**3.3 Undescended thymus**

*Thymus*

**2. Development of the thymus**

immune system [12].

**3.1 Thymic cyst**

**3.2 Ectopic cervical thymus**

The thymus, a retrosternal lymphoid tissue, develops from the third and fourth pharyngeal pouches like the parathyroid glands at the sixth week of gestational age [7–10]. During the next weeks of development, the thymus migrates through a path, called thymopharyngeal duct, to its final destiny which is the anterior mediastinum [11]. The liver and bone marrow are the primary organs responsible for production of lymphoid cells. These cells migrate to the thymus gland which results

The thymus is usually located in the anterior mediastinum, although it can be found anywhere in the thymopharyngeal path. The thymus has a bilobed or quadrilateral shape; however, it can be found in other shapes. The thymus is commonly found in chest radiographs of infants and children as a large mediastinal mass. During adolescence, the thymus encounters a fibrofatty change as the age increases. Growth of the thymus continues from birth to 2–3 years of age, when it reaches its highest weight, while sex hormones make the thymus smaller during adolescence. Appropriate function of the thymus in childhood guarantees the condition of the

Limited information is available about the precise epidemiology of thymic congenital anomalies. Since these anomalies are not symptomatic, it may be more common than the available reports [13–15]. There are various reports available about the prevalence of thymic diseases and anomalies ranging from 4.45 to 30% [16–18]. It could be concluded that thymic anomalies are common, but their symptomatic manifestation is uncommon. According to the previous studies, thymic congenital anomalies and diseases are three times more prevalent in men than women [13]. Also it has been reported that two thirds of these lesions are usually found in the

As one of the uncommon lesions of the thymus gland, thymic cysts may be seen in various age groups. Congenital forms of thymic cysts can be found anywhere along with the thymopharyngeal path. In addition, thymic cysts may be developed following thoracotomy or chemotherapy [20]. In imaging, these lesions have a thin wall with no solid components and do not enhance with intravenous contrast administration. These cysts may contain protein or hemorrhagic fluid. On histologic examination, the wall of cyst is lined by the columnar or stratified epithelium.

The exact incidence of ectopic cervical thymus remains unclear because of the asymptomatic nature of these masses. Cervical thymus is usually detected incidentally [21, 22]. Ultrasonography is the choice method for imaging especially in children requiring no contrast or sedation. Echo characteristics of an aberrant cervical thymus are easily defined by ultrasonography. Ultrasonic features of cervical thymus, echogenic linear structures surrounded by hypoechoic rims, are similar to those of the mediastinal thymus [23, 24]. In cases with large ectopic thymus, diagnosis is more challenging where fine needle aspiration cytology may

in differentiation of the thymus into a cortex and medulla [7].

**3. Epidemiology of congenital anomalies of the thymus**

first decade of life [19]; the oldest reported age of presentation is 71.

**70**

be helpful.

As a rare lesion, undescended thymus is usually presented as a midline neck swelling in a child. Thyroglossal duct cyst, thyroid or parathyroid lesions, and cystic hygroma or cystic teratoma are among other differential diagnoses. A variety of imaging modalities are useful for diagnosing undescended thymus such as MRI, nuclear scan (Gallium 61), computed tomography, ultrasonography, or conventional radiography [25].
