Central nervous system tumours
Introduction
Tumours of the central nervous system (CNS) are an especially important group of tumours because of their disproportionate impact on patients’ wellbeing and longevity. As with tumours elsewhere in the body, CNS tumours may be classified as primary or secondary and as benign or malignant. In adults, the majority are secondaries from systemic cancers, however in children most are primary brain tumours.
Brain metastases develop in about 15% (10-30%) of cancer patients, although incidence is significantly higher in post-mortem series. In adults, the commonest tumours metastatic to the brain (in decreasing rate of incidence) are lung, breast, melanoma and colorectal cancers. Prognosis in patients with brain metastases depends predominantly on performance status, age, number of metastases and whether systemic disease is controlled or not. Treatment depends on symptoms, prognosis, site and number of metastases. Treatment may comprise surgical resection with control rates of 40-50%, radiosurgery (a highly focused and accurate form of radiotherapy) with control rates of 70-80% or whole brain radiotherapy. For very frail patients, supportive care alone may be the most appropriate management. Dexamethasone is helpful for most patients to counteract brain oedema commonly associated with metastases.
Primary CNS tumours represent just 2% of adult tumours but about 25% of childhood tumours and around 15% of adolescent tumours. In adults, brain tumour incidence increases with age.
Glioma
The commonest primary brain tumours of adults are gliomas which comprise about two-fifths of all primary brain tumours. Gliomas span a spectrum from benign to malignant and are graded pathologically on a scale of one to four according to the WHO classification. Gliomas of grades II-IV are conventionally considered ultimately incurable although life may be significantly prolonged by treatment.
Glioblastoma Multiforme (GBM) account for 80% of gliomas. GBM is the most aggressive type and hence grade four. Median survival in treated patients is about 17 months but better for younger, fitter patients. Treatment consists of maximal safe surgery followed by radiotherapy, which is given concurrently with a chemotherapy tablet in fitter patients. Chemotherapy is thereafter continued for an additional six cycles. Radiotherapy is the main component of treatment, responsible for more than doubling survival times. As with all brain tumours, surgery may be limited or contraindicated if tumour impinges on eloquent areas. In patients who are of limited performance status, supportive care along with dexamethasone may be most appropriate.
Low grade glioma is grade II on the WHO scale. It is the second commonest primary malignant brain tumour. Median survival is about 7-9 years with better prognosis associated with oligodendroglioma morphology compared to mixed or pure astrocytoma. Surgery is the primary therapeutic modality although radiotherapy and chemotherapy will be required at some point in almost all patients. WHO Grade III or anaplastic gliomas have median survivals of 3-5 years with oligodendrogliomas again tending to live longer than astrocytomas or mixed forms. Primary treatment is surgery followed by radiotherapy. This is usually followed by chemotherapy especially if 1p19q deletions are present. Again, almost all patients will require all three modalities over the course of their disease. Grade I gliomas are benign and usually cured with surgery although radiosurgery may be used if lesions are irresectable or multiply recurrent.
Medulloblastoma
Medullobalstoma and primitive neuro epithelial tumours (PNET) are the next commonest primary CNS malignancies. They are commoner in children than adults. The entire neuraxis is at risk and treatment typically includes resection followed by craniospinal radiotherapy and chemotherapy.
Other neoplasms
Just under half of primary adult brain tumours are benign. Unlike other tumour sites, even benign tumours in the CNS may pose risk to life or functional impairment due to uncontrolled growth in the cranial cavity or spinal canal, and most require definitive treatment at some point. This usually takes the form of surgery or radiosurgery. The commonest benign brain tumours are meningiomas, pituitary adenomas and vestibular schwannomas.
Meningioma
Meningiomas are usually benign (WHO grade 1). They comprise about a fifth of primary brain tumours. Surgery is the conventional therapy, although radiotherapy is an alternative especially for skull base tumours since complete resections are usually difficult to achieve without morbidity. Adjuvant radiotherapy is required after surgery for grade II-III meningiomas.