Bone marrow involvement in NPM-ALK-positive lymphoma: report of two cases

Two cases of NPM-ALK-positive anaplastic large cell lymphoma (ALCL) with bone marrow involvement are reported. These cases were recognized within a group of NPM-ALK-positive ALCLs (n = 6) by using immunohistochemistry with the ALK1 monoclonal antibody. In case 1, the bone marrow showed diffuse infiltration of round to spindle-shaped lymphoma cells with moderate fibrosis. In case 2, lymphoma cells intermingling with hematopoietic cells could only be identified by immunohistochemical staining. In contrast to the four NPM-ALK-positive ALCL cases, which showed a cohesive growth pattern in the lymph nodes, the two cases reported here displayed lymphoma cells of smaller size, and they were classified as lymphohistiocytic variants histologically. ALK1 stained small-sized components more clearly than did CD30 (HRS-4). These results suggest that bone marrow involvement of NPM-ALK-positive ALCL may be frequently associated with a histological variant showing a small-sized cell component, and that ALK1 immunostaining is a useful tool to investigate lymphomas for bone marrow involvement.