CERVICAL THYMIC CYST
The thymus begins its development during the sixth week of embryonic life as outpouchings from the ventral aspects of the third pharyngeal pouches. The dorsal aspects of the third pharyngeal pouches become the inferior parathyroid glands. These dorsal portions elongate into tubular structures called the thymopharyngeal ducts. The fourth pharyngeal pouches develop into the superior parathyroid glands but may also contribute partially to thymic development. As the thymic primordia migrate inferiorly from the future area of the pyriform sinuses along the course of the carotid sheaths, the proximal portions of the thymopharyngeal ducts atrophy and are obliterated. The pharyngeal attachments of the ducts are usually severed by seven weeks gestation. By the eighth week, the thymic primordia fuse in the midline of neck before descending into the superior mediastinum. In the third month of gestation, the thymic epithelium becomes surrounded by mesoderm and accumulates lymphocytes. The epithelium forms Hassall's corpuscle and then disappears.
Disturbance of this process of thymic development and migration can result in aberrant thymic tissue in the cervical region or in other locations such as the mediastinum, skull base, or tracheal bifurcation. In the cervical region, ectopic thymic tissue can arise anywhere along the line of usual descent from the angle of the mandible to the superior mediastinum. It can present as a solid or cystic mass as seen by imaging of the neck. Sporadic reports of solid or cystic ectopic cervical thyroid tissue can be found dating as far back as the early twentieth century; however, most reports consist of only a few cases. The incidence of this entity is quite low, causing it to often be low in (or absent from) the differential diagnosis of patients being evaluated for neck masses. Additionally, a pre-operative diagnosis of ectopic thymus or cervical thymic cyst is rarely established prior to resection. Fine needle aspiration will often yield only small, mature lymphocytes and is usually inadequate to make a definitive diagnosis of thymic tissue as opposed to a low-grade lymphoproliferative process. Entities which are more commonly considered in the clinical and radiographic work-up of these lesions include branchial cleft cyst, cervical lymphangioma, and cervical teratoma.
Ectopic thymic tissue often undergoes hyperplasia in the first decade of life, usually in response to infection or vaccination. For this reason, cervical thymic masses are largely diagnosed in the pediatric population. A small proportion of cases are diagnosed in the second or third decade of life. Some studies have shown a slight male predilection and an increased incidence of left-sided lesions; however, other studies have not supported these findings. Patients with ectopic cervical thymic tissue or cervical thymic cysts are often asymptomatic; however, patients may present with stridor or dysphagia due to compression of the trachea or pharyngeal space, respectively. Symptomatic cases are more often seen in the neonates as opposed to older children. Grossly, cervical thymic cysts can be unilocular or multilocular and may be filled with clear, serous fluid or thicker brown fluid if hemorrhage has occurred. Specks of cholesterol may be evident. The cyst walls can vary from thin and fatty to thick and fibrous. The inner wall can be smooth or trabeculated. The histologic diagnosis of a cervical thymic cyst is often not difficult; however, the diagnosis needs to be kept in mind since the entity is rare and a pre-operative clinical diagnosis is often not established. The cystic spaces may be lined by cuboidal, columnar, or stratified squamous epithelium. The epithelium may be focally replaced by fibrous or granulation tissue with cholesterol clefts and giant cell reaction. In order to make the diagnosis of cervical thymic cyst, thymic tissue must be identified within the cyst wall.
In mediastinal thymic cysts, one must be certain to rule out thymoma with cystic degeneration, which may be difficult to discriminate from a benign cyst. However, malignant degeneration of cervical thymic cysts has never been reported, and no recurrences have been documented following complete excision. In conclusion, cervical thymic cyst is a rare entity that presents as a unilateral neck mass in a pediatric patient. Its behavior is uniformly benign, and complete resection is curative. Due to the very low incidence (or possibly a lack of documentation in literature), cervical thymic cysts are almost never considered in the differential diagnosis of a patient with a neck mass, and they are only rarely definitively diagnosed pre-operatively. Therefore, the pathologist must keep this entity in mind when evaluating resection specimens from patients with undiagnosed neck lesions.
Contributed by James T. Edinger, MD and Sarangarajan Ranganathan, MD