Diagnosis of a mediastinal mass

Some 75% of mediastinal masses are benign, and more likely to be so in an adult. Thymomas, thyroid masses, lymph nodes, and benign cysts are the commonest mediastinal masses in adults. In children, over 80% of masses are neurogenic tumours, germ cell tumours, or foregut cysts.

A slow‐growing, benign, mediastinal mass may be asymptomatic and found incidentally on a chest X‐ray. As the mass enlarges, it can cause symptoms of cough, chest pain, and breathlessness. If the mass compresses adjacent structures, such as the trachea, oesophagus, or superior vena cava, it can cause stridor, dysphagia, and superior vena cava obstruction (SVCO). The patient may also develop systemic symptoms, depending on the mass.

Anterior mediastinal mass

The anterior mediastinum is behind the sternum and in front of the pericardium (Figure 16.1, Figure 16.2, Figure 16.3). On a CXR, the hilum overlay sign (one can see the hilar vessels through the mass), displacement of the anterior junction line, obliteration of the retrosternal space and a hazy cardio‐phrenic angle suggest an anterior mediastinal mass (Box 16.3).

Thymoma is a tumour of epithelial cells arising in the thymus. It is commonest in men over 50 years and rare in patients younger than 20 years. It is associated with myasthenia gravis in 30–40% of cases, and 20% of patients presenting with myasthenia gravis (MG) are found to have a thymoma. Patients with MG have positive acetylcholine receptor autoantibodies which bind to acetylcholine receptors at the post‐synaptic motor endplate causing nerve fatigue. Patients with thymoma and MG complain of pain, dyspnoea, dysphagia, and muscle weakness with repeated contraction, and the inability to maintain an upward gaze.

Thymomas contained within the thymic capsule tend to be relatively benign. If a thymoma is suspected radiologically, the case should be discussed with a thoracic surgeon and oncologist at the lung multidisciplinary meeting prior to a fine needle aspiration of the mass.

The treatment is with thymectomy, usually through a median sternotomy scar. A VATS procedure may be possible for small thymomas. A transcervical thymectomy may also be indicated in patients with myasthenia gravis without a thymoma as this can improve symptoms in many and result in complete remission in 30–40% of cases. Best results are obtained in younger patients with detectable acetylcholine receptor antibodies who present with early disease. Symptoms of myasthenia gravis should also be treated with pyridostigmine and with immunosuppressants, such as prednisolone or azathioprine.

Malignant thymomas extend outside the capsule and spread by ‘seeding’, invading local structures and spreading to the pleural space (Figure 16.4, Figure 16.5). The most widely used staging system for thymoma is the Masaoka system, which is based on the degree of capsular invasion (Table 16.2). The WHO system, which is based on the microscopic appearance of the cells, has subtypes A, B and C. Type C tumours are thymic carcinomas, which have the worst prognosis.

Thymic cysts can be congenital or acquired secondary to inflammation. They are asymptomatic unless large and cause symptoms of compression, in which case they should be excised. Thymic lipoma and thymic hyperplasia can also appear as an anterior mediastinal mass. Thymic carcinoids can behave aggressively, with local invasion and distant metastases. These tumours are not associated with myasthenia gravis but may be associated with Cushing’s syndrome. Treatment is with surgery, chemotherapy, and octreotide.

Germ cell tumours are a diverse group of benign and malignant tumours which account for 10–15% of anterior mediastinal neoplasms in adults. They result from a failure of immature germ cells to migrate during embryogenesis, with the mediastinum being the commonest extragonadal site. Benign, mature cystic teratomas (dermoid cysts) are the commonest germ cell tumours in adults, accounting for 60–70% of cases, and occurring in the third decade. These well‐differentiated tumours contain fat, skin, hair, eyes, nails, sweat glands, cartilage, and teeth. They usually present with symptoms of compression and rarely with expectoration of hair (trichoptysis), sebum or fluid from a connection that forms between the tumour and the airways. Tumour markers are negative with a teratoma. CXR and CT thorax will reveal a well‐circumscribed, multi‐loculated, cystic mass with a fat‐fluid level and calcification. Teeth and hair may be present. The prognosis is excellent with surgery.

The majority of malignant mediastinal germ cell tumours occur in men who will be symptomatic. Serum levels of β‐hcg and/or AFP will be elevated in 80–85% of cases. Seminomas account for about half of malignant germ‐cell tumours and affect men in their twenties and thirties. The CXR will depict a large, lobulated, well‐defined anterior mediastinal mass. Local invasion of mediastinal structures is uncommon, although lymph node, lung, and bone metastases can occur. Treatment is with radiotherapy, chemotherapy and surgery, and the prognosis has improved significantly in the past decade. Measurement of tumour markers can be helpful in monitoring the disease.

Other malignant germ cell tumours affecting young men include choriocarcinoma, embryonal cell carcinoma, endodermal sinus tumour, and mixed germ‐cell tumour. These too may secrete AFP and β‐hcg. Some 20% of these men have Klinefelter’s syndrome, and the tumour may also be associated with haematological malignancies. These lesions appear as a large, irregular, heterogeneous mass with central necrosis, haemorrhage, and cyst formation. Invasion of adjacent structures with pleural and pericardial effusions can occur. Distant metastases occur at an advanced stage. Treatment is with chemotherapy and surgery, and the five‐year survival rate is 50%.

Lymphomas can arise from lymph nodes in the anterior or middle mediastinum. B‐cell Hodgkin’s lymphoma is the commonest type of lymphoma. Patients may present with B symptoms (fever, night sweats, and weight loss), and LDH levels may be elevated significantly. A surgical biopsy is recommended to confirm the histological diagnosis. The patient should be referred urgently to the haemo‐oncologist for chemotherapy and further management.

A large, retrosternal thyroid goitre can appear as an antero‐superior mediastinal mass. Patients are usually euthyroid, but may experience symptoms of dyspnoea and dysphagia because of compression of the trachea and oesophagus. The CXR and CT thorax may show tracheal deviation (Figure 16.6). The contrast CT scan may reveal enhancement of an encapsulated mass with haemorrhagic and cystic changes, and possible calcification. Further imaging with radioactive iodine (¹²³I or ¹³¹I) scan and a thyroid ultrasound will be required, as well as thyroid function tests and measurement of thyroid antibodies. Surgery should be considered if the patient is symptomatic.

Parathyroid adenomas occur in elderly women and should be considered in those who have persistent hyperparathyroidism and hypercalcaemia despite parathyroidectomy. These benign functioning ectopic adenomas occur in the anterior mediastinum near the thymus. They may be too small to be detected on a CXR. A contrast CT thorax will show an encapsulated mass and there will be increased uptake with ^99mTc sestamibi scintigraphy. Management is with surgical excision.

Middle mediastinal mass

The middle mediastinum is the area between the anterior and posterior mediastinum. A middle mediastinal mass may appear in the aorto‐pulmonary window with widened paratracheal stripes, displacement of the azygo‐oesophageal recess on the right, and a pseudoparavertebral line on the left (Box 16.4).