Low-grade gliomas account for approximately 20% of all cerebral gliomas (grade I and grade II neuroepithelial tumors).
Symptoms will depend on the site of the tumour. Prognosis depends on age at diagnosis, length of preoperative symptoms, epilepsy, and extent of resection (Salcman 1995; Piepmeier et al. 1996).
WHO grade I gliomas include rare entities like:
- pilocytic astrocytoma and
- subependymal giant cell astrocytoma.
- dysembryoplastic neuroepithelial tumors (DNET),
- pleomorphic xantho-astrocytoma (PXA)
Postoperative radiotherapy is not required. If there is suboptimal resection, these tumours grow so slowly that it is unlikely that early cranial radiotherapy has any advantage over a wait and watch policy, particularly since radiation-induced side-effects increase with the passage of time and there can be extended survival, even in the group with subtotal resection.
In symptomatic cases where the risks of resective surgery are considered too great, radiation therapy can produce long-term symptomatic benefit. Approximately 80 per cent of patients with pilocytic astrocytomas are alive at 15 years (Shaw 1995; Shaw et al. 1997).
WHO grade II gliomas commonly present with seizures without neurological deficit:
- fibrillary astrocytomas,
- protoplasmic astrocytomas,
- diffuse astrocytoma,
- mixed oligoastrocytoma.
Age and grade are important prognostic factors (Eyre et al. 1993). Patients under the age of 40 years have a median survival of 8 years, compared with 5.5 years for patients aged between 40 and 50 years and 1.6 years if older than 50 years. If patients have a good performance status, the median survival is 7.4 years, compared with 1.6 years if they have a poor performance status (Eyre et al. 1993).
Most tumours are situated in the frontotemporal regions and are frequently diffuse, extending throughout a lobe at presentation. The diffuse infiltrating astrocytomas clearly can not be resected. In the more focal, low-grade astrocytomas in non-eloquent areas, resection may be feasible. There is uncertainty of how best to treat patients with low-grade gliomas. It is probably advisable to biopsy lesions suggestive of low-grade glioma because there can be foci of higher grade despite the lack of enhancement. However, if seizures are the only symptom, a wait and watch policy is preferred by some clinicians and patients. There are no randomized controlled trials of resection versus biopsy in low-grade glioma, and it is unlikely that such a study would see completion because of the excellent survival with astrocytomas (46% 5-year survival) and oligodendroglioma (73% 5-year survival) (Shaw 1995; Shaw et al. 1997).
Radiation can reduce the size of a tumour but, because of the good survival, this has to be balanced with the increased likelihood of developing delayed radiation-induced toxicity. Early radiotherapy delayed time to progression but survival was identical (Karim et al. 1998).
Chemotherapy therefore has no proven place in initial treatment. At the time of recurrence most tumours have changed to higher grades of malignancy and chemotherapy may then have something to offer (Muller et al. 1977).