Thymic carcinomas may be classified according to a histological grading system into either low grade subtypes or high grade subtypes.^[1]. 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.

• Thymic carcinomas may be classified according to a histological grading system into either low grade subtypes or high grade subtypes, which include:^[1]

• 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:^[2]

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