Mediastinal liposarcoma of thymic origin

Clinical History

A 45-year-old female patient presented with dyspnoea, non-productive cough, chest discomfort and weight loss (5 kg) for the past five months. These symptoms were insidious in onset and gradually progressive. There was no history of dysphagia, wheezing, haemoptysis, fever or tuberculosis. She had no family history of malignancy and was a non-smoker.

Imaging Findings

On routine evaluation with a chest radiograph (not shown), she was found to have a mass lesion in the left hemithorax and was referred for a contrast-enhanced CT of the thorax. The CT revealed an opacity in the left lung upper and mid-zone, which did not project above the clavicle. The descending thoracic aorta could be clearly seen through it suggesting an anterior mediastinal mass (Fig. 1).

Non-contrast CT (Fig. 2) showed a large well-marginated soft tissue density lesion of approximate size 12×9×8 cm in anterior mediastinum containing areas of fat attenuation (HU -70 to -100). No evidence of calcification or haemorrhage was noted. The lesion abutted the arch of the aorta, left pulmonary artery and anterolateral chest wall without any definite invasion of mediastinal structures. The soft tissue component showed heterogeneous enhancement (Fig. 3). There was associated pericardial effusion. Screening CT of the brain and abdomen revealed no evidence of metastasis. CT-guided FNAC revealed mixed type (well-differentiated and myxoid) liposarcoma of thymic origin.

Discussion

Liposarcomas are most common soft tissue sarcomas in adults constituting about 15-20% of all sarcomas. They usually have their origin in the extremities or the retroperitoneum. However they can also be found in other locations like abdomen, vulva, buttocks and mediastinum. Primary mediastinal liposarcomas are very rare accounting for less than 1% of all mediastinal tumours and 9% of all primary mediastinal sarcomas [1, 2, 3, 4].
Mediastinal liposarcomas are commonly found in posterior mediastinum and are rare in anterior mediastinum. Mediastinal liposarcoma containing thymic tissue have also been found and termed as thymic liposarcoma [1, 3, 5].

Around 15% of cases can be asymptomatic and detected incidentally on chest imaging. But they can exhibit clinical symptoms due to their larger size and direct invasion of the adjacent structures like pericardium and superior vena cava. The most common symptoms are dypnoea, tachypnoea, wheezing and chest pain. They can also present with signs of superior vena caval obstruction [3, 5, 6].
They originate from the primitive mesenchymal cells or the adipose tissues of thymus. Histologically, they are classified into five subtypes: well-differentiated, dedifferentiated, myxoid, round cell and pleomorphic. The well-differentiated subtypes are low grade in nature, rarely metastasize and have a better prognosis. The pleomorphic and dedifferentiated subtypes have a poor prognosis [3, 4].
On conventional chest radiographs, mediastinal liposarcomas are seen as widened mediastinum with possible deviation of trachea and vessels. The appearance of mediastinal liposarcoma on CT varies from a predominantly fat-containing mass to a solid mass. On MRI, fatty tissue with high signal intensity is seen in T1-weighted imaging and diminished signal intensity in T2-weighted imaging. Marked heterogeneous internal structure is seen in the pleomorphic subtype [3, 5, 6].

The preferred treatment is surgical excision. However, in more advanced and infiltrating tumours, partial excision or debulking of the tumour may be useful in relieving the compressive effect. Radiotherapy and chemotherapy can also be used as an adjunct. The five year survival rate is about 38%. Recurrence is common in deep-seated liposarcomas and usually manifests within the first 6 months with a recurrence rate of 40%; probably due to incomplete excision at the time of primary surgery. Therefore, careful long-term follow up is necessary. [1, 3, 5-7]

Masses that can mimic these lesions include: thymolipomas, teratomas, germ cell tumours, lymphomas, herniated peritoneal fat and diaphragmatic hernias [5, 7]

Teaching Point

Though mediastinal liposarcoma is rare, it should be kept in the differential diagnosis of fat-containing mediastinal lesions.

Differential Diagnosis List

Final Diagnosis

Primary mediastinal liposarcoma (thymic)

URL:	https://www.eurorad.org/case/13791
DOI:	10.1594/EURORAD/CASE.13791
ISSN:	1563-4086