Anaplastic large cell lymphoma: another entity in the differential diagnosis of small round blue cell tumors.

We saw in consultation a biopsy specimen from a 6-year old girl with anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma (ALCL). The tumor arose in soft tissue of the neck, and diagnostic tissue was obtained by core needle biopsy. Histologically, the neoplasm was cellular without pattern. Immunohistochemical workup with a large panel of antibodies at another institution showed immunoreactivity for NB84 and neuron specific enolase (dim). Antibodies specific for CD3, CD20, and CD45/LCA were negative; CD30 or ALK were not assessed. Electron microscopy showed cytoplasmic structures thought to be neurosecretory granules. The neoplasm was interpreted initially as a neuroblastoma. At the time of our review, we considered the possibility of ALCL. Immunohistochemical analysis for CD30 showed bright, uniform expression and ALK was positive in a nuclear and cytoplasmic pattern, confirming the diagnosis of ALK+ ALCL. The purpose of this review is to discuss ALK+ ALCL and many of the other entities included under the rubric of small round blue cell tumor, with a focus on tumors that occur in children.