Supratentorial Primitive Neuroectodermal Tumor (sPNET)

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  • 86 Controversies in Neuropathology

    Neuro-Oncologist: Youve got to help me with this case. It was called PNET at one institution but malignant glioma at another. Its a frontal lobe tumor in a 5-year-old.

    Pathologist: Well, I see the problem. Take a look. Its a highly cellular blue cell tumor all right. Theres focal necrosis with maybe some pseudopalisading, but not really. Theres mild nuclear pleo-morphism, and no vascular proliferation, and no neuronal or neu-roblastic features other than trapped cortical ganglion cells. Its cer-tainly malignant, but Im not sure I can tell you what it is. Theres some infiltration, and maybe a bit of perineuronal satellitosis, but Im not sure that helps. Is the diagnosis of small cell malignant tumor good enough?

    Neuro-Oncologist: No. If its PNET the whole neuraxis will be irradiated, but only the tumor area if its malignant glioma. Che-motherapy would not be the same either. Treatment protocols are very different. Theres no evidence of CSF dissemination so far in this case, but Im concerned about under treatment if its PNET and I treat it like a malignant glioma.

    Pathologist: I understand the protocol issue, but has it ever been established that supratentorial PNETs are more prone to CSF dis-semination than malignant gliomas?

    Neuro-Oncologist: I guess the assumption has been that neur-axis radiation is standard for medulloblastomas and it should be for supratentorial PNETs as well, but I dont know of any controlled study. In any case, I have little flexibility since neuraxis treatment is basically standard-of-care for PNET. The parents are devastat-ed. They had a hope of cure with the original diagnosis of PNET, but now if its malignant glioma, they know what that means. I thought it was easy to distinguish PNETs from malignant glio-mas.

    Pathologist: Not really, at least not to me. Theres little agree-ment between pathologists. Some lean toward malignant glioma in this situation or just malignant tumor, particularly if theyre more experienced in adult CNS tumors. Others are liberal with the diagnosis of PNET. A glioblastoma with features of the lesion as it occurs in adults is distinguished easily from a supratentorial neuroblastoma with Homer-Wright rosettes. A small cell tumor that is diffusely immunoreactive for synaptophysin would qualify readily as PNET as well. But its much different with lesions such as your case without specific histological or immunohistochemical features. I must say, that all of these pediatric malignant tumors

    tend to overlap. No two cases seem exactly the same and they often dont fall into established diagnostic categories. Its hard to get trac-tion. I know its important to you, but its just not clear.

    Neuro-Oncologist: Was immunohistochemistry done?

    Pathologist: Several times. You can see this synaptophysin stain-ing here that is consistent with PNET, but its focal and could well be that of trapped cortex. GFAP staining is prominent as in these perivascular astrocytes overrun by the tumor. Theres reactivity over here too, but its hard to sure whether its of reactive astrocytes or tumor cells. Even if tumor cells were positive thered be the difficult issue of small cell malignant glioma versus PNET with glial differ-entiation, and I have no idea how to distinguish these two.

    Neuro-Oncologist: Can anything else be done?

    Pathologist: Well, we could take tissue from the paraffin block and look for neuroblastic differentiation in the form of microtu-bules and neurosecretory granules, but Im not optimistic that the findings would be definitive.

    Neuro-Oncologist: What about molecular or cytogenetic fea-tures?

    Pathologist: Sounds good, but theres little known. Theres lots of papers about PNETs, but they basically all describe medullo-blastomas. Even if there were a large series of supratentorial tumors Id be skeptical of the conclusions given the problem of defining PNET at the histological level. I mean, would the present case be included or not? In some problem cases, like yours, almost every tumor cell is immunoreactive for p53, but whether this favors gli-oma or PNET is unclear.

    Neuro-Oncologist: How confusing. I had no idea. What do you suggest?

    Pathologist: It wont help you here but someone needs to iden-tify individual prognostic factors in pediatric small cell supratento-rial tumors, rather than evaluate outcome on the basis of tumors diagnosed by a priori definitions. The latter seem clear on paper but are hard to apply in practice. It would take large number of patients, and would be a HIPPA nightmare, but without such a study cases such as this will continue to be decided on an institu-tion-to-institution basis.

    Neuro-Oncologist: Thanks. I guess.


    Supratentorial Primitive Neuroectodermal Tumor (sPNET)

    Peter C. Burger, M.D.

    Department of Pathology, Johns Hopkins University, Baltimore, Md.

    Editors Note: Suptratentorial primitive neuroectodermal tumor is a controversial entity, one often difficult to differentiate from a high grade glioma. The following hypothetical dialogue between a Neuro-Oncologist and his Pathologist colleague illustrates some of the difficulties that arise in the diagnosis and treatment options for affected patients.