Tumour classification is based on the presumed cell of origin and biological grade of malignancy, which is derived from the microscopic appearances. Most neuropathologists use the World Health Organisation classification system for CNS tumours [2] which has a biological grading system from I-IV, with I being very slow growing and unlikely to spread, to IV being rapidly growing and ability to spread extensively within the CNS (spread outside of the CNS is rare for most primary tumours of the CNS). It should be noted that these biological WHO grades relate to the natural behaviour of the tumour without treatment, so that some WHO Grade IV tumours, (e.g. medulloblastomas), can have a prolonged survival with current therapy. In addition, different genetic subtypes of tumours of the same WHO histological type and grade, vary in their prognosis, which is likely to be reflected in future versions of the WHO classification system.

Although tumours from elsewhere in the body may spread to the CNS, approximately two thirds of intracranial tumours are primary, the majority of which are either of astrocytic origin, meningiomas, pituitary tumours or peripheral nerve sheath tumours. The frequency of each type of tumour varies with age and gender, for example gliomas are more common in men and meningiomas more common in women, and certain tumours such as medulloblastomas, ependymomas, pilocytic astrocytomas, craniopharyngiomas and choroid plexus tumours are more common in childhood.

Tumours may present with a variety of neurological symptoms including signs of raised intracranial pressure (sometimes due to hydrocephalus, particularly in the case of posterior fossa tumours), focal neurological deficits and epilepsy. The aetiology of tumours of the CNS is unknown in the vast majority of cases. Recent data suggests an association between long-term mobile phone and some cases of ipsilateral schwannoma, but evidence for an association with other types of tumour is inconclusive [3, 4]. Tumours may occur as a delayed complication of radiotherapy (particularly meningeal tumours and gliomas), Epstein Barr virus infection may cause lymphoma and there are a number of rare inherited genetic syndromes that predispose to neurological tumours (see Table 6.1).

It is important that specimens are sent to pathologists with adequate clinical information to allow accurate interpretation of the histological appearances (see Box 3.1). Information that a pathologist requires includes age, gender, family history (particularly of tumours), past medical history (including any treatment such as chemotherapy, radiotherapy and dexamethasone, or previous tumours), symptoms (including duration) and imaging findings (including location of tumour and pattern of contrast enhancement). If preoperative embolisation has been used to reduce bleeding during surgery e.g. for meningiomas, the pathologist must be informed as this can cause tumour necrosis and an increased mitotic rate [5], which can be confused with malignant change. In many cases it may be helpful to the neurosurgeon to have a rapid intraoperative diagnosis to confirm that the tissue is diagnostic and/or to give a provisional histological analysis to guide the extent of surgical resection that is appropriate (see Box 3.2). If an intraoperative diagnosis is planned it is important to warn the pathologist in advance so that laboratory staff can be prepared to deal with a specimen promptly. It is important to inform the laboratory if there is any potential that the patient may harbour a category 3 or 4 pathogen, so that additional precautions can be taken to prevent risk of infection to laboratory staff. A system should be agreed with a local laboratory on how intraoperative specimens are sent, but in general, the specimen should be sent immediately to the laboratory fresh, and should not be allowed to dry out in the process as this will hinder interpretation. Rapid intraoperative diagnoses are made by either freezing the tissue and then cutting ‘frozen sections’ from this onto a glass slide, or by squashing a small sample of tumour onto a glass slide and making a ‘smear preparation’. Tissue for ‘permanent’ histology is placed in formalin and may be sent to the laboratory separately. In an increasing number of cases fresh tissue may also be required to allow more extensive genetic testing to be undertaken on the tumour, so local laboratory advice should be taken on how tumours should be sent.