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thymoma 1 thymoma2 
thymoma3 thymoma4

Case history

An 84-year-old woman presents with a mediastinal mass.  Representative sections are shown.

What is the diagnosis?

  1.         Solitary fibrous tumor
  2.         Small lymphocytic lymphoma
  3.         Thymoma, Type A
  4.         Thymoma, Type AB

Answer: D. Thymoma, Type AB

Brief explanation of the answer: 

The histologic findings show a lesion composed of spindle cells and lymphocytes.  The spindle cells are monotonous without significant nuclear atypia.  These findings are consistent with thymoma, type AB or mixed thymoma.  The World Health Organization divides thymic tumors into the following categories:

Type A: Spindle cell/medullary, composed of spindle cells

Type AB: Mixed type, composed of spindle cells and lymphocytes

Type B1: Lymphocyte rich, resembling normal thymus

Type B2: Composed of polygonal epithelial cells with abundant cytoplasm and vesicular nuclei with lymphocytes

Type B3: Predominantly epithelial cells with minor lymphocyte population

Type C: Thymic carcinoma

Thymic carcinomas tend to be unencapsulated with obvious anaplastic features.

Case contributed by Diana Lin, M.D., Assistant Professor, Anatomic Pathology, UAB Department of Pathology