The histological properties and possible origin of cervical thymus with cysts - A case report and hypotheses about its development

Ectopic accessory thymic tissue usually presents as an asymptomatic neck mass found at any level corresponding to the embryonic descent of the thymus. This tissue may contain smaller or larger cysts. However, the exact pathogenesis of "enigmatic" cervical thymic cysts remains controversial. A 7-year-old boy was referred to our workplace for the evaluation of a cervical mass. An ultrasound suggested a multi-loculated cystic mass, while CT and MRI indicated a left-sided, anteriorly located cervical mass beneath the sternocleidomastoid muscle. Following the radiological findings, surgical excision revealed a cystic mass. The mass of tissue was covered by a capsule. In H&E staining, the cervical mass had the same structure as normal thymus. Additionally, immunohistochemical findings suggest that the cellular microenvironment of cervical thymus also displays a place for development of T-lymphocytes. Within the parenchyma multiple cysts lined with cytokeratin-positive thymic epithelial cells were found. Inside the cysts, there were CD68-positive multinucleated giant cells and cholesterol clefts. A tendency to cystic degeneration inside the thymic tissue occurs more often in cervical thymuses than in normally located ones. The reason for the formation of cysts is unknown. We summarized seven possible histological, embryological and evolutional backgrounds for the development of these thymic cysts.