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Significance of grading and general concepts in low grade brain tumors
Grade describes specific features in a tumor that are linked with specific outcomes.
Rapidly growing tumors are given a higher grade which are associated with a poor prognosis, while the converse is true with the lower grades of tumors.
Low-grade gliomas have specific features and are graded by features visualized from the tumor tissue with the following criteria as recognized by the World Health Organization (WHO):
Grade I: These are the least malignant tumors, associated with long-term survival, and appear in shape and size like normal cells microscopically. Surgery is curable for these tumours. Examples include pilocytic astrocytoma, craniopharyngioma, gangliocytoma, gangliogioma
Grade II: These tumors appear slightly abnormal, have the ability to spread locally, and can recur possibly as a higher grade. Examples include astrocytoma, ependymoma, or oligodendroglioma.
Grade III: These tumors are not very different from Grade II tumors, but can actively reproduce quickly, spreading more frequently to other brain tissues, and are likely to recur as Grade IV tumors. Examples include anaplastic astrocytoma, anaplastic oligodendroglioma,gemistocytic astrocytoma and anaplastic ependymoma.
Grade IV: These are most malignant tumors. They reproduce rapidly, have a peculiar appearance under the microscope, and invade surrounding brain tissue. New blood vessels are observed that make multiplication of abnormal cells easy. Glioblastoma multiforme is an example.
Low grade gliomas (LGGs) represent up to 30% of gliomas and affect patients at a younger age than high-grade gliomas. LGGs are commonly located in and around those areas of the brain involved in motor, language, visuospatial and memory function. The 5-year overall and progression-free survival rates range from 58% to 72% and 37% to 55% respectively. Studies have indicated that these patients may survive for up to 20 years. However, some of these tumors grow continuously and tend to progress to a higher grade, leading to loss in neurological function associated with the specific location and can end in death.
Seizures are the most common (90%) presentation and can be intractable in half of the cases. Seizures are commonly seen in cortically based tumors, particularly in frontal, temporal, insular/parainsular location and with oligodendroglial tumors.
Grading of the tumor does not seem to be associated with the epileptic severity. Focal neurological deficits take years to manifest, however, symptoms of raised intracranial pressure may sometimes be observed. Poorer prognostic factors include age>40 years with neurological deficit and large tumors crossing the midline. Oligodendrogliomas have a better prognosis than astrocytomas, whereas oligoastrocytomas have an intermediate outcome. At the molecular level, IDH1 codon 132 mutation has been recently suggested as an independent favorable prognostic factor.
Investigations include MRI and LGGs appear as low-signal mass lesions on T1-weighted MRI and high signal on T2- weighted and FLAIR sequences. Contrast enhancement only occurs when there is a focal area of high-grade transformation, although some tumors, particularly oligodendrogliomas can have a stable patchy enhancement. Large tumors crossing the midline carry a poorer prognosis. PET scanning is usually not indicated.
Treatments for LGGs include antiepileptic treatment though prophylactic therapy with them is not successful. Total resection provides a good survival; radiotherapy improves symptom control as well as shrinks tumors.According to the new WHO classification that includes the molecular markers;high risk LGGs should receive radiation +/-oral temozolamide as an adjuvant after surgery to prevent recurrences or progression. Chemotherapy is an option in high-risk patients including those with large tumors, recurrence after surgery or radiation.