Thymic carcinoma classification

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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]

Overview

Thymic carcinomas may be classified according to a histological grading system into either low grade subtypes or high grade subtypes.

Classification

• Thymic carcinomas may be classified according to a histological grading system into either low grade subtypes or high grade subtypes, which include:

• 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 nuclear:cytoplasmic ratio.

• Mucoepidermoid carcinoma: The characteristic features for this rare subtype is the presence of squamous cells and mucus producing cells, with moderate nuclear 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.

• The table below lists the histological 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