Radiotherapy in the Management of Low-Grade Gliomas

Abstract

Low-grade gliomas account for approximately 10%-15% of all primary brain tumours in adults (Burger et al. 1982). The tumours are slowly progressing and are therefore often considered ¡°benign¡±. They affect the brain by two mechanisms: (1) infiltration of tumour cells may alter neurological function; (2) this is usually accompanied by a space-occupying effect leading to compression of adjacent neural tissue and subsequently causing increased intracranial pressure.Due to the different locations within the brain of the disease, the clinical symptoms and signs vary considerably and many patients have a long medical history at the time of diagnosis. The most common features are seizures (66%), focal deficits (51%),headache (44%), neuropsychological disorders (16%) and visual deterioration (16%) (Laws et al.1984). According to the histological criteria of the World Health Organization (WHO) gliomas are classified into grades I and II (Zulch 1979). In the most recent WHO classification of gliomas (Kleihues et al. 1993), low-grade tumours are subdivided into ¡°ordinary¡± astrocytoma (fibrillary, protoplasmic, gemistocytic); oligodendroglioma; mixed oligoastrocytomas displaying features of diffuse infiltration; and the circumscribed pilocytic astrocytoma. Less common variants include pleomorphic xanthoastrocytoma,subependymal giant astrocytomas, and subependymomas, the latter group being related to the pilocytic astrocytomas. In all of these systems, oligodendrogliomas are mostly considered as lowgrade gliomas, although their histology and clinical course can vary from that of other low-grade gliomas. The hallmark of the histological picture of lowgrade glioma is the absence of necrosis, while there is some overlap between anaplastic astrocytoma, the intermediate grade of the three-tiered system (Ringertz 1950; Burger et al. 1991), and grades 2 and 3 of the four-tiered classification (Kernohan and Sayre 1952; Daumas-Duport et al. 1988) regarding vascular proliferation. In order to improve uncertainties in classification, attempts have been made to identify distinct subgroups of patients with low-grade glioma. A labelling index of <1% was associated with a lower recurrence rate than a labelling index of >1% (Hoshino et al. 1988). It was also found that chromosome number and structure abnormalities may adversely influence survival (Jenkins et al. 1992).