LAUSR

Most ALK+ LBCL are CD30-negative/weak, whereas ALK+ ALCL are strongly and uniformly CD30 positive, making the differential diagnosis theoretically very simple. Nevertheless, as seen in the present case, strong uniform CD30 expression is possible in ALK+ LBCL. Together with an absence of significant expression of common B cell markers (CD20, PAX-5, CD79a) and strong expression of CD4 and cytotoxic molecules, this may lead to a misdiagnosis of ALK+ ALCL, especially in the presence of an NPM–ALK rearrangement (more characteristic of ALK+ ALCL). A useful hint to the correct diagnosis lies in cell morphology: it is mainly immunoblastic/plasmablastic in ALK+ LBCL and highly pleomorphic, often with ‘hallmark’ forms in ALK+ ALCL. Hence, when confronted with atypical features in an ALK+ tumour, determining the right lineage (B or T/NK) by additional immunohistochemical and/or molecular studies is key to the correct diagnosis. It is also worth noting that this distinction is not purely academic, as the two entities have a different prognosis and are treated differently.