Thymoma

Gross Features

-    Lobulated, firm, gray-white masses, up to 15-20 cm in the largest dimension; malignant thymoma tends to be fleshy in appearance

-    Sometimes have areas of cystic necrosis and calcification

-    Majority are encapsulated; up to 25% infiltrate capsule and invade perithymic tissues and structures

-    Malignant thymoma infiltrates capsule into surrounding tissues; metastases to lungs may be seen

Microscopic Features

-    Consist of jigsaw puzzle-type lobules separated by fibrous bands

-    All are made up from epithelial cells and a variable infiltrate of non-neoplastic thymocytes

-    Benign thymomas often composed of medullary-type epithelial cells, or a mixture of cortical and medullary-type epithelial cells

-    Medullary-type thymomas have elongated, spindle-shaped epithelial cells with a sparse infiltrate of thymocytes

-    Mixed thymomas have an admixture of plumper, rounder, cortical-type epithelial cells and a denser infiltrate of lymphocytes

-    Type I malignant thymomas possess epithelial cells mainly of the cortical variety (rounder, plumper, with abundant cytoplasm and round, vesicular nuclei) accompanied by immature thymocytes that express TdT

-    Palisading of epithelial cells around blood vessels is sometimes seen; minor areas of spindled “medullary-type” epithelial cells are commonly seen

-    Type II malignant thymomas are microscopically well-differentiated or poorly-differentiated squamous cell carcinomas

-    Next most common variant is lymphoepithelioma-like carcinoma composed of sheets of cells with indistinct borders similar to nasopharyngeal carcinoma

-    Other variants also exist (sarcomatoid, basaloid, clear cell variants)

Immunohistochemistry

-    Cam 5.2, CD5, BCL-2, CD15 positive

-    TTF-1, CD20, PLAP negative