Primitive neuroectodermal tumors including the medulloblastoma: glial differentiation signaled by immunoreactivity for GFAP is restricted to the pure desmoplastic medulloblastoma ("arachnoidal sarcoma of the cerebellum").

Immunoreactivity of tumor cells for glial fibrillary acidic protein (GFAP) is usually regarded as sign of astrocytic histogenesis and/or differentiation. The present study aimed at a systematic evaluation of the significance of GFAP-containing cells in primitive neuroectodermal tumors (PNETs) with special reference to the controversial entity of desmoplastic medulloblastoma (so-called "circumscribed arachnoidal sarcoma of the cerebellum"). Fifty-three PNETs, including 17 pure desmoplastic medulloblastomas were investigated, using GFAP antisera and the peroxidase-antiperoxidase (PAP) technique. Seventy percent of the pure desmoplastic medulloblastomas showed GFAP immunoreactive cells, in 47% indistinguishable from adjacent nonreacting tumor cells. Most immunoreacting cells were found in the reticulin free islands, showing in 6 cases a gradual transition of immunoreacting cells from tumor cells to larger cells shaped like astrocytes. The classical medulloblastomas showed only larger immunoreacting cells which were interpreted as reactive astrocytes. Therefore, the so-called circumscribed arachnoidal cerebellar sarcoma or pure desmoplastic medulloblastoma merits a separate place in the group of PNETs as a tumor with frequent signs of astroglial differentiation; this interpretation appears to be clinically correlated by a difference in age incidence and prognosis of that special tumor-type in comparison with classical medulloblastoma.