Leptomeningeal metastases develop when seedlings of cancer, from tumours elsewhere into the body, get into the cerebrospinal fluid and settle on either the arachnoid or pia mater to form secondary cancers.
It might be helpful to understand a bit more about the meninges. The meninges are made up of three layers covering the brain and the spinal cord. The outermost is a thick, tough membrane called the dura mater. Lining the inside of the dura mater is a thin, soft layer, called the arachnoid mater, and inside this is a similar layer, called the pia mater. The two thin, soft layers, the arachnoid and pia, are called the leptomeninges. Between the arachnoid and the pia there is a space, called the subarachnoid space, and the cerebrospinal fluid, CSF, which bathes the brain and the spinal cord, circulates through this space.
The tumours most likely to form leptomeningeal metastases are acute leukaemias, some types of non-Hodgkin lymphoma (NHL), breast cancer, small cell lung cancer, and malignant melanoma. Nearly half the people who develop leptomeningeal metastases will also have secondary cancers in the brain itself, these are known as cerebral secondaries.
Symptoms of leptomeningeal involvement include loss of nerve function (this may show itself by weakness of the eye muscles, or difficulty in speech, or weakness of one or more of the arms or legs). Headache is common, and seizures (fits) occur in about 1 in 10 people who have the condition.
The diagnosis is usually made by examining samples of cerebrospinal fluid (which are almost always abnormal, and often contain cancer cells), and by carrying out an MRI (magnetic resonance imaging) scan, which almost always confirms any secondaries.
The treatment of leptomeningeal metastases varies, depending on where in the body the cancer started (the primary cancer), but will often involve radiotherapy to the affected area, possibly combined with chemotherapy. The chemotherapy is usually given intrathecally, that is, through a lumbar puncture needle into the spinal canal. This is so the drugs go straight into the CSF and will circulate to where the cancer is, giving drugs into the bloodstream, through a vein in the arm, is less effective.
Because leptomeningeal metastases are quite common in acute leukaemias, and small cell lung cancer, treatment is often given prophylactically - that is it is given before the metastases have been found, this is to try to stop them developing. Giving treatment in this way has reduced the risk of leptomeningeal metastases in these two types of cancer.
Unfortunately leptomeningeal metastases are a serious development and without treatment few people would survive for more than a few weeks. The aim of any treatment is to control the disease; it depends on where the cancer started as to how well this can be done. In acute leukaemias and non-Hodgkin lymphomas it is usually possible to control the condition in most people although for other cancers it can be more difficult and the outlook may be less good.