Mediastinal lymphoblastic lymphoma with non-lymphoblastic histologic features.

The clinical and pathologic features of an unusual case of mediastinal lymphoma in a 46-yr-old man are presented. Histology of the tumor was that of a diffuse, non-large cell lymphoma. The nuclear chromatin was coarse, suggesting a relatively mature stage of lymphocyte maturation, and the lymphoma was provisionally classified as diffuse small-cleaved cell lymphoma. Immunohistologic study and cell surface marker analysis revealed a common thymocytic phenotype (CD3, CD1a, CD4, CD8, and TdT positive), however, and DNA flow cytometric analysis revealed a high S+G2 M fraction of 29%. With the immunostaining profile accepted as definitive for lymphoblastic lymphoma, histologic features were reassessed in retrospect. Cells having nuclear features typical of lymphoblasts were not recognized. Occasional cases of lymphoblastic lymphoma may not be recognizable when evaluated only by histology using the generally accepted criteria for diagnosis.