Type B1 Thymoma (Lymphocyte-rich) : Definition
Type B1 thymoma closely resembles normal thymus cytologically and architecturally. It is usually an encapsulated mass with fibrous bands (seen here) separating it into nodules or lobules. It has areas resembling normal thymic cortex which are composed of scattered epithelial cells in a dense background of immature T-cells. Foci of medullary differentiation (seen here as lighter areas) with or without Hassallís corpuscles are always present. Given the abundance of lymphocytes, it is also known as lymphocyte-rich thymoma or lymphocytic thymoma and the differential diagnosis includes T lymphoblastic lymphoma.