Unique composite hematolymphoid tumor consisting of a pro-T lymphoblastic lymphoma and an indeterminate dendritic cell tumor: evidence for divergent common progenitor cell differentiation.

Until recently, hematopoietic neoplasms were considered monoclonal proliferations belonging to one cell lineage. In the last years, evidence for transdifferentiation from one cell lineage to another or divergent common progenitor cell differentiation has accumulated, mainly based on composite hematolymphoid tumors, sharing common genetic abnormalities. We report the case of a 59-year-old woman with a composite pro-T lymphoblastic lymphoma (LBL) and indeterminate dendritic cell tumor infiltrating the lymph nodes, bone marrow and stomach. Genetic analyses revealed that both cell populations bore +21, while a G13D mutation of the NRAS gene and monosomy 18 were detected only in the pro-T LBL. The synchronous appearance of two distinct uncommon hematolymphoid tumors in the same patient, recurrent at three different anatomic locations, with an identifiable common genetic denominator, namely +21, but also with unique genetic anomalies in the pro-T LBL raises the hypothesis of a divergent common progenitor cell differentiation.