Teaching Case

A 55-year-old female non-smoker presented with a two month history of shortness of breath preceded by a 4-month history of myasthenia gravis. She has had previous aortocoronary bypass surgery but had been stable from a cardiac perspective. Apart from previous cosmetic breast augmentation, there was no other significant clinical history.

The chest radiographs (Fig. 1a-b) demonstrate a lobulated anterior mediastinal mass with further ill-defined shadowing overlying the right lower and left mid zones. Bilateral pleural effusions are also present.

Post-contrast axial CT images confirmed a large lobulated anterior mediastinal mass with bilataral paramediastinal extension. This mass is homogeneously iso-attenuating to chest wall musculature. The selected axial CT image (Fig. 2) also demonstrates SVC compression accompanied by subtle irregularity along its concave compressed margin towards the right (arrow); the latter feature is concerning for vascular invasion.

Additional pleural based metastases are shown on the image more inferiorly (Fig. 3) and some of these demonstrate evidence of chest wall invasion. The sagittal image (Fig. 4) demonstrates contrast within the paraspinal veins from collaterals draining into the azygos venous system (arrow).

No lung retraction or pleural plaques are seen. Percutaneous biopsy of one of the pleural based lesions revealed metastatic thymic carcinoma.

A thymic carcinoma is a neoplasm which lies at the malignant end of the spectrum of thymic epithelial tumours (classified as a type C thymic epithelial tumour under the WHO classification system) [1].

It is one of the imaging differential considerations for an invasive anterior mediastinal based intrathoracic mass lesion.

At least ten histological types have been described with the most common subtypes being squamous cell carcinoma and lymphoepithelial-like carcinoma (previously classified as atypical thymoma) [2, 3].

As a general rule, thymic carcinomas tend to be larger in size than their benign counterparts [4].
On CT, a thymic carcinoma is typically seen as a large, poorly marginated soft tissue attenuating mass. It is often seen invading adjacent structures on imaging. While areas of necrosis, haemorrhage, calcification and/or cyst formation may be seen [5], most tumours (as in this case) are homogenous and isoattenuating to chest wall musculature [6].

An irregular contour +/- irregular invasion into adjacent organs (inclusive of the great vessels) are features which have been described as being useful in differentiating a thymic carcinoma from more benign thymic epithelial tumours [6, 7, 8]. Pericardial or pleural involvement and pleural effusions can also be frequent findings at diagnosis.

Hilar lymph nodal enlargement, diaphragmatic elevation (suggesting underlying phrenic nerve palsy) [9] and presence of lung nodules (suggestive of metastases) are additional findings that would favour towards thymic carcinoma over an invasive thymoma on imaging.

Thymic carcinomas are biologically aggressive and can frequently metastasise haematogenously to lungs, liver, brain and bone. Most patients (as in this case) typically present around the fifth decade of life. The presenting symptoms are generally secondary to mediastinal compression and/or invasion or related to distant metastases. Patients with thymic carcinoma have a very poor prognosis with mean survival rates at around 60% at 10 years [10].

The presence of an anterior mediastinal based mass with gross invasion of vascular structures and pleural based metastases makes thymic carcinoma highly likely on imaging grounds. It is important for the radiologist to know that there is a very wide spectrum of thymic epithelial tumours (ranging from benign to malignant) as well as have an understanding on the significance of vascular invasion.

Although the incidence of associated paraneoplastic syndromes such as myasthenia gravis has been described to be low (usually seen with more benign thymic epithelial tumours), it was interestingly present in this particular case as an initial presenting symptom.