Case 585 -- Posterior fossa tumor

Contributed by Turki Omar Al-Hussain, MBBS, and Mohammad Anas Dababo, MD
Department of Pathology and Laboratory Medicine, King Faisal Specialist Hospital and Research Center, Riyadh, 11211, Saudi Arabia


A 2-year-old girl was admitted to the department of neurosciences, King Faisal Specialist Hospital and Research Center, Riyadh, Saudi Arabia, with three weeks history of deterioration of walking, then became unable to walk and later she developed a projectile vomiting mostly in early morning. There was no history of seizures or loss of consciousness. At admission her neurological examination revealed bilateral papilledema, nystagmus and truncal ataxia with intention tremor. Magnetic Resonance Images (MRI) of the brain showed a 3x4 cm posterior fossa enhancing mass extending from the base of the cerebellum to the roof of the fourth ventricle without calcification (Figure. 1, sagittal T-1 weighted with contrast) and (Figure 2: axial T2-weighted with contrast). Then, the patient underwent midline suboccipital craniotomy with a near total resection of the tumor.


Gross examination of the tumor showed white-tan soft friable tissue. Microscopically, the tumor consisted of highly cellular clusters of small to medium-sized cells with hyperchromatic round to oval nuclei and indistinct cell borders (Figure 3) admixed with areas showing fine fibrillary, paucicellular, neuropil-like matrix (Figure 4). Also noted were areas containing true rosettes (ependymoblastomatous type) with well-formed central lumina (Figure 5). These rosettes consisted of multilayered cells with hyperchromatic nuclei, numerous mitotic Figures and apoptotic bodies (Figure 6). The lumina of some rosettes contained granular eosinophilic material. Periodic Acid Schiff (PAS) special stain was positive along the luminal border of the true rosettes and highlighted some of the granular material within the lumina (Figure 7). Immunohistochemical studies were characterized by positivity for synaptophysin (Figure 8) and CD56 (Figure 9) in the neuroblastic tumor cells and neuropil-like areas. Glial fibrillary acidic protein (GFAP) was only positive in scattered cells in these areas (Figures 10 & 11). Neurofilament protein was negative. The true ependymoblastomatous rosettes were negative for CD56, GFAP and synaptophysin but they strongly expressed vimentin (Figure 12). The Ki-67 was much higher in the ependymoblastomatous rosettes (over 90%) than in the neuroblastic areas (approximately 15%), as seen in (Figure 13). The patient was then treated with chemotherapy which included courses of cyclophosphamide and vincristine, cisplatin and etoposide (VP16). Unfortunately, the patient developed recurrent disease 6 months after resection and chemotherapy.


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