Tumors of the thymus can either be benign (non-cancerous) or malignant (cancerous). There are certain tumors that are classified as atypical (uncertain or unspecified behavior), meaning that some behave in a benign manner or some behave in a malignant manner. A majority of the thymic tumors diagnosed via lab studies (biopsy) are benign.

The thymus is a specialized primary lymphoid organ in the human body, located behind the breastbone in the upper part of the chest. It plays a crucial role in the development and maturation of a specific type of white blood cell known as T-lymphocytes or T cells, which are vital for the immune system. The thymus is particularly active during childhood and adolescence, gradually decreasing in size and activity with age, a process known as involution.

The World Health Organization (WHO) is an authoritative body that compiles, prepares, and publishes histological and molecular classification of tumors, via their Blue Books, based on research inputs from experts worldwide. These reference books chiefly aid in global cancer research, provide gold standards for tumor diagnosis, and help in treating cancer patients.

According to the WHO Classification of Thoracic Tumors (2021), 5th Ed., Vol.5, benign and malignant tumors of the thymus are classified in the following manner:

Tumors of the thymus:

Epithelial tumors

• Thymoma
  • Type A thymoma (including atypical subtype)
  • Type AB thymoma
  • Type B1 thymoma
  • Type B2 thymoma
  • Type B3 thymoma
  • Micronodular thymoma with lymphoid stroma
  • Metaplastic thymoma
  • Lipofibroadenoma of the thymus
• Thymic carcinoma
  • Squamous cell carcinoma of the thymus
  • Basaloid carcinoma of the thymus
  • Lymphoepithelial carcinoma of the thymus
  • NUT carcinoma of the thorax
  • Clear cell carcinoma of the thymus
  • Low-grade papillary adenocarcinoma of the thymus
  • Mucoepidermoid carcinoma of the thymus
  • Thymic carcinoma with adenoid cystic carcinoma-like features
  • Enteric-type adenocarcinoma of the thymus
  • Adenocarcinoma NOS of the thymus
  • Adenosquamous carcinoma of the thymus
  • Sarcomatoid carcinoma of the thymus
  • Undifferentiated carcinoma of the thymus
  • Thymic carcinoma NOS

Thymic neuroendocrine neoplasms

• Thymic neuroendocrine neoplasms
• Neuroendocrine tumors
  • Carcinoid/neuroendocrine tumor of the thymus
• Neuroendocrine carcinomas
  • Small cell carcinoma of the thymus
  • Large cell neuroendocrine carcinoma of the thymus

Additional thymic tumor articles (on DoveMed) that are not part of WHO Classification of Thoracic Tumors:

Ectopic Hamartomatous Thymoma: It is an infrequent benign tumor of the soft tissues, mostly occurring around the shoulder bones, below the neck. The tumor is generally seen in middle-aged men.

Intrapulmonary Thymoma: It is an epithelial lung tumor that is very similar to thymomas arising from the mediastinum. These tumors are generally observed in middle-aged adults.

Lymphoma of Thymus Gland: It is an uncommon lymphoma that is generally observed in older adults. The thymus gland is a lymphoid organ located in the chest cavity between the lungs.

MALT Lymphoma of Thymus Gland: It is a rare form of non-gastric type of extranodal marginal zone lymphoma. It affects the thymus gland in older adults. The condition is also known as Marginal Zone Lymphoma of Thymus Gland.