Prethymic adult lymphoblastic lymphoma. A clinicopathologic and immunohistochemical analysis.

The clinical, histologic, and immunohistologic features of three cases of pre-T-cell (CD7+/CD2-) lymphoblastic lymphoma in adults are reported. The patients were adults over age 50 years who had a relatively indolent nodal disease, partial involvement of lymph nodes, and primitive immunophenotype. The phenotype of the three cases was TdT+, HLA-DR+, CD34+, CD71+, CD38+, and CD7+, most resembling the normal prothymocyte, and in contrast to normal thymocytes, which generally coexpress CD1+, CD4+, and CD8+. The prethymic T-cell character was further supported by germline T-cell receptor beta and gamma chain genes. In contrast to most reported cases with this early immunophenotype, these patients had nodal disease but not peripheral blood involvement. Two of the three cases were associated with Langerhans' cell histiocytosis (histiocytosis X), a previously unreported association. Because of the Langerhans' cell histiocytosis and the relatively indolent clinical presentation, the differential diagnosis in all cases included both a benign process and a lower-grade lymphoma. Recognition of this unusual form of adult lymphoblastic lymphoma is essential for correct diagnosis and treatment.