Medulloblastoma

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Contents

Epidemiology

Most common malignant brain tumor in children (20% of all pediatric brain tumors with 65% of all pediatric brain tumors occurring in the posterior fossa)

Clinical Presentation

Symptoms usually related to hydrocephalus and include morning headache and vomiting; bilateral papilledema is present in most children at the time of diagnosis; unilateral CN 6 palsy is the most common cranial nerve deficit

Treatment

  1. Surgery:
    1. external ventricular drain (EVD) is typically placed at the same time as the craniectomy/craniotomy for tumor removal
    2. 10-30% of patients will require post-operative shunts and is more common in the younger patients and those with larger tumors and large ventricles pre-operatively
    3. occipital sinus in the midiline between the Torcula and the foramen magnum must be temporarily occluded as the dura is divided in the midiline then permanently occluded by coagulation or sutures; circular sinus at the foramen magnum is frequently patent until the age of 1 year and can also be occluded
    4. tonsils are separated and if possible a patty is inserted at the floor of the brainstem underneath the tumor (about 1/3 of medulloblastomas invade the floor of the 4th ventricle)

Outcomes

  1. preoperative deficits worsen in 40% of patients and only 14% of those new deficits get better
    1. around 15% of patients develop posterior fossa syndrome characterized by mutism, whining or abulia, pseudobulbar palsies with drooling or inability to swallow with incrasing ataxia, paresis or visual impairment
      1. thought to be due to edema of the dentatorubrothalamic tract and to splitting of the inferior vermis
  2. medulloblastoma is staged through the Chang staging system (T and M); those older than 4 and those with < 1.5 cm of residual tumor have better outcomes
  3. radiation includes 2400-3600 cGy to the craniospinal axis
  4. chemotherapy is still in the trial stages

Neuropathology

  • i. Nearly all medulloblastomas are sporadic with an allelic loss of chromosome 17p found in 50% and <5% having p53 mutations
    • 1. most common malignant brain tumor in children
    • 2. more common in males
  • ii. Composed mainly of undifferentiated cells with hyperchromatic nuclei and sparse cytoplasm; densely cellular; derived from external granular layer of the cerebellar cortex
  • iii. Patients with Gorlin’s basal cell nevus syndrome or Turcot’s syndrome (chromosome 5q) are prone to medulloblastomas; Remember: think of medulloblastoma and Gor(e)T(ex) which may be used to close the meninges after a medulloblastoma case
  • iv. Neuronal differentiation may take the form of rosettes that lack a central canal or capillary (Homer-Wright rosettes); medulloblastomas are reactive to synaptophysin
  • v. Medulloblastomas do not typically arise from the floor of the fourth ventricle and do not show widespread pseudorosettes and GFAP immunoreactivity like ependymomas do and can be differentiated on that basis; most arise from the cerebellar hemisphere
  • vi. Desmoplastic medulloblastoma differs from other medulloblastomas histologically by reticulin free islands of monomorphic round cells among the reticulin rich areas
  • vii. CT: typical medulloblastoma is pear or heat shaped in the midline of the vermis that displaces the fourth ventricle anteriorly; calcification may occur in up to 50%; contrast enhancement is heterogenous and does not occur at all in 3%
  • viii. MRI: typically fills the fourth ventricle through the foramen of Magendie into the cisterna magna (cerebellomedullary cistern); obstructive hydrocephalus is common; signal characteristic is heterogeneously hypointense to gray matter with cysts seen in 75% of cases; contrast enhancement is variable
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