCES:
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