Answer for BIR CoW 16 March 2025
Invasive Thymoma
Findings
MRI: A well-defined lobulated lesion in the anterior mediastinum extends to the superior and posterior mediastinum (D1–D6 vertebral levels). The lesion appears T1 isointense and T2 heterointense with internal T2 hyperintense cystic areas. It involves the mediastinal pleura, superior pericardium, and right atrial appendage, showing diffusion restriction with reduced ADC values. The mass abuts the ascending aorta and superior vena cava (SVC), causing significant SVC compression but without altered signal intensity within the SVC. Partial encasement of the right main bronchus and trachea is also noted.
Impression: Invasive heterogeneous anterior mediastinal lesion with SVC compression and right atrial invasion—likely invasive thymoma.
Histopathology (HPE): Thymoma (Modified Masaoka Stage IV).
Discussion
Thymoma is the most common primary anterior mediastinal neoplasm, with other differential diagnoses including thymic cyst, thymic hyperplasia, lymphoma, and germ cell tumors. The World Health Organization (WHO) classifies thymomas into types A, AB, B1, B2, and B3, based on epithelial cell morphology and lymphocyte-to-tumor cell ratio. The Masaoka staging system assesses tumor invasion and is a key prognostic factor influencing treatment.
Imaging Modalities
References
Findings
MRI: A well-defined lobulated lesion in the anterior mediastinum extends to the superior and posterior mediastinum (D1–D6 vertebral levels). The lesion appears T1 isointense and T2 heterointense with internal T2 hyperintense cystic areas. It involves the mediastinal pleura, superior pericardium, and right atrial appendage, showing diffusion restriction with reduced ADC values. The mass abuts the ascending aorta and superior vena cava (SVC), causing significant SVC compression but without altered signal intensity within the SVC. Partial encasement of the right main bronchus and trachea is also noted.
Impression: Invasive heterogeneous anterior mediastinal lesion with SVC compression and right atrial invasion—likely invasive thymoma.
Histopathology (HPE): Thymoma (Modified Masaoka Stage IV).
Discussion
Thymoma is the most common primary anterior mediastinal neoplasm, with other differential diagnoses including thymic cyst, thymic hyperplasia, lymphoma, and germ cell tumors. The World Health Organization (WHO) classifies thymomas into types A, AB, B1, B2, and B3, based on epithelial cell morphology and lymphocyte-to-tumor cell ratio. The Masaoka staging system assesses tumor invasion and is a key prognostic factor influencing treatment.
Imaging Modalities
- Chest Radiograph: Often the first imaging modality, it may show a soft tissue mass in the anterior mediastinum. Features include:
- Thickening of the anterior junction line.
- The "silhouette sign" where the tumor obscures adjacent structures.
- Lateral views can confirm a prevascular mass.
- Computed Tomography (CT): The preferred imaging method due to high resolution. CT helps assess:
- Tumor morphology, margins, size, density, and enhancement.
- Involvement of adjacent structures.
- Differentiation between solid and cystic lesions.
- Magnetic Resonance Imaging (MRI): Used in specific cases to:
- Evaluate solid vs. cystic components.
- Assess invasion into the pericardium, vessels, and cardiac structures.
- Identify microscopic fat using chemical shift imaging.
- Avoid radiation exposure in younger patients or those with CT contrast contraindications.
- Positron Emission Tomography (PET/CT): Its role in thymoma evaluation is limited due to false positives from infection, thymic hyperplasia, or fibrosis. However, it can help:
- Detect occult metastases.
- Assess tumor aggressiveness (higher FDG uptake in thymic carcinoma vs. thymoma).
- Differentiate thymic carcinoma (SUVmax >6) from low-grade thymomas.
- CT: Typically a smooth or lobulated mass, often unilateral. About one-third show heterogeneous enhancement due to necrosis, hemorrhage, or cystic changes.
- MRI: Prevascular masses with low-to-intermediate T1 and high T2 signal intensity. Fat suppression improves tumor delineation.
- PET/CT: Higher metabolic activity in aggressive tumors, but with limited differentiation between thymic subtypes.
- 68Ga-DOTATATE PET/CT: Superior to FDG PET/CT in detecting neuroendocrine thymic tumors and identifying candidates for peptide receptor radiotherapy (PRRT).
- Thymic carcinoma appears larger, with irregular margins, greater necrosis, cystic changes, hemorrhage, and significant local invasion.
- Presence of pleural/pericardial nodules or effusion favors thymic carcinoma.
- Superior vena cava (SVC) syndrome is a common complication, leading to facial/neck swelling, cough, headache, and dyspnea.
- MRI excels in assessing cardiac involvement and delineating tumor invasion.
References
- Strange, C. D., Ahuja, J., Shroff, G. S., Truong, M. T., & Marom, E. M. (2022). Imaging evaluation of thymoma and thymic carcinoma. Frontiers in Oncology, 11, 810419.
- Benveniste, M. F. K., Rosado-de-Christenson, M. L., Sabloff, B. S., Moran, C. A., Swisher, S. G., & Marom, E. M. (2011). Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma. RadioGraphics, 31(7), 1847–1861. doi:10.1148/rg.317115505
Note:
We do not discourage differential diagnosis. But all the differentials must satisfy the findings noted in the case.
If you feel you have answered rightly but cannot find your name in the above list, please call 09551942599.
Did you Know?
The order in which the names appear in this winner's list is based on the time of submission. The first person to send the correct answer gets his/her name on top of the list!
We do not discourage differential diagnosis. But all the differentials must satisfy the findings noted in the case.
If you feel you have answered rightly but cannot find your name in the above list, please call 09551942599.
Did you Know?
The order in which the names appear in this winner's list is based on the time of submission. The first person to send the correct answer gets his/her name on top of the list!