Thymic carcinomas are a heterogeneous group of malignant epithelial tumors with diverse morphology that resemble carcinomas encountered outside the thymus (Table 106.1).¹ Thymic carcinomas represent a small group of thymic epithelial tumors.^2,3,4 In the database of the International Thymic Malignancy Interest Group, 14% of all thymic tumors are thymic carcinomas.⁵

Squamous cell carcinomas are the most common thymic carcinoma, representing 41% to 79% in more recent larger series with histologic data.^{5,6,7,8,9,10,11}

There is a wide variation in the frequency of remaining subtypes. Undifferentiated carcinomas are often the second most frequent,^6,7,10,11 but only represent 3% in the largest series.⁵ The other types include lymphoepithelioma like, mucoepidermoid, basaloid, clear cell, sarcomatoid, and adenocarcinomas (including papillary and mucinous). Basaloid carcinomas are sometimes included as a subset of squamous cell carcinomas.¹¹

Neuroendocrine carcinomas are generally classified separately from thymic carcinomas and include typical and atypical carcinoids and small cell and large cell neuroendocrine carcinomas. Neuroendocrine carcinomas, when included in series of other thymic malignancies, constitute about 7%⁷ and carcinoid tumors up to 15%.¹⁰ The somewhat arbitrary classification is also reflected in the consideration by some of mucoepidermoid carcinomas as distinct from thymic carcinomas.¹¹ Other salivary gland-type carcinomas, such as adenoid cystic carcinoma, rarely arise in the thymus.¹²

There are rare reports of nuclear protein in testis (NUT) carcinoma and hepatoid carcinoma of the thymus.^13,14

The mean age at presentation is about 60 years, although children and adolescents may be affected.^5,6,8,15 There is a slight male predominance. From one-third to three-quarters of patients present with advanced disease (Masaoka stage 3 or 4).^5,6,8,9,15