Thymoma and Thymic Carcinoma
Background
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 (MN), 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 MN?
Anterior: LNs, thymus, mesenchymal tissues
Middle: heart and great vessels, trachea, esophagus, most mediastinal LNs, vagus and phrenic nerves
Posterior: paraspinal tissues, sympathetic and peripheral nerves
What proportion of tumors of the MN 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 median age and gender predilection for thymomas?
The median age for thymomas is 40–60 yrs. There is no gender predilection (male = female).
Are thymomas common in children?
No. Thymomas are extremely rare in children, but if present they are extremely aggressive with poor survival.
Pathologically, what is the most important defining feature of thymomas?
Coexistence of nonneoplastic lymphoid cells with neoplastic epithelial cells (spindle to polygonal types)
How do thymic carcinomas differ from thymomas?
Much less prevalent (<1% of thymic tumors), very aggressive, with poorer survival (5-yr OS 30%–50%)
What are the WHO designations of thymomas vs. thymic carcinomas?
WHO type is based on shape and the lymphocyte/epithelial ratio.
WHO type A–AB: benign thymoma, medullary, spindle cell
WHO type B1–B3: malignant thymoma, lymphocytic, cortical, epithelial
WHO type C: highly malignant, thymic carcinoma, clear cell/sarcomatoid types
What is the LN metastatic rate of thymomas vs. thymic carcinomas?
Thymoma: ~1%–2%
Thymic carcinoma: ~30%
(Kondo & Monden review of 1,320 pts with thymic tumors [Ann Thorac Surg 2003])
What is the hematogenous dissemination of thymomas vs. thymic carcinomas?
Thymoma: ~1% (mostly to lung)
Thymic carcinoma: 12% (lung > bone, liver)
Workup/Staging
What is the DDx of a mediastinal mass by location in the ant, middle, and post MN?
Anterior: thymoma, thymic carcinoma, carcinoid, germ cell tumors, lymphomas (Mnemonic TTT: Thymoma, Teratoma, Terrible lymphoma)
Middle: cysts > lymphoma, teratomas > sarcomas (osteosarcoma, fibrosarcoma, angiosarcoma, rhabdomyosarcoma of the heart), granuloma
Posterior: neurogenic tumors (PNET, schwannoma, neurofibroma, neuroblastoma, ganglioneuroma), pheochromocytoma
What clinical presentations are common for pts with mediastinal tumors?
50% are diagnosed incidentally on imaging studies. Sx are caused by a mass effect resulting in cough, shortness of breath, pain, stridor, Horner, superior vena cava (SVC) syndrome, HTN (catecholamine), myasthenia gravis (MG) (thymoma)
How do pts with thymomas or thymic carcinomas usually present?
50% are incidental findings; but if there are Sx, they reflect either locally advanced Dz, metastatic sequalae, or paraneoplastic disorders (in 50%–60% of thymomas but hardly seen in thymic carcinomas).
What paraneoplastic disorders are commonly seen in thymomas?
MG (35%–50% of cases), red cell aplasia (5%), immune deficiency syndromes like hypogammaglobulinemia (5%), autoimmune disorders (collagen vascular, dermatologic, endocrine, renal Dz), and other malignancies (lymphomas, GI/breast carcinomas, Kaposi sarcoma)
What workup should be employed for a mediastinal mass?
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