The thymus, along with the lower pair of parathy-roid glands, is derived from the third and fourth pharyngeal pouches. The right and left halves of the gland fuse to form a pyramid-shaped organ enclosed by a thin fibrous capsule. The thymus has a vital location adjacent to the important organs of the mediastinum. The gland usually sits in the anterosuperior portion of the mediastinum, with the base of the thymus sitting on the pericar-dium and the upper poles of each lobe extending superiorly into the neck.
Your dissection should start with what is per-haps the most important step in examining the thymus—a careful examination of the surface of the specimen. Is the organ well encapsulated, or is there evidence of a tumor with invasion into adjacent structures? Are pieces of lung, pericar-dium, or blood vessels present? Document the degree of encapsulation. Next, weigh the speci-men, measure it in all three dimensions, and ink the surfaces of the gland. The gland can then be sectioned at 2- to 3-mm intervals. In infants, the cut surface of the thymus is pink, but by adulthood much of the parenchyma has been replaced by yellow fat. Describe the cut surface of the gland. Is it uniform and lobated, cystic or solid? Is there evidence of necrosis or fibrosis? Any grossly identifiable lesions should be measured, and sec-tions should be submitted from each lesion to
demonstrate the relationship of the tumor to adja-cent structures, to the inked margins, and to any attached tissues. Because invasion into adjacent organs is a critical feature used to identify malig-nant thymomas, sampling should be directed to areas suspicious for capsular invasion. Moreover, because thymomas can be histologically hetero-geneous, it has been suggested by Moran and Suster21 that a minimum of five sections should be submitted from all thymomas. If no grossly iden-tifiable lesions are noted, submit four representa-tive sections for histology. Save the specimen because you may have to go back to it, depending on the patient’s clinical history.
• What procedure was performed, and what structures/organs are present?
• What are the size and weight of the thymus?
• Is the thymus partially or totally encapsulated?
• What are the type and grade of any neo-plasms identified?
• If a tumor is present, does it extend to a margin or into adjacent structures?