Neuro-oncology: Molecular Classification Guides Prognosis
Integrate histology with molecular markers
Modern brain tumor questions expect molecular stratification. Adult diffuse gliomas are classified by IDH status and 1p/19q codeletion.
Adult gliomas. Glioblastoma is IDH wild-type and grade 4, typically with central necrosis and ring enhancement; IDH-mutant astrocytoma carries a substantially better prognosis. Oligodendroglioma is IDH mutant with whole-arm 1p/19q codeletion and often calcifies. MGMT promoter methylation predicts benefit from temozolomide. Recurrent contrast enhancement within about 12 weeks of radiation may be pseudoprogression rather than true progression; perfusion imaging and serial scans help distinguish them. Radiation necrosis typically appears months later in the prior high-dose field.
Other tumors. Meningiomas are extra-axial, dural-based, and enhance homogeneously with a dural tail. Metastases favor the gray-white junction and are often multiple. Primary central nervous system lymphoma occurs in immunocompromised patients and in older adults, shows homogeneous enhancement with restricted diffusion, and may regress with corticosteroids; obtain biopsy before initiating steroids when possible. Vestibular schwannoma causes unilateral sensorineural hearing loss and is imaged with thin cuts through the internal auditory canal.
Clinical problems. Seizures are common with cortical tumors; use nonenzyme-inducing antiseizure medications when possible. Carcinomatous meningitis produces headache, cranial neuropathies, and multifocal symptoms with enhancing leptomeningeal disease. Pituitary apoplexy causes sudden headache, visual field loss, and ophthalmoplegia, and is an endocrine emergency.
Traps: hydrocephalus from a posterior fossa mass can produce symptoms out of proportion to headache; a single enhancing lesion in a patient with known malignancy is not automatically a metastasis.
Rule of thumb: new enhancement within 12 weeks of radiotherapy is pseudoprogression until serial imaging or perfusion studies prove otherwise.