Dorsal Exophytic Brainstem Tumors

Dorsal exophytic brainstem gliomas are a distinct subgroup of neoplasms that account for 20% of all brainstem tumors. They generally have a relatively benign histology and slow growth characteristics, and do not carry the near universal dismal prognosis seen in the intrinsic pontine gliomas. However, some do have higher grade histology, but do not share the same dismal prognosis of the intrinsic pontine glioma.

These tumors may present in early infancy and generally appear before puberty. They most often grow out of the floor of the fourth ventricle and into the ventricle itself, as opposed to invading the brainstem. The tumor may obstruct the flow of CSF. The MRI appearance is hypointense on T1 and hyperintense on T2, but unlike the more malignant intrinsic pontine glioma, these tumors have sharp borders and enhance intensely with contrast. And unlike many other brainstem tumors, they are sometimes amenable to surgical excision with a positive outlook (55).

Symptoms can be insidious, and patients may have a mild set of symptoms for months or years before the diagnosis is made. The initial symptoms may be caused by intermittent obstruction of CSF flow. Occasional vomiting and failure to thrive without overt neurologic signs may result in prolonged evaluations by specialists without correct diagnosis. Because of the location, the child may have only mild or intermittent cranial nerve paresis, and may not have any pyramidal tract findings unless the tumor is invasive. Symptoms will vary by age, with failure to thrive and chronic vomiting seen in infants, whereas older patients present with a combination of headache, vomiting, and ataxia (4).

The treatment for these tumors is primarily surgical, although a complete surgical excision is usually not feasible because there is not a plane between the tumor and brainstem. In addition, the tumor can invade a large area of the brainstem surface, in which case only the exophytic component can be removed. CSF flow may be re-established with successful debulking, but ifthis is not possible a ventriculo-peritoneal shunt or third ventriculostomy will be necessary.

These tumors most often have a low-grade glial histology and tend to be grade I or grade II astrocytomas. However, higher grade astrocytomas and gangliogliomas have been reported and therefore surgical consideration needs to be considered with each patient. The small percentage of tumors that recur typically remain histologically benign and are amenable to repeat tumor excision. Following a surgical excision, even if there is remaining tumor, it is reasonable to withhold radiotherapy or other treatments in favor of expectant observation with interval MR imaging and examinations (55-57).

Radiation therapy has been generally reserved for patients with either HGG or progression that is not amenable to repeat surgery. For those patients with high-grade neoplasms, it is recommended that surveillance MRI of the spine be performed to properly stage the patient's disease. Unless there is evidence of tumor spread, involved-field radiotherapy using standard dose and dose-fractions is recommended (55-57).

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