Malignant thymoma (thymic carcinoma)

Clinical History

The patient presented with cough, chest pain and dyspnoea. She had lost 10 kg of weight in the past 3 months.

Imaging Findings

Biplane chest radiograph showed a dense mass in the anterior middle mediastinum without displacement of the tracheal band. There was left pleural effusion and high-riding diaphragm suggesting phrenic nerve palsy (Fig. 1). On MSCT, there was a bulky mass with heterogeneous enhancement in the left anterior mediastinum, which invaded the pericardium and bulged out against the S3 segment of the left lung (Fig. 2). At bronchoscopy, tumour invasion of the left S3 bronchus was found and biopsy was performed. The specimens were classified as thymic carcinoma (WHO type C). The extent of the disease was attributed stage IV according to the Masaoka system [1]. Given the advanced stage of the disease and the age and general state of the patient, primary radiotherapy was chosen as the most appropriate treatment option. Follow-up imaging of the thorax one year after the first diagnosis showed partial remission of the tumour (Figs. 3, 4).

Discussion

The normal thymus is composed of epithelial and stromal cells deriving from the 3rd and 4th pharyngeal pouches, and lymphocytes, deriving from the mesoderm. It is the organ, where lymphoid precursors from the bone marrow migrate to complete their differentiation into T-cells. After puberty, it gradually involutes and gets replaced with fatty tissue. Neoplasms arising from the epithelial cells of the thymus are termed thymomas. They are more common than neoplasms from the lymphoid constituents. In fact, thymomas are the most common space-occupying lesions of the anterior mediastinum in adults (about 40%). The disease affects men and women alike, mostly in the 5th to 6th decade of life. In up to 40%, it is associated with various autoimmune diseases, myasthenia gravis being the most common. However, up to one half of patients are asymptomatic, and myasthenia is rather a favourable prognostic indicator, for it is found more frequently in less advanced stages. Other symptoms are unspecific, and the tumour is usually detected on routine radiographs of the chest [2]. Overall incidence of thymic neoplasms in the US is about 0.15 per 100, 000 [3], carcinoma accounting for probably less than 1%.
The aetiology of thymic neoplasms is unknown. The WHO has devised a classification system based on histologic features [4] that requires sufficient amounts of tissue to be collected. Fine-needle aspiration can often not even differentiate thymoma from lymphoma [2]. The extent of the disease is traditionally determined at surgery, using the Masaoka staging system [1], modified by the French Study Group on Thymic Tumors (GETT) [5]. CT has the ability to differentiate between invasive and non-invasive thymomas [6, 7]. Hence, it can be used as a pretherapeutic staging modality and may predict local unresectability. Histologic type and Masaoka stage are probably independent predictors of survival [8].
Treatment of thymomas should be operative, whenever feasible. In advanced stages like the present case, a multimodality approach is preferred in eligible patients [9]. Radiotherapy alone or in combination with chemotherapy is also an acceptable option, if the patient is unable or unwilling to undergo a surgical procedure [10].

Differential Diagnosis List

Final Diagnosis

Thymic carcinoma (malignant thymoma)

URL:	https://www.eurorad.org/case/9189
DOI:	10.1594/EURORAD/CASE.9189
ISSN:	1563-4086