Development of peripheral T-cell lymphoma not otherwise specified in an HTLV-1 carrier

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL) after a long latency period of about 60 years. As the mature T-cell neoplasms that emerge in patients infected with HTLV-1 are often ATL, T-cell neoplasms developing in such patients tend to be diagnosed simply as ATL without further investigation. However, not all T-cell neoplasms that develop in HTLV-1-infected cases are ATL. Mature T-cell malignancies other than ATL should be carefully excluded in patients infected with HTLV-1, as these sometimes closely resemble ATL in their clinical, morphological, and histological features. Here, we present a case of peripheral T-cell lymphoma not otherwise specified (PTCL-NOS) in an HTLV-1 carrier. Confirmation of monoclonal integration of the virus with Southern blotting leads to a definite diagnosis of ATL. Although we did not detect the monoclonal integration band of HTLV-1 in this case, the high HTLV-1 proviral load complicated the diagnosis. Multicolor flow cytometric analysis clearly showed that HTLV-1 was not integrated in the tumor cells, and facilitated discrimination of PTCL-NOS from ATL.