Thymus
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surface of the thymus is pink, but by adulthood much of the parenchyma has been
replaced by yellow fat.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?
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