Thymic Involution

Axial CT image shows a normal-appearing thymus (arrows) with minimal soft tissue strands and mostly fat replacement of the gland. The gland has concaved margins.

Facts: Thymic Involution

• Thymus undergoes a gradual loss of cortical lymphotcytes and atrophy of epithelial cells and replacement by fat
• Believed to start at puberty but new data suggest that it actually starts from the first few years of life
• By the age of 40 to 45 years, more than 50% of thymus is replaced by adipose tissue
• Speed of involution increased with stress and other factors

Imaging

• Normal/involuted thymus: normal-shaped, flat or concaved margin, fat replacement increases with age and by the age of 40 the thymus is usually mostly fatty
• Thymic hyperplasia: convex margin, diffusely enlarged gland but normal shape, homogeneous attenuation similar to normal thymic tissue
• Thymoma: homogeneous, well-defined soft tissue mass, calcification, homogeneous enhancement

Our case-- thymic involution in a patient with myasthenia gravis, confirmed at surgery.

Reference:

1. Bogot NR, Quint LE. Imaging of thymic disorders. Cancer Imaging 2005; 5:139-149.

2. Greer JP, et al. Wintrobe's Clinical Hematology, 12th ed, 2009.