Thymic rebound
Summary
Benign thymic regrowth after systemic stress (classically post-chemotherapy) that can mimic anterior mediastinal relapse.
When it happens
- Commonest in children and young adults (can occur in adults)
- Triggers: chemotherapy, steroid withdrawal, major illness/surgery, radiotherapy
- Timing: often months after treatment (variable)
Imaging features
CXR
- Smooth anterior mediastinal fullness
- Usually no aggressive mass effect
CT
- Diffuse, symmetric soft tissue in thymic bed
- Preserves normal thymic shape (triangular/quadrilateral), smooth margins
- No invasion: no vascular encasement, no destructive features
- Fat content can be mixed (age dependent)
MRI
- Chemical shift: signal drop on opposed-phase supports hyperplasia (microscopic fat)
- DWI: thymus can show restriction even when benign, do not overcall on DWI alone
FDG PET/CT
- Mild to moderate, fairly diffuse thymic uptake
- Focal very intense uptake or new extra-thymic lesions shifts concern to relapse
Warning
Red flags: nodular thymus, progressive enlargement, very focal intense FDG, or new disease elsewhere. In these, do not label "rebound" as definite.
