A broad range of rare tumours can develop in the thymus, including those of non-thymic type (eg. lymphoma, germ cell tumours). This page relates to tumours of the thymic epithelium. This forms the most common tumour of the medastinum in adults. The WHO classifies thymoma as grades A, AB, B1, B2 and B3 which exhibit thymic differentiation. Thymic carcinoma (grade C) may show differentiation along multiple other lines (eg. squamous cell carcinoma, neuroendocrine carcinoma, etc).
Epidemiology
Thymomas are rare tumours that are nevertheless the most common tumour of the thymus in adults, followed by mediastinal lymphoma. Incidence is about 5/1,000,000 per year.
Aetiology
The exact cause is not known. A number of thymomas are associated with myasthenia gravis, and other autoimmune disorders may also be associated with a thymoma.
Natural History
Tumours on the benign end on the spectrum (A/AB/B1) tend to be localised and encapsulated (T1); breach of the capsule can occur with invasion into fat (T2). B2 and B3 carcinomas, and thymic carcinomas, exhibit local invasion into adjacent structures (T3). It is also possible for thymic tumours to seed the pleural and pericardial cavities (T4 disease). Nodal spread is rare.
Clinical Features
Patients may be found to have a thymoma incidentally, due to local mass effect, or during investigation for autoimmune disease (usually myasthenia gravis). Local effects are more common with more aggressive tumours. Examination may be unremarkable, or may detect pleural effusion if the pleural cavity has been seeded.
Tumour Features
Type A Thymoma (15%)
These tumours are usually encapsulated and early stage (mostly I, some II, rarely III). The cut surface may be faintly lobulated. They are made up of either spindle or oval cells that may be arranged as sheets or in a storiform pattern. There is no cellular or nuclear pleomorphism. Prognosis is excellent with 100% 10 year survival if the tumour is completely resected.
Type AB Thymoma (40%)
A more common variant is to see type A histology (lymphocyte poor) mixed with type B (lymphocyte rich), although the B areas are different from pure B thymoma. They have a lobulated cut surface and have a lobulated pattern microscopically as well. The A component is similar to pure A thymoma. The B regions contain small polygonal cells associated with lymphocytes. They are typically stage I (70%) but may be stage II or III. Prognosis is also excellent (90-100% 10 year survival).
Type B1 Thymoma (10% but 50% associated with myasthenia gravis)
This uncommon variant is comprised of cells identical to the normal thymus. They contain a small number of malignant thymic epithelial cells (small and polygonal) that are surrounded by normal T lymphocytes. The cortical areas are significantly increased when compared to normal thymus but medullary areas are present. About half are stage I, one quarter stage II and the remainder more advanced.
Type B2 Thymoma (25%, 50% associated with myasthenia gravis)
This is a common type of thymoma that has a more aggressive course. They tend to be more locally advanced and about 15% are unresectable. The malignant epithelial component is more extensive and cells are larger than B1 types. 5 year survival is about 75%.
Type B3 Thymoma (10%)
This variant of B thymoma mostly consists of large polygonal epithelial cells with minimal but some lymphocytes present; they often grow in a sheet like pattern. They lack encapsulation and infiltrate locally. Most are stage II or III. 10 Medullary islands are rarely seen. 10 year survival is 50-60%.
Thymic Carcinoma
Thymic carcinoma is a broad category of epithelial malignancies that arise within the thymus but do not resemble thymic tissue. The most common variant is squamous cell carcinoma, indistinguishable from squamous cell carcinoma of other sites. Other variants such as mucoepidermoid, lymphoepithelioid carcinoma, papillary adenocarcinoma and clear cell carcinoma. The uniting feature is a poor prognosis and aggressive behaviour.
Thymic Neuroendocrine Tumours
This includes well differentiated, intermediate (atypical carcinoid) and poorly differentiated (small cell) variants. Atypical variants make up the majority of cases and have a relatively aggressive behaviour when compared to carcinoid in other locations. 10 year survival is 30%. Small cell carcinoma of the thymus is highly aggressive and most patients die within 2 years. Unlike thymoma, these tumours typically spread to regional lymph nodes (stage IV disease).
Staging
Thymoma is staged using TNM as well as the Masoaka criteria, which correlate well.
- Stage I (T1 N0): Localised within the thymus - Cure 100% except thymic carcinoma
- Stage II (T2 N0): Infiltrates local fat - Cure approaches 100% except B3 and thymic carcinoma
- Stage III (T3 N0): Invades local structures - Grade dependent survival
- Stage IV (T4 or N1 or M1): Pleural/pericardial nodules (T4), lymph node metastases (N1) or distant metastases (M1).
