Thymic rebound hyperplasia is considered a form of true thymic hyperplasia.
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Pathology
In periods of bodily stress the thymus may acutely shrink to 40% of its original volume (depending on the severity and duration of the stress). During the recovery phase it can grow back to its original size or even larger (up to 50% larger). This "rebound effect" is known as thymic rebound hyperplasia. It typically takes the thymus 9 months to return to its original size 1.
Examples of acute stress events that can trigger thymic atrophy and later rebound include:
pneumonia
corticosteroid therapy
radiation therapy
chemotherapy
surgery
burns
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post-chemotherapy
10-25% will undergo rebound hyperplasia, usually occurring within 2 years of treatment 2
Rebound hyperplasia is typically seen in children, but can also occur in adults 2.
Radiographic features
CT
Thymic rebound hyperplasia typically shows diffuse enlargement, a fine mixture of fat and lymphoid tissue, a smooth contour, and normal vessels 2.
PET
The normal thymus is typically barely visible on PET. However, in rebound hyperplasia it show intense FDG uptake. This can lead to confusion, and misdiagnosis of lymphoma 3.