Background Flashcards
What is the embryonic derivation of the thymus?
The embryonic derivation of the thymus is the 3rd pharyngeal pouch.
Where is the thymus located, and what is its function?
The thymus is in the ant mediastinum (ME), involved in the processing and maturation of T lymphocytes to recognize foreign antigens from “self” antigens.
What structures are located in the ant, middle, and post ME?
Ant: LNs, thymus, mesenchymal tissues
Middle: Heart and great vessels, trachea, esophagus, most mediastinal LNs, vagus and phrenic nerves
Post: Paraspinal tissues, sympathetic and peripheral nerves
What proportion of tumors of the ME are malignant?
One-third of mediastinal tumors are malignant.
How prevalent is thymoma relative to other mediastinal tumors?
Thymoma comprises 20% of all mediastinal tumors but 50% of all ant mediastinal tumors.
What is the sex and ethnic/racial predilection for thymomas?
There is no sex predilection (male = female). Asian/Pacific Islanders and African Americans in the United States have a higher incidence of thymomas. (Engels, J Thorac Oncol, 2010)
What age group has the highest incidence of thymomas?
Pts in the 7th decade of life (Engels, J Thorac Oncol, 2010)
Are thymomas common in children?
No. Thymomas are extremely rare in children, but if present they are extremely aggressive with poor survival.
Do thymic malignancies usually present as a result of radiation-induced secondary malignancy?
No. Development of a thymic malignancy is rare after radiation therapy. There is currently no environmental or infectious risk factor associated with development of thymic malignancies.
How do thymic carcinomas differ from thymomas?
Thymic carcinomas are much less prevalent (<1% of thymic tumors), very aggressive, with worse survival. They are usually not associated with paraneoplastic syndromes (e.g., myasthenia gravis [MG]). Thymic carcinomas do not have immature T lymphocytes. They lack the lobulated pattern separated by thick fibrous bands usually seen in thymoma. They also lack histologic features such as Hassall corpuscles, medullary differentiation, and perivascular spaces.
What is the LN metastatic rate of thymomas vs. thymic carcinomas?
Thymoma: ∼1%–2%
Thymic carcinoma: ∼30%
(Kondo K et al., Ann Thorac Surg 2003 [review of 1,320 pts with thymic tumors])
What is the probability of hematogenous dissemination of thymomas vs. thymic carcinomas?
Thymoma: ∼1% (mostly to lung)
Thymic carcinoma: 12% (lung > bone, liver)
Pathologically, what is the most important defining feature of thymomas?
Coexistence of nonneoplastic lymphoid cells with neoplastic epithelial cells (spindle to polygonal types)
What are the WHO histologic classification divisions of thymomas and thymic carcinomas?
WHO type is based on shape and the lymphocyte/epithelial ratio
Thymoma types (frequency %)
WHO type A (4%–7%): Spindle-oval cells with few to no lymphocytes
WHO type AB (28%–34%): Spindle cells mixed with immature lymphocyte-poor and lymphocyte-abundant areas
WHO type B1 (9%–20%): Immature T-cells with areas of both normal thymic cortex and medulla
WHO type B2 (20%–36%): Large polygonal epithelial cells with even mixture of lymphocytes
WHO type B3 (10%–14%): Sheets of polygonal epithelial cells with mild–moderate atypia and scant lymphocytes
Micronodular thymoma with lymphoid stroma: Epithelial nodules surrounded by lymphoid stroma containing mature B- and T-cells but devoid of epithelial cells
Metaplastic thymoma: Alternating epithelial cells and bland slender spindle cells with absence of immature T-cells
Thymic carcinomas
Squamous cell, basaloid, mucoepidermoid, lymphoepithelioma-like, sarcomatoid, clear cell, adenocarcinomas, NUT, and undifferentiated carcinomas.
What value does the WHO histologic classification system have in clinical decision-making for thymomas?
Controversial d/t histologic heterogeneity, but some studies have shown that histologic subtype is an independent prognostic factor in early-stage Dz as WHO Type A, AB, and B1 thymomas purport lower-risk Dz compared to B2, B3 (Chen et al., Cancer 2002). However, stage is still the most important clinical factor. WHO classification assists in the ability to recognize the variable appearance as thymomas, thereby providing standardization for pathologists.
