The size of the thymus peaks in late childhood and begins involuting during late adolescence, ultimately demonstrating varying degrees of fatty replacement. In adults, it typically appears as a small triangular-shaped fatty density.
Normal Thymic Metabolic Activity:
- In children, diffuse thymic uptake is normal.
- In adults, the normal thymus typically demonstrates little or no FDG uptake.
- Mild diffuse thymic uptake is considered normal.
Morphology of the Thymus:
- Before considering any thymic uptake “normal”, it important that the thymus have a relatively normal CT appearance — somewhat triangular on the axial and coronal views, with straight or concave borders.
- When prominent residual thymic tissue is present, it typically demonstrates a “fluffy” appearance (a rather unprofessional descriptor, but quite illustrative).
- If the thymus is irregularly enlarged, has nodular component or is expanding the mediastinal borders (which may appear convex), the possibility of malignancy should be considered.
Superior Extension of the Thymus (Anatomic Variant):
Resulting from incomplete fetal migration, a soft tissue nodule can be seen adjacent to the left brachiocephalic vein. In cases where the thymus is hypermetabolic, this nodule may also be FDG-avid (should be of similar intensity on PET and of similar density on CT).
Thymic Rebound:
With stress (e.g. infection, surgery, burns, chemotherapy), the thymus may shrink to 2/3 its normal size. Upon recovery, the thymus typically 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 often demonstrates significant metabolic activity, often intensely FDG-avid.
The enlarged gland should largely maintain its normal triangular configuration, without significant mass effect on the antero-lateral borders of the anterior mediastinum (“Thymic Rebound Hyperplasia” is discussed in greater detail, here).
