Normal thymus: CT characteristics in subjects under age 20.

The chest computed tomographic (CT) scans of 40 subjects aged 20 or younger were analyzed retrospectively with special attention to the anterior mediastinum and the thymus. Thirty-four of the patients were considered to have normal CT appearances of the anterior mediastinum; neoplasm was suspected at this site in the other six. In the normal group, the gland was consistently identified as a characteristic anterior mediastinal structure. It had smooth, undulating, lateral contours tending to be convex laterally in the very young and concave laterally at an older age, although there was overlap in many in whom the shape of the lateral contours was a combination of convex, concave, or straight. A sharp angular contour to the lateral margin of the thymus was seen occasionally and is also thought to be characteristic of normal thymus. The posterior border of the gland molded to the heart and great vessels in all patients. The anterior contour was molded to the anterior chest wall or had a pointed shape directed toward the sternum. Molding to the mediastinum and chest wall was not found or was present to only a limited degree in the patients with anterior mediastinal tumor. Attenuation values varied between different normal glands and within the same gland on both pre- and postcontrast scans. Recognition of normal variation in the CT appearance of the thymus in young patients may prevent false-positive diagnosis of neoplasm.