Histopathologic examination revealed the neck mass was composed of multiple, coalescent expansile, nonencapsulated, and noncircumscribed sheets of dark, basophilic, neoplastic round cells, interspersed with islands and clusters of normal epithelial cells (Hassall corpuscles) within distorted, expanded thymic tissue (Fig 3A).
Results of immunohistochemistry confirmed that the cervical mass sections were thymic in origin, with Hassall corpuscles providing an internal positive control, by showing strong positive cytoplasmic immunoreactivity for A1/AE cytokeratin.
The cyst wall contained abundant adipose tissue intermixed with atrophic thymic tissue composed of lymphoid cells, epithelial cords, and Hassall corpuscles. A small parathyroid gland was found in the cyst wall.
The congenital theory is based on the hypothesis that cervical thymic cysts develop from the persistence of the thymopharyngeal duct, while the acquired theory is based on the hypothesis that cervical thymic cysts develop as a result of the degeneration of Hassall corpuscles within remnants of ectopic thymic tissue.
The presence of thymic parenchyma, lymphoid tissue, and Hassall corpuscles within the adipose cyst wall is considered to be pathognomonic.
Interestingly, sections from the presumed parathyroid gland turned out to also contain ectopic thymic tissue with distinct cortex, medulla, and Hassall corpuscles
. A unique image of thyroid, parathyroid, and thymus is depicted in one field at X10 magnification (Figure).
Histology identified Hassall corpuscles, and on that basis, a diagnosis of a thymic cyst was made.
Lobules of thymic tissue containing Hassall corpuscles were seen in the cyst wall.
In addition, several nests of cytokeratin-positive epithelial cells that formed Hassall corpuscles were scattered throughout the lesion (Figure 2).
Lymphocyte invasion occurs at 10 weeks of gestation, whereas subsequent endodermal regressions form the Hassall corpuscles. Growth and development of the thymus continue after birth until puberty.
Hassall corpuscles of the thymic medulla were evident centrally within lobules.
The latter are recognizable by the presence of Hassall corpuscles and lymphocytes.