Intrathecal chemotherapy is a way of delivering cancer-fighting drugs directly into the fluid surrounding your brain and spinal cord. It exists because most chemotherapy given through an IV cannot reach the central nervous system effectively. The brain is protected by a tightly sealed network of blood vessels called the blood-brain barrier, which blocks the vast majority of drugs circulating in the bloodstream from entering brain tissue. By injecting medication into the spinal fluid itself, doctors bypass that barrier entirely and put the drug exactly where it needs to be.
Why Standard Chemotherapy Can’t Reach the Brain
The blood-brain barrier is both a physical and biochemical wall between the bloodstream and the brain. It’s essential for keeping toxins and pathogens out, but it also keeps out most chemotherapy drugs. This makes treating cancers that have spread to the central nervous system extremely difficult with standard IV infusions alone. Even drugs that work well against tumors elsewhere in the body often can’t accumulate in brain tissue at concentrations high enough to kill cancer cells.
Once injected into the spinal fluid, the drug travels along with the natural circulation of cerebrospinal fluid (CSF). It can also seep into nearby brain tissue through a combination of passive diffusion and a system of tiny channels running alongside blood vessels, sometimes called the glymphatic system. There is a practical limit, though: simple diffusion only moves the drug about 2 millimeters into brain tissue from the fluid pathway. Deeper penetration depends on that glymphatic flow, which carries the drug further along paravascular spaces into the brain.
Cancers Treated With Intrathecal Chemotherapy
The most common reason for intrathecal chemotherapy is leptomeningeal disease, a condition where microscopic cancer cells spread into the cerebrospinal fluid surrounding the brain and spinal cord. This can happen with solid tumors or blood cancers. Breast cancer that has metastasized to the brain is one of the most frequent indications, along with lung cancer and melanoma.
Blood cancers like leukemia and lymphoma are also treated this way, especially acute lymphoblastic leukemia (ALL) in children. In pediatric ALL, intrathecal chemotherapy is often given preventively, before cancer cells have a detectable presence in the spinal fluid, because the central nervous system is a common hiding place for leukemia cells that survive standard treatment. A landmark study at St. Jude Children’s Research Hospital showed that intensifying preventive intrathecal chemotherapy in children with ALL brought the five-year risk of an isolated central nervous system relapse down to just 1.2%.
Drugs Used in Treatment
Only a handful of drugs are used intrathecally because the central nervous system is sensitive to chemical irritation. The three most common are methotrexate, cytarabine, and hydrocortisone. When all three are given together in a single injection, it’s called “triple intrathecal therapy.” The hydrocortisone serves partly as an anti-inflammatory, helping reduce irritation to the tissues lining the spinal canal. Newer approaches are also being explored, including delivering immunotherapy agents directly into the spinal fluid. Early studies using this method for metastatic melanoma have shown promise.
How the Procedure Works
There are two main ways to deliver intrathecal chemotherapy: a lumbar puncture or an Ommaya reservoir.
A lumbar puncture is the more straightforward option. You either lie on your side with your knees pulled up toward your chest and chin tucked down, or sit hunched forward over a table. Both positions open up the spaces between the vertebrae in your lower back. A doctor inserts a thin spinal needle into the midline of the lower spine, advancing it slowly and checking every few millimeters for the flow of cerebrospinal fluid, which confirms the needle is in the right space. Once positioned, the chemotherapy is injected slowly over one to two minutes. The entire procedure typically takes about 15 to 30 minutes, including preparation.
An Ommaya reservoir is a small, dome-shaped device surgically implanted under the scalp. A thin tube called a catheter connects it to one of the fluid-filled chambers (ventricles) inside the brain. Once in place, a doctor can deliver chemotherapy by inserting a needle through the skin of the scalp into the reservoir, which feeds the drug directly into the ventricular fluid. This avoids the need for repeated lumbar punctures, which matters when treatment spans weeks or months.
Ommaya Reservoir vs. Lumbar Puncture
The two methods don’t distribute the drug identically. The natural flow of cerebrospinal fluid moves preferentially toward the ventricles. When a drug is injected into the lumbar space at the base of the spine, it has to travel against this current to reach the brain’s ventricles, where cancer cells often reside. As a result, drug concentrations in the upper brain may be lower with lumbar delivery. An Ommaya reservoir delivers the drug directly into the ventricles, which can produce more consistent exposure across the central nervous system. Studies have observed better treatment responses in patients receiving intraventricular chemotherapy compared to lumbar injection, though both remain in routine use depending on the clinical situation.
What Recovery Looks Like
After a lumbar puncture delivery, you’ll typically be asked to lie flat for a few hours. This helps prevent post-procedure headaches, which happen because the puncture site in the spinal membrane can leak small amounts of fluid, temporarily lowering the pressure around the brain. Most people go home the same day. Soreness at the injection site is common and usually fades within a day or two.
Side Effects and Risks
The most frequent side effect is headache, which can result from the lumbar puncture itself or from chemical irritation of the membranes surrounding the spinal cord. This irritation, called chemical arachnoiditis, causes headache, nausea, vomiting, and sometimes fever. It is more common with certain drug formulations, particularly a slow-release form of cytarabine.
More serious but less common complications include:
- Nerve damage in the lower spine. Some patients develop numbness, tingling, or weakness in the legs, a condition related to irritation of the nerve roots at the base of the spinal cord. In rare cases, this has led to bladder or bowel incontinence.
- Encephalopathy. This refers to altered mental function, which can include confusion, drowsiness, or difficulty concentrating. It typically resolves but can be alarming.
- Hydrocephalus. In rare instances, the treatment can cause a buildup of fluid pressure inside the brain, which requires medical intervention.
- Hearing loss. This has been reported in isolated cases, particularly with repeated treatments.
The risk of severe neurological complications is low for most patients, but it increases with higher doses, more frequent treatments, and certain drug combinations. In one study evaluating a slow-release form of cytarabine alongside intensive systemic chemotherapy, serious complications like incontinence occurred in about 6% of patients, a rate the investigators considered unacceptable for that particular regimen.
How Effective It Is
For preventive use in childhood ALL, intrathecal chemotherapy has been one of the most significant advances in treatment. Before it became standard practice, central nervous system relapse was a major cause of treatment failure. The St. Jude Total Therapy study demonstrated that early, intensified triple intrathecal therapy reduced the five-year risk of any CNS relapse to 3.2%, with overall event-free survival reaching about 80%.
For leptomeningeal disease from solid tumors like breast cancer or melanoma, the goals are often different. Complete cure is less common in these cases, and treatment focuses on controlling symptoms, slowing disease progression, and preserving neurological function for as long as possible. Response rates vary widely depending on the type of cancer, how extensively it has spread, and how well the drug distributes through the spinal fluid.