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Seth G.S. Medical College and K.E.M. Hospital, Mumbai , India

Case of the Month

< Case No. 77 September 2026 >

Beyond the Mediastinum: A Giant Invasive Thymoma with Cardiac Extension

Contributed by : Prajwal Tayde

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Introduction :

Thymomas are the most common primary epithelial neoplasms of the thymus and typically arise in the anterior mediastinum. They are usually diagnosed in adults, most commonly between the fourth and sixth decades of life. Although thymomas may show relatively indolent behavior, some demonstrate capsular invasion and extension into adjacent mediastinal structures.

Invasive thymomas are characterized by extension beyond the thymic capsule into surrounding mediastinal fat, pleura, pericardium, lung, diaphragm or, in advanced cases, And produce encasement/invasion of the great vessels. Large invasive tumors may cause significant mass effects and can present with respiratory symptoms, chest discomfort, cough or constitutional symptoms.

Contrast-enhanced CT is the principal imaging modality for evaluating a mediastinal mass, as it helps determine the tumor center, internal composition, local invasion, vascular relationships, pleural/pericardial involvement and distant disease. Histopathology is essential for definitive diagnosis and classification.

Case presentation :

A 42-year-old man presented with complaints of progressively increasing dyspnea, abdominal distension, reduced appetite, generalized weakness and intermittent non-productive cough for one year.

He also complained of a feeling of chest heaviness and progressive reduction in exercise tolerance. The symptoms had gradually increased in severity over the preceding four months.

There was no history of fever, significant weight loss or night sweats. There was no history of previous thoracic surgery or chronic pulmonary disease.

On examination: there was reduced air entry over the right hemithorax. The abdomen appeared distended.

Radiological Findings :

The frontal chest radiograph (Figure 1) demonstrated near-complete opacification of the right hemithorax with crowding of the right-sided ribs. The right costophrenic angle was obscured as was the right heart border.

The appearance raised a suspicion for a large space-occupying lesion involving the right hemithorax/mediastinum, with associated mass effect.

Fig 1

Figure 1. Chest radiograph: Frontal chest radiograph showing near-complete opacification of the right hemithorax with crowding of the right-sided ribs and obscuration of the right costophrenic angle and right heart border, suggestive of a large right mediastinal/hemithoracic mass.

Contrast-enhanced CT of the chest (Figures 2-7) showed a large invasive right mediastinal mass with extensive pleural and adjacent structure involvement. This included vascular and intracardiac extension, chest-wall extension, pleural deposits, and associated hepatic congestion with ascites.

Figure 2, 3

Figure 2. Contrast-enhanced CT chest: Axial contrast-enhanced CT image showing a large nodular, ill-defined, heterogeneously enhancing soft-tissue mass centered in the right mediastinal pleura with contiguous extension along the right costal and diaphragmatic pleura (black arrow). Few internal calcific foci are seen (white arrow).Figure 3. Contrast-enhanced CT chest: Axial contrast-enhanced CT image demonstrating extensive encasement of the great vessels (black arrow) with significant compression of the lower trachea (white arrow). No definite bronchial cut-off is identified.

Figure 4,5

Figure 4. Contrast-enhanced CT chest: Axial contrast-enhanced CT image showing loss of the intervening fat plane (white arrow) with the dilated right atrium and contiguous enhancing soft-tissue extension from the right atrium into the right ventricle across the tricuspid valve, suggestive of intracardiac invasion.

Figure 5. Contrast-enhanced CT chest: Axial contrast-enhanced CT image demonstrating contiguous extension of the mass through the right 10th intercostal space (white arrow) into the overlying subcutaneous tissues, indicating trans-thoracic extension.

Figure 6,7

Figure 6. Contrast-enhanced CT chest: Axial contrast-enhanced CT image showing heterogeneous mottled enhancement of the liver producing a characteristic “nutmeg” appearance, consistent with hepatic venous congestion, with gross ascites.

Figure 7. Contrast-enhanced CT chest: Axial contrast-enhanced CT image demonstrating multiple well-defined soft-tissue nodules (black arrow) scattered along the anterior chest wall, consistent with pleural/chest-wall metastatic deposits.

Radiological Diagnosis :

The important differentials for a large invasive mediastinal mass include:

1. Invasive thymoma

This was thought most likely given the lesion was centered in the mediastinum, and, adult age group and aggressive local extension. Thymomas may invade surrounding mediastinal fat, pleura, pericardium and adjacent structures.

2. Thymic carcinoma

This is an important differential, particularly in a large heterogeneous mass with marked invasion, necrosis and possible lymphadenopathy. Thymic carcinomas generally demonstrate more aggressive behavior than conventional thymomas.

3. Mediastinal lymphoma

A bulky mediastinal lymphoma can produce extensive mediastinal widening and mass effect. Associated lymphadenopathy and involvement of multiple nodal stations may favor lymphoma.

4. Mediastinal germ-cell tumor

These are particularly relevant in younger adult men. Tumor markers such as AFP and β-hCG may help in the evaluation of suspected germ-cell tumors.

5. Mediastinal sarcoma

The rare, should still be considered in a very large aggressive mediastinal mass with heterogeneous enhancement and necrosis.

Radiological Diagnosis

Imaging diagnosis: Invasive thymoma.

Pathological Diagnosis

A CT guided biopsy was performed.

Pathological Diagnosis :

A CT guided biopsy was performed.

Histopathological examination demonstrated a lobulated thymic epithelial neoplasm composed of epithelial cells mixed with a prominent population of small immature lymphocytes.

The epithelial cells showed polygonal morphology with oval nuclei and relatively mild-to-moderate cytological atypia. Fibrous septa were seen separating the tumor into lobules.

Microscopic evidence of invasion through the thymic capsule into the surrounding mediastinal soft tissues was identified, confirming the invasive nature of the lesion.

The lymphocytic component was composed predominantly of immature T lymphocytes.

Final diagnosis :

Invasive Thymoma

Timeline

Management:

In view of the extensive local invasion, intracardiac extension, vascular encasement and metastatic/pleural deposits, the tumor was considered unresectable. The patient is currently under palliative care with serial clinical and radiological follow-up for disease monitoring and symptom management.

Discussion :

Invasive thymomas are characterized by extension beyond the thymic capsule into adjacent mediastinal structures. The extent of invasion is important for staging, assessment of resectability and treatment planning [1].

On contrast-enhanced CT, thymomas typically appear as lobulated anterior mediastinal masses. Large tumors may show heterogeneous enhancement due to necrosis or hemorrhage [2]. Features suggesting invasion include irregular margins, obliteration of fat planes, pleural/pericardial nodules, vascular encasement and direct extension into adjacent organs [1,3].

Pleural dissemination is relatively characteristic of advanced invasive thymoma and may manifest as pleural nodules, masses or diffuse pleural thickening. Pericardial and great-vessel involvement indicate advanced disease and may significantly limit surgical resectability [1,4]. In the present case, extensive vascular involvement with intracardiac extension represented particularly advanced local disease.

Histopathology is essential for definitive diagnosis. Thymoma characteristically comprises thymic epithelial cells admixed with immature T lymphocytes, with the relative proportion of these components varying among WHO subtypes [4, 5]. In this case, the combination of lobulated architecture, thymic epithelial cells, prominent immature lymphocytes and capsular invasion confirmed invasive thymoma.

The main differentials include thymic carcinoma, lymphoma and mediastinal germ-cell tumor. The combination of a mediastinal epicenter, extensive contiguous invasion and characteristic histopathological findings favored invasive thymoma [3,4].

Management depends on stage and resectability. Complete surgical resection is preferred when feasible, while chemotherapy and/or radiotherapy may be used as part of multimodality treatment. Extensive unresectable disease may require systemic and palliative treatment [6].

This case highlights the role of contrast-enhanced CT in demonstrating the extensive local, vascular, pleural and intracardiac involvement of a giant invasive thymoma.

References:

1. Marom EM. Imaging thymoma. J Thorac Oncol. 2010;5(10 Suppl 4):S296–S303. doi:10.1097/JTO.0b013e3181f209ca.

2. Carter BW, Benveniste MFK, Truong MT, Marom EM. State of the Art: MR Imaging of Thymoma. Magn Reson Imaging Clin N Am. 2015;23(2):165–177. doi:10.1016/j.mric.2015.01.005.

3. Marom EM. Advances in thymoma imaging. J Thorac Imaging. 2013;28(2):69–80. doi:10.1097/RTI.0b013e31828609a0.

4. Marx A, Chan JKC, Chalabreysse L, et al. The 2021 WHO Classification of Tumors of the Thymus and Mediastinum: What Is New in Thymic Epithelial, Germ Cell, and Mesenchymal Tumors? J Thorac Oncol. 2022;17(2):200–213. doi:10.1016/j.jtho.2021.10.010.

5. Carter BW, Benveniste MF, Madan R, et al. IASLC/ITMIG Staging System and Lymph Node Map for Thymic Epithelial Neoplasms. RadioGraphics. 2017;37(3):758–776. doi:10.1148/rg.2017160096.

6. Falkson CB, Vella ET, Ellis PM, et al. Surgical, Radiation, and Systemic Treatments of Patients With Thymic Epithelial Tumors: A Clinical Practice Guideline. J Thorac Oncol. 2022;17(11):1258–1275. doi:10.1016/j.jtho.2022.08.007.

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