True histiocytic lymphoma following B-acute lymphoblastic leukaemia: case report with evidence for a common clonal origin in both neoplasms

True histiocytic lymphoma (THL) is a very rare type of non-Hodgkin's lympho ma (NHL) in which neoplastic cells exhibit markers of histiocytic different iation. Some cases of THL have been reported in patients with previous acut e lymphoblastic leukaemia (ALL), especially in children and young adults, i n whom the acute leukaemia was of T-cell origin. The relationship between t he initial lymphoid tumour and the secondary THL remains unclear, as a comm on monoclonal origin shared by both neoplasms has never been definitively d emonstrated. We report a patient with B-ALL who developed a nodal and extra nodal tumour with histological and immunohistochemical features of THL 4 ye ars after the initial diagnosis. Genotypic study showed that both neoplasms contained the same immunoglobulin heavy gene rearrangement, which has not been reported previously.