Benign intracranial tumours cause harm by the pressure they exert on adjacent structures. The commonest benign tumours arise from cells lining the brain or close to the cranial nerves emerging from the brain. They can pose significant treatment challenges because of their sometimes intimate relationship to vital structures.
How Meningiomas Develop
Meningiomas arise from meningothelial cells in the arachnoid layer covering the brain. This discrete layer is closely applied inside the dural covering. They account for about 20% of all primary brain tumours. The vast majority are benign (WHO grade 1) but atypical types which are more prone to recurrence (WHO grade II) and even frankly malignant tumours (WHO grade III) are encountered. Meningiomas may occasionally involve the skull bone and cause it to thicken. Very rarely the scalp itself may become involved by the tumour.
Molecular Classification
Meningiomas are increasingly described not only in terms of the observable markers of cell division but also in terms of gene mutations detectable within the tissue of the tumour itself. In particular this molecular diagnosis helps to predict the response to radiotherapy.
Neurofibromatosis type 2 is a disease resulting from a gene mutation affecting the manufacture of a protein called Merlin. This protein seems to suppress the development of tumours in the lining of the nervous system and when it malfunctions tumours of the vestibular nerves (schwannomas) and meningiomas develop. Other consequences include clouding of the lens of the eye, or low-grade intrinsic brain tumours.
In one half of cases the gene mutation has been inherited from one parent. There are occasions when the mutation occurs spontaneously and is not inherited. There is also a milder phenotype called mosaic NF2 where the effects of the abnormal protein are limited to a particular area of the body. This occurs when not every cell in the body carries the NF2 gene mutation.
Hormonal Assosciations.
Meningiomas are more common in women although the reasons for that is not entirely understood. Female sex hormones can influence the development and growth rates of meningiomas as observed during pregnancy. Certain hormone medications can cause them to form or increase their rate of growth as has been seen with people taken progestin therapy (cyproteron acetate).
There are conflicting studies on the safety of hormone based therapy for those diagnosed with meningioma. It is currently reccomended that mechanical contraception or IUD be favoured in that case. It is also suggested that hormone replacement therapy treatment for menopause be avoided although it is often valuable to discuss the risk/benefit balance with an expert gynaecologist and in some case therapy might still be appropriate with careful clinical surveillance.
Environmental Assosciations.
Ionising radiation may damage the genetic material in cells causing tumours such as meningioma to develop many years after the radiation exposure itself.
Management
Small meningiomas which are not growing over time and not causng symptoms may be safely left alone. It is however advisable to monitor them for a period of time. The duration of such surveillence will be affected by the size and location of tumour as well as the patient's time in life. Surgical removal offers the prospect of cure for most meningioma although it is usual to monitor the patients with CT or MR scans for some years after surgery. It is our experience that if WHO grade I lesions recur they will usually do so within two years.
Surgical Excision
The best prospect for cure is to remove not only the tumour but also the section of dura to which it is attached. This is not always possible especially with tumours arising along the skull base or enclosing important venous sinuses. Treating the dura that remains with electrocautery can reduce the chances the tumour will come back. If recurrent but benign tumour is an ongoing problem radiosurgery or radiotherapy to the origin can be effective at controlling growth.
Below is an image pre-operatively and another on the right taken with an intra-operative MRI after the remvoal of the tumour.
Stereotactic Radiosurgery
For some small, symptomatic meningiomas of the skull base radiosurgery may be considered as primary treatment. Good control rates are possible with good safety. Some have advocated such treatment for small asymptomatic meningiomas although it would important to acknowledge that any treatment will carry some risk.
Radiosurgery may also provide an adjunctive treatment where an element of the tumour involves structures where the tumours dissection would entail causing disability. Having made the tumour smaller any residual in such an area may be adequately treated by radiosurgery afterwards. This is an excellent strategy to minimise treatment related complications in larger meningiomas that are intimately attached to critical structures.
