The most likely diagnosis in this patient is Hodgkin lymphoma. This patient's chest radiograph shows a mass that originates from the mediastinum, given the convex angles resulting from the mass impinging on the pleura. The lateral film localizes the mass to the anterior mediastinum. Thymomas are the most common mediastinal lesion in adults and are also located in the anterior mediastinum. However, they occur more commonly in patients between the ages of 40 and 50 years old and can present with paraneoplastic syndromes such as myasthenia gravis. The second most common cause of anterior mediastinal masses is lymphoma; affected patients, such as this one, are typically younger at the time of presentation. Hodgkin lymphoma is the most common lymphoma to involve the mediastinum, followed by lymphoblastic lymphoma and primary mediastinal diffuse large B-cell lymphoma. Other causes of anterior mediastinal masses include germ cell tumors, including teratomas. However, Hodgkin lymphoma is the most common cause of anterior mediastinal masses in patients aged 20 to 30 years, such as this patient.

Bronchogenic cysts are congenital anomalies that develop in the middle mediastinal compartment and most commonly present in the second decade of life. Although they may be found incidentally as rounded lesions on imaging, they may be symptomatic if large owing to compression of normal tissues. Other anomalies such as pericardial cysts and esophageal duplication cysts may also be seen in this compartment. The mass seen on this patient's chest radiograph does not arise from the middle mediastinum compartment.

A mass in the posterior mediastinum is usually a neurogenic tumor. In children, these typically arise from the sympathetic ganglia (for example, neuroblastomas), whereas in adults neurogenic tumors tend to arise from the nerve sheaths (for example, schwannomas). The mass on this patient's chest radiograph is located in the anterior mediastinal compartment.