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Presented by Pedram Argani, M.D. and prepared by Jon Davison, M.D.
Case 2: 72 year-old male with a mediastinal mass.
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1. Question
Week 225: Case 2
72 year-old male with a mediastinal mass.images/JMD_5-30-05_SPWC/Case_2/1.jpg
images/JMD_5-30-05_SPWC/Case_2/2.jpg
images/JMD_5-30-05_SPWC/Case_2/3.jpg
images/JMD_5-30-05_SPWC/Case_2/4.jpgCorrect
Answer: Invasive thymoma
Histology: This tumor has an invasive growth pattern, manifested as large tongues of tumor cells infiltrating into the perithymic soft tissue. The neoplasm is composed of polygonal epithelioid cells without significant pleomorphism or mitotic activity, with an admixed population of lymphocytes. By immunohistochemical stains, the lymphocytes have the phenotype of immature thymic lymphocytes (CD3-positive, TdT positive, CD99-positive) but the epithelial cells do not label for CD5.
Discussion: Thymic carcinoma would be characterized by epithelioid tumor cells demonstrating pleomorphism, and lacking in immature thymic lymphocyte component. Cytologically, an encapsulated thymoma and invasive thymoma are identical; this distinction is made based upon invasive architecture. Metastatic carcinoma (for example, from the lung) would involve a lymph node and would demonstrate greater pleomorphism.
This thymoma would be best classified as B3 in the 2004 WHO Classification. This neoplasm has an abundance of epithelioid thymic epithelial cells (“bioactive”), and a paucity of thymic lymphocytes. Tumors with this histology tend to present with invasive disease, and some have termed these “well differentiated thymic carcinoma”.
Incorrect
Answer: Invasive thymoma
Histology: This tumor has an invasive growth pattern, manifested as large tongues of tumor cells infiltrating into the perithymic soft tissue. The neoplasm is composed of polygonal epithelioid cells without significant pleomorphism or mitotic activity, with an admixed population of lymphocytes. By immunohistochemical stains, the lymphocytes have the phenotype of immature thymic lymphocytes (CD3-positive, TdT positive, CD99-positive) but the epithelial cells do not label for CD5.
Discussion: Thymic carcinoma would be characterized by epithelioid tumor cells demonstrating pleomorphism, and lacking in immature thymic lymphocyte component. Cytologically, an encapsulated thymoma and invasive thymoma are identical; this distinction is made based upon invasive architecture. Metastatic carcinoma (for example, from the lung) would involve a lymph node and would demonstrate greater pleomorphism.
This thymoma would be best classified as B3 in the 2004 WHO Classification. This neoplasm has an abundance of epithelioid thymic epithelial cells (“bioactive”), and a paucity of thymic lymphocytes. Tumors with this histology tend to present with invasive disease, and some have termed these “well differentiated thymic carcinoma”.
