ALK-positive diffuse large B-cell lymphoma: report of three cases

Diffuse large B-cell lymphoma positive for anaplastic lymphoma kinase (ALK(+) DLBCL) is a rare variant of diffuse large B-cell lymphoma, with characteristic morphological, immunohistochemical and cytogenetic features. Only 34 cases of ALK-positive diffuse large B-cell lymphoma have so far been reported in the literature. We examined three new cases, which showed similar characteristics to previously reported cases, but with peculiar nuclear-membrane staining for ALK protein in one patient and a 5'-ALK gene deletion in another. All of them had stage IV disease at initial presentation, with poor outcomes. The tumour cells showed immunoblastic/plasmablastic histology and were positive for ALK and Oct2, but negative for CD3, CD20, CD79a, CD30 and PAX5. The staining pattern of ALK protein was cytoplasmic in two patients and associated with the nuclear membrane in one patient. Fluorescence in situ hybridization (FISH) analysis using the ALK break-apart probe revealed ALK gene rearrangements in all three patients, with a 5'-ALK gene deletion in one patient. These three cases suggest that different types of cytogenetic aberrations may involve the ALK gene in ALK-positive diffuse large B-cell lymphoma leading to peculiar immunohistochemical staining patterns.