Benign thymic pathologies such as hyperplasia or thymic cysts are rarely studied. In the published literature there are few studies focused on the clinical and radiological manifestations of benign entities of the thymus. Usually they are grouped as benign thymic disorders .
Thymic cysts are relatively rare pathology that can be seen in any age group. They represent 1–3% of all mediastinal masses . They may be congenital or acquired [4, 5]. The congenital cyst can be found anywhere along the thymopharyngeal duct . They usually unilocular, have a thin wall, contain clear fluid and appears in the pediatric age group . The acquired cyst, may develop after chemotherapy, radiotherapy, HIV infection, thoracotomy, or associated with thymic tumors that disrupt and compress normal thymic tissue. [4, 5]. These are multilocular contain proteinaceous or hemorrhagic material which results in soft-tissue attenuation of the mass and can create a diagnostic dilemma .
The most common mediastinal cysts are the bronchogenic cyst, which usually occurs in the posterior and middle mediastinum. They account for 10 to 15% of all primary mediastinal masses . The identification of bronchogenic cyst has increased because of more advanced diagnostic methods, and regular medical checkups were performed in recent years . The site of the bronchogenic cyst determined by the embryological period of growth at which the abnormality occurs. When the abnormal buddings happens in the course of the early growth, the cyst apt to be located along the tracheobronchial tree. Cysts that developed later are more peripheral and may be sited within the lung parenchyma. . The majority of the cysts are mediastinal bronchogenic cysts and closely related to the tracheobronchial tree near the tracheal carina [1, 4, 7].
Infrequently, the cysts have occurred in rare sites, including skin and subcutaneous tissues, neck, pericardium, diaphragm, mesentery of the bowel, and the intramedullary part of the spine. They have also been reported to extend from the mediastinum through the diaphragm into the abdomen as dumbbell cysts .
Rarely, bronchogenic cyst can occur within the thymus gland. The first reported case was published by Taniwaki and colleagues in 1997 . After which few cases were also reported. These cysts are asymptomatic and discovered incidentally in imaging studies done for another reasons. Usually they are of very little clinical importance, and can go undetected except if they produce compressive symptoms [4, 8]. Multi-slice CT is used to delineate the cyst shape, size, location, attenuation and relation to the adjacent structures. Nevertheless, typical imaging characteristics can only be seen in 10–40% of patients . Soft-tissue attenuation on chest CT caused by protein-rich cyst fluid can make the preoperative differentiation from solid neoplasm mainly thymoma very difficult. This has been reported in 40–63% of the reported cases [4, 6]. MRI can be beneficial in demonstrating the cystic nature of what appeared to have a soft-tissue attenuation on the CT . Unless there is no accompanied inflammation in the bronchogenic cyst or underlying intrathymic pathology like hyperplasia or invasive tumor the intrathymic bronchogenic cyst has no fludeoxyglucose (FDG) uptake on the (PET/CT) .
The definitive diagnosis of intrathymic bronchogenic cyst is usually made after complete excision of the lesion by illustration of the histological characteristics of the bronchogenic cyst like ciliated columnar epithelium with or without infiltration by chronic inflammatory cells, presence of cartilage, smooth muscle, bronchial glands and nerve tissue .