Thymic Rebound Hyperplasia (“Thymic Rebound”)
This phenomenon, seen in up to 25% of patients after chemotherapy (and other stresses, including infection, surgery and burns), is not infrequently mistaken for malignancy (active lymphoma, metastatic adenopathy or primary thymic cancer) by the inexperienced radiologist.
During chemotherapy, the thymus may shrink to 2/3 of its normal size. After chemotherapy, the thymus usually returns to its normal size.
About 25% of patients, however, will experience a “rebound” overgrowth (hyperplasia) of the gland — up to 50% greater than its normal size. This enlarged gland frequently demonstrates significant metabolic activity, often intensely FDG-avid.
The enlarged gland should largely maintain its normal morphology (triangular configuration, with straight or concave lateral margins), without significant mass effect on the lateral borders of the anterior mediastinum.
Thymic rebound hyperplasia typically occurs within the first 9 months after chemotherapy, but has been reported up to five years after treatment.
Diagnostic Criteria:
- Requires history of chemotherapy (or similar stress).
- Enlarged gland largely maintains its normal morphology (triangular configuration, with straight or concave lateral margins), without significant mass effect on the lateral borders of the anterior mediastinum (if irregularly enlarged or causes mass effect on the mediastinal borders, the possibility of malignancy must be considered).
- Don’t let a diagnosis of thymic rebound be deterred by intense metabolic activity
