Anterior Mediastinal Masses

Table 40.1. Mediastinal masses

Anterior/Superior Compartment

Middle Compartment

Posterior Compartment

Thymomas

Bronchogenic cyst

Neurogenic tumours-neurofibroma, neurilemmoma (schwannoma),ganglioneuroma, ganglioneuroblastoma, malignant schwannoma, neuroblastoma, paraganglioma

Thymolipomas

Enteric cyst

Malignant lymphoma

Carcinoid tumours

Pericardial cyst

Gastroenteric cysts

Thymic cyst

Malignant lymphoma

Germ cell tumours

Primary cardiac tumours

Malignant lymphoma

Metastatic carcinoma

Teratomas

Metastatic carcinoma

Thyroid/parathyroid lesions

Mesenchymal lesions - Lipoma,

Haemangioma, Lymphangioma

Aberrant thyroid

Thyroid goitre

Unilocular thymic cysts: of developmental origin and occur more often in the neck than the mediastinum. The lining may be flattened, cuboidal, columnar or (rarely) squamous epithelium with thymic tissue in the wall.

Multilocular thymic cysts: acquired and thought to be secondary to inflammation. Some cases are seen in HIV infection. They can mimic an invasive thymic tumour and occur in about half of thymuses with nodular sclerosing Hodgkin's disease or seminoma. They also occur in other tumours such as thymoma and large cell lymphoma, though less frequently. Exceptionally, true squamous cell carcinoma arises from these cysts.

Thymic hyperplasia: strongly associated with autoimmune disease especially myasthenia gravis. There is extreme variability in size and weight of the thymus with formation of germinal centres, principally in the medulla that expand and cause cortical atrophy.

Thymoma: the most common primary neoplasm of the mediastinum. Seventy-five per cent present in the anterior mediastinum but they can also occur in other compartments (neck, thyroid, pulmonary hilum, lung parenchyma, pleura). It is a mixture of neoplastic thymic epithelial cells and non-neoplastic lymphocytes. Tumours are evaluated on the basis of the morphology of the neoplastic epithelial cells (spindle, plump) and the relative number of these cells, compared with the non-neoplastic lymphocytic component.

Medullary thymomas: composed of epithelial cells that resemble those of the medulla and are elongated or spindle shaped. They are benign.

Mixed thymomas: show a mixture of spindle cells and plumper, rounder, cortical-type epithelial cells. They act in a benign fashion.

Predominantly cortical (organoid) thymomas: have a less prominent epithelial component with lymphocyte-rich organoid corticomedullary areas. Local invasion is common.

Cortical thymomas: have a lesser component of lymphocytes with large round or polygonal epithelial cells. They are frequently locally invasive.

Well-differentiated thymic carcinoma: composed predominantly of epithelial cells with mild nuclear atypia and few lymphocytes. It is locally invasive.

Thymic carcinoma: an epithelial tumour exhibiting cytological features of malignancy. Cytoarchitectural features are no longer specific to the thymus but are analogous to those seen in carcinomas of other organs. No immature lymphocytes are present. Microscopic types of thymic carcinoma include squamous cell carcinoma, non-keratinizing squamous cell carcinoma and lymphoepithelioma-like carcinoma. These account for over 90% of cases.

Lymphoma: accounts for 10-14% of mediastinal masses in adults and is the commonest primary neoplasm of the middle mediastinum. Lymphoma of any type may occur, generally as part of widespread disease.

Mediastinal Hodgkin's disease: the nodular sclerosing variety occurs most frequently with mediastinal involvement in 80% of cases There is a nodular growth pattern, collagen bands and lacunar cells.

Non-Hodgkin's lymphoma: usually high grade. T-lymphoblastic (young patients) or large B-cell and occasionally low grade (MALToma).

Mediastinal large B-cell lymphoma: thought to be of thymic B-cell origin. Histological examination shows a diffuse proliferation of cells, which are compartmentalized into groups by fine bands of sclerosis. There may be thymic remnants. There is an association with nodular sclerosis Hodgkin's lymphoma (composite lymphoma). Biopsy samples are often small and may be obscured by profuse sclerosis with associated cellular crush artefact.

Germ Cell Tumours: make up 20% of mediastinal masses.

Mature cystic teratoma: the most common type of mediastinal germ cell neoplasm comprising a disorganized mixture of derivatives of the three germinal layers - ectoderm, mesoderm and endoderm.

Immature teratoma: a germ cell tumour similar to mature teratoma but also containing immature epithelial, mesenchymal or neural elements.

Seminoma: the most common malignant germ cell tumour to occur in the mediastinum. These arise almost always within the thymus.

Non-seminomatous malignant germ cell tumours: include malignant teratomas, malignant tera-tocarcinomas, yolk sac tumours, endodermal sinus tumours, choriocarcinomas and embryonal cell carcinomas.

Malignant germ cell tumours are usually treated with chemotherapy and radiotherapy. If a residual mass is left, it is usually a benign teratoma or necrotic tumour mass that can potentially degenerate and redevelop malignancy. Excision may be carried out.

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