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.. Primary thymic carcinomas are rare malignant neoplasms of the mediastinum, and combined thymic carcinomas are even less common. Although tumor stage is the single most important prognostic factor in thymoma, a combination of stage and histologic subtype should be considered in predicting survival.

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