Thymic cyst: A clinicopathological, immunohistochemical, and molecular study of 36 cases focusing on squamous- and ciliated-type epithelium

BACKGROUND: Thymic cysts are benign mediastinal lesions lined by various epithelial types, but the distribution of each type is unknown. In this study, we aimed to evaluate the clinicopathological and immunohistochemical features of thymic cysts. DESIGN: We retrieved 36 resected primary thymic cysts. The lesions were initially evaluated morphologically, focusing on the type of epithelium. Immunohistochemical staining was performed for cytokeratin 7 (CK7), CK13, CK20, high molecular weight cytokeratin (HMWCK: CK34βE12), p63 and β5t. In addition, we investigated 15 occult cystic lesions (OCLs) associated with thymic epithelial tumors. RESULTS: Primary thymic cysts were classified as squamous/cuboidal (SC-type, N = 18), ciliated columnar (CC-type, N = 15), mucinous (N = 2), and mixed (N = 1) types. All cysts were CK7-positive. Linear-luminal pattern CK7 staining was observed more frequently in SC-type cysts (100%) than in CC-type cysts (40%) (P < 0.0001). CK20 was negative in all the cases except for the two mucinous cysts. Full-layer positivity for HMWCK was observed more frequently in SC-type cysts (78%) than in CC-type cysts (6.7%) (P < 0.0001). SC-type cysts were more frequently multilocular than CC-type (10/18, 55.6% vs 1/15, 6.7%; P = 0.0083). CK13 positivity was observed in 70.6% of SC-type cysts and 21.4% of CC-type cysts (P = 0.0113). Radiological findings showed that SC-type cysts were larger than CC-type cysts (P = 0.015), and the computed tomography (CT) values of SC-type cysts were markedly lower than those of CC-type cysts (P < 0.0001). The majority of OCLs were classified as SC-type (13/15). CONCLUSIONS: Based on morphological features, thymic cysts mainly comprise two types, SC-type and CC-type, which differ in their sizes, locularity, immunophenotypes, and CT features.