Primitive neuroectodermal tumor (PNET) is a soft tissue tumor that arises from primitive neural crest tissue. It shares the same cytogenetic translocation as Ewings sarcoma, a tumor of the bone.
When present in cerebrospinal fluid cytospins, PNET cells tend to occur as singlets or doublets rather than large tumor clumps.
Notice how these two cells are "stuck" to each other. In a cytospin with this low number of cells, it would be unusual for two cells to be touching each other unless they were actually fused together- this is an important and distinctive feature of this condition.