Thymic carcinoma pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]Parminder Dhingra, M.D. [3]

Pathophysiology

The thymic carcinoma is a tumor of the thymic epithelium, but it is associated with lack of immature lymphocytes and often presents with more invasive or metastatic disease.

Genetics

Thymic carcinoma, specially the squamous cell subtype, is associated with genetic alterations. It has been reported chromosomal gains like +1q, +17q, +18 and chromosomal losses of -3p, -6, -13q,-16q, -17p to a relation with this subtype.

G-banded metaphase spread shows a complex karyotype, including der(16)t(1;16)(q12;q12.1).

Patologic Classification

Thymic carcinomas are categorized in low and high grade histological subtypes, which includes squamous cell carcinoma, basaloid carcinoma, mucoepidermoid carcinoma, lymphoepithelioma-like carcinoma, sarcomatoid carcinoma (carcinosarcoma), clear cell carcinoma, papillary adenocarcinoma, carcinoma with t(15;19) translocation, neuroendrocrine carcinomas, undifferentiated carcinoma and combined thymic epithelial tumors.

• Squamous cell carcinoma: This subtype of thymic carcinoma is the most common and exhibits atypia with a clear-cut aspect of keratinization with keratin pearls as seen in squamous cell carcinomas. Squamous cell carcinoma lack of capsule and presents with necrosis and hemorrhage.

• Basaloid carcinoma: This subtype consists of solid lobules of tumor cells with marginal palisading, without keratinization and a basophilic pattern due to an elevated nucleocytoplasmic ratio.

• Mucoepidermoid carcinoma: The characteristic features for this rare subtype is the presence of squamous and mucus producing cells, with moderate atypia. It has a mucinous macroscopic appearance.

• Lymphoepithelioma-like carcinoma: The morphology of this subtype resembles the nasopharingeal lymphoepitelioma, with syncytial growth of undifferentiated malignant cells.

• Sarcomatoid carcinoma (carcinosarcoma): It is also known as spindle cell thymic carcinoma, is an infiltrative neoplasm with large areas of coagulative necrosis and without a capsule. It is an uncommon tumor that affects patients between 40-80 years old.

• Clear cell carcinoma: Consist of cells with minimal nuclear atypia with a characteristic lucent cytoplasm and a lobulated architecture without sinusoidal vasculature (in contrast with the renal clear cell carcinoma)

• Papillary adenocarcinoma: It has a tubulopapillary pattern growth with cuboidal cells and Psammoma bodies may be present. Type A thymoma may be the origin of this subtype of carcinoma due an expression of malignant transformation.

• Carcinoma with t(15;19) translocation: It is an aggressive tumor with a translocation t(15;19)(q13:p13.1 )that has the characteristic presence of undifferentiated cells with high mitotic activity and squamous morphology.

• Neuroendocrine Carcinomas: The neuroendocrine thymic carcinomas are classified in 4 categories: typical, atypical, small cell and large cell carcinomas. The typical and atypical are categorized as well differentiated neuroendocrine carcinomas, and the small cell and large cell carcinomas are categorized as poorly differentiated.

Classification of Neuroendocrine Carcinomas

Well Differentiated		Poorly Differentiated
Typical Carcinoid	Atypical Carcinoid	Small Cell	Large Cell
No necrosis; <2 mitoses per 2 mm2 (10 HPF)	Necrosis present and/or 2-10 mitoses per 2 mm2 (10 HPF)	Small cell cytology	Non-small cell NEC with >10 mitoses per 2 mm2 (10 HPF)
Morphological Variants Spindle cell type Pigmented type With amyloid (extrathyroidal medullary carcinoma) Oncocytic/oxyphilic type Mucinous Angiomatoid type Combinations of the above variants		Variants SCNEC combined with Non-NECs	—

HPF: High power field, SCNEC: Small cell neuroendocrine carcinoma, NEC: Neuroendocrine carcinoma

Associated Conditions

Thymic carcinoma has not been reported in association with myasthenia gravis, in contrast with thymoma. Paraneoplasic syndromes had been reported in the neuroendocrine subtype, but is not as common as in thymomas.