20% of all childhood cancers start in the central nervous system (CNS), which consists of the brain, the spinal cord, and the surrounding fluid (cerebrospinal fluid, or CSF), lining tissues (meninges) and bone (cranium and vertebrae). Brain tumors are the most common solid tumor of childhood with 24.5 cases per 1 million children, per year and about 20% of those brain tumors found in children less than 3 years old.
In the past several years, the incidence of pediatric CNS tumors has been increasing. This increase is partially explained by medical advances, including improved imaging, which has led to earlier detection and diagnosis of brain tumors.
There are many different types of pediatric CNS cancers, all of which have long and rather complicated names. Two things determine the diagnosis, or type of tumor: 1) where in the CNS the tumor starts, or the location, and 2) how the tumor looks under a microscope, also known as the histology. Primary brain tumors are tumors that arise in the brain, while primary spinal tumors grow in the spinal cord. However, some brain tumors can spread to involve parts of the spine, and vice versa. On occasion, certain types of tumors can even spread to areas outside of the CNS, such as distant bones or bone marrow.
This is where the concept of "staging" the tumor comes into play, and each tumor type has its own specific staging system. The purpose of assigning a cancer stage is to help predict outcome as well as to guide treatment by appropriately applying what has been learned in pediatric cancer clinical trials. Ultimately, every cancer treatment plan is individualized for every patient, and takes into account not only the stage and clinical data, but also the goals and desires of the patient and his or her family.
Medulloblastoma is a type of brain tumor that occurs in infants and young children. It represents about 20% of all pediatric CNS cancers. By definition, medulloblastomas occur in the cerebellum, which is the back part of the brain that controls walking, balance and fine motor coordination, among other things.
Medulloblastoma is a long name made up of three smaller words: medulla= Latin for marrow, meaning inner substance or core; blastos = Greek word for germ, meaning young, primitive, not fully developed; and oma = Greek for tumor. In other words, this is a tumor of primitive, undeveloped cells located inside the cerebellum.
Medulloblastoma almost always occurs in children less than 15 years old, and most commonly between the ages of 5-6 years. About 20% of the cases occur in infants less than two years old. There are approximately 400 cases diagnosed each year and the disease appears to be more common in boys than girls .
The exact cause(s) of medulloblastoma are not known. It does not appear to "run in families" or to be directly inherited from the parents. However, medulloblastoma is associated with certain chromosomal abnormalities that probably occur at some point during a child's development. This includes not only development after birth, but also the prenatal period before a baby is born, while it is still a growing embryo or fetus. One possible cause that has been suggested is exposure of the fetus to certain causative viruses or environmental agents, but this theory remains under investigation.
The most common symptom of this tumor is frequent, severe vomiting. Less commonly seen are morning headaches, nausea, confusion, and visual changes, such as double vision. Typically, children with medulloblastoma come to the attention of parents and teachers because of unsteady walking and clumsiness with holding things. All of these symptoms are consistent with a problem in the cerebellar region of the brain. For example, constant vomiting results from excessive pressure in the brain due to tumor blockage of important pathways for cerebrospinal fluid flow.
Please note that the symptoms mentioned here do not necessarily or automatically mean a child has a brain tumor, but further medical evaluation is required to rule out the possibility of a cerebellar tumor, such as medulloblastoma.
A magnetic resonance imaging (MRI) scan of the brain with gadolinium contrast-enhancement is the gold standard for detecting medulloblastoma. It has a very characteristic appearance on MRI scan: a well-defined, solid-looking mass located in the cerebellum, with fairly uniform contrast enhancement.
Medulloblastoma is a tumor type that is classically associated with spread from the brain to the spine and/or cerebrospinal fluid (CSF) bathing the CNS. Thus, part of the diagnostic workup should also include a gadolinium-enhanced MRI of the spine. The search for disease spread to the spine can be supplemented with a lumbar puncture. This is a procedure in which a thin needle is inserted into the lower back in order to obtain a sample of the cerebrospinal fluid, looking for floating tumor cells (cytology).
Of course, the only way to know the diagnosis of a suspicious mass is to biopsy it, meaning to "cut out a piece of the mass". Although performing a biopsy on the brain may sound like a bad idea, it is actually pretty easy to do for tumors located towards the front or the top of the brain using modern neurosurgical techniques.
However, medulloblastomas, as mentioned before, occur in the cerebellar region, which is technically rather difficult to access for simple biopsy. Thus, diagnosis is typically made from radiology studies, with or without results from a lumbar puncture. Surgery is undertaken with the goal of complete mass removal (called gross total resection), rather than just biopsy. This is further explained below.
As mentioned earlier, staging is a way of grouping cancer patients with similar diagnoses and similar extent of disease. Most CNS tumors remain where they started, (ie: in the brain or in the spine), often referred to as "localized disease". However, medulloblastomas are notorious for spreading from the cerebellum down to the spine, or "metastasizing". They typically invade the tissues that line the CNS (called meninges) before gaining access to the cerebrospinal fluid (CSF) which bathes both the brain and the spine. Once there, tumor cells can travel through the CSF and deposit themselves, or "seed", in any part of the spine, resulting in "metastatic disease". Approximately 30% of medulloblastomas have spread (metastatic) to the CSF at diagnosis. Rarely, these tumor cells can gain access outside of the CNS and metastasize to distant bone or bone marrow.
The original Chang staging system was devised in the late 1960's, before the widespread use of radiology scans, relying primarily on information about tumor size and spread that is obtained during actual surgery, with the naked eye. A modified version of this system is now used for medulloblastoma, incorporating the presence or absense of metastases. Based on what the surgeon sees at the time of the surgery, the tumor is placed in one of the following categories (T referring to tumor):
In addition to "T" staging, medulloblastoma staging has been modified by including "M" staging, where the "M" stands for metastasis. Remember, this is a word that describes how far the tumor cells have spread from the original location, if at all. The M stage is determined not only by the surgeon's observations, but also in combination with MRI scans and lumbar cytology, and consists of 5 possible groups:
Each patient is assigned a combination of one T stage and one M stage. As mentioned in the introduction, one of the reasons staging is important is that it helps predict how a patient might do in the long run, or how "curable" their cancer is, in a way. For medulloblastomas, the M stage is considered far more important in determining ultimate patient outcome and survival than the T stage. In other words, regardless of what the T stage may be, children who are in the M0 group do far better than those in M1, who tend to fare better than M2 kids, who in turn do better than M3 or M4 children.
Generally, treatment regimens are tailored to treat two groups: those with average risk and those with high risk disease. Average risk patients are those that are older than 3 years, have no metastatic disease, and have less than 1.5 cc of residual tumor after surgery. If a child has any of these features, he or she is considered high risk.
Of course, each case of medulloblastoma is different, and while staging attempts to group patients by risk, any one individual patient may not follow the "rules", so to speak.
Surgery is typically the first component of therapy, although it is no longer acceptable as the only component. Long-term results of patients treated as far back as the 1930's have taught us that surgery alone does not cure this tumor.
It is, however, very important to perform as complete a surgery as possible, with the goal being removal of all visible tumor while sparing as much surrounding brain tissue as possible. This is then confirmed with a post-operative MRI scan of the brain to look for any leftover, or residual tumor. Based on what the MRI shows, the surgery is classified as one of the following:
The ideal situation is a gross total resection, but this is not always possible. For example, sometimes the tumor is invading into or stuck onto other parts of the brain, making safe surgery very difficult. Nonetheless, the overall goal is to take as much of the tumor out as possible without risking severe brain deficits, as the long-term survival of medulloblastoma patients is directly influenced by the degree of surgery. In other words, the more complete the surgery, the better the long-term outcome. This is also known as a maximal safe resection.
The surgeon attempts to balance the need to remove as much tumor as possible without damaging healthy brain tissue. Some children will develop a syndrome called as posterior fossa syndrome after brain surgery in the area of the cerebellum. This is associated with difficulty swallowing, balance dificits, mutism (a speech disorder leaves the child unable to speak), poor muscle tone and emotional lability. These neurologic deficits may remain for months, but often improve slowly. Although historically this was seen in up to 15% of children, the rate of this has been noted to occur less commonly at pediatric neurosurgical centers.
After surgery, external radiation to the entire CNS (craniospinal irradiation, or CSI) is recommended to prevent the tumor from coming back in this area (recurrence, or relapse). Even if complete surgery is performed, low-dose radiation to the brain and spine is very important for local control, local meaning within the CNS. This is the region that is most at-risk for the tumor returning.
Data from old clinical studies of medulloblastoma patients clearly shows that as many as 60-70% of children who had good surgery and no other treatment had recurrence of the tumor. In contrast, patients who had good surgery followed by CSI had lower rates of tumor recurrence. The long-term disease-free survival (patient living without tumor) with the modern radiotherapy techniques of today is as high as 60-65%.
After radiation of the entire cranium (brain) and spine with a lower dose of radiation, the area of the original tumor, including where the surgeon operated, continues to receive radiation to a higher final dose (so-called radiation "tumor bed boost"). This is because the region in the brain where the tumor first started is the most likely place to still have some lingering, unseen, microscopic tumor cells. Historically, the entire cerebellum has been boosted to this higher radiation dose, putting normal structures including the inner ear and memory structures at risk for radiation damage. In recent studies, only the tumor bed has been boosted with similar success, which allows for protection of normal tissues. The development of intensity modulated radiation therapy (IMRT), a more precise form of radiation, also allows decreased radiation dose to important structures near the posterior fossa.
Much attention has understandably been paid to the possible long-term complications of radiation therapy to the brain and spine of a growing child. These can include deficits in memory, learning, social/emotional adjustment, hormone levels, hearing, and growth problems. The development of such side effects depend on many factors, including extent of pre-radiation surgery, amount and location of brain that is treated with radiation, age of the child at diagnosis, and how much radiation dose is given, among others. Because the side effects are very dependent on age (which is reflective of their stage of development), CSI is often deferred, or delayed by the use of chemotherapy, for children under the age of 3 years old.
However, modern radiotherapy techniques and proper attention to minimizing the radiation dose to important brain structures, whenever feasible, can allow for safe and effective treatment, even in younger children. While no therapy is without side effects, radiation therapy can be planned and delivered in such a way as to minimize potential long-term side-effects. This is best accomplished at a major radiation oncology center where physicians and staff are familiar with pediatric patients and technologically capable of treating childhood cancers.
Even though the actual craniospinal volume that is irradiated is small, a large volume of normal tissue is exposed to radiation with photon radiotherapy (traditional radiation therapy), including the heart, lung, bowel, gonads, and vertebral bodies. Proton radiotherapy offers a major potential advantage over photons for the radiation of the spinal canal given its biologic properties. Photons (which are used in traditional radiation) enter the body, reach the tumor and continue through the body, exiting on the opposite side. This exposes the tissues in front and behind the tumor to radiation and, in turn, the risk for damage. Proton therapy works differently; entering the body and reaching it's peak dose at the tumor and stopping, exposing tissues in front of the tumor to lower doses of radiation and tissues behind the tumor to virtually no radiation. This translates into improved dose to the target volume while sparing normal tissues and reducing side effects and toxicity. Proton therapy can also prevent radiation dose to normal tissues for the cranial boost, especially for sensitive structures, such as the middle ear. Despite its theoretical advantages, proton therapy is very challenging, requires weeks of planning for CSI and is only available in limited areas.
Studies have begun evaluating survival and side effects after proton therapy compared to what is traditionally seen with radiation. Massachusetts General Hospital in Boston used proton therapy to treat 60 children with medulloblastoma from 2003 to 2009. This included both high risk and average risk children, with an average age of 6 years old (range 3-22 years old). At a median follow up of 16 months, the 3 year overall survival was 90% with a 3 year progression free survival of 80%. These results are promising with control rates similar to traditional photon radiation.
Several proton centers have reported on rates of hearing loss in pediatric medulloblastoma patients treated with proton therapy, as this is an important side effect given its ability to affect quality of life, communication skills and development. These centers found lower rates of hearing loss in patients treated with proton therapy when compared with what was historically seen with photon therapy.
At the present time, the role of chemotherapy in medulloblastoma is the standard of care as part of tri-modality therapy, along with surgery and radiation, to help increase long-term disease-free survival. Chemotherapy is standard adjuvant (after surgery) therapy for all patients, regardless of their risk stratification. Clinical trials for average risk medulloblastoma document disease-free survival at 5 years from diagnosis of about 85%. While this rate sounds good, it was achieved with chemotherapy drugs such as cisplatin, vincristine, and CCNU, all of which have associated side effects, both short and long term. The use of chemotherapy has allowed for decreased radiation dose to the brain and spinal cord. This is very important in the attempt to decrease the toxicity of radiation. In younger patients, chemotherapy has an even more critical role; delaying radiation until the child is older and their developmental level is less susceptible to radiation damage.
A series of clinical trials known as the Head Start trials were undertaken to evaluate delaying or avoiding radiation in very young patients. Head Start I and II showed promising results in the 1990's and early 2000's with all patients undergoing very high doses of chemotherapy followed by stem cell transplants. Head Start III was undertaken with the goal of further improving upon those results seen in the first two Head Start trials. The authors recently reported the results for 92 patients treated on Head Start III from 2003 to 2009. 52% of young patients under 6 years old were able to avoid radiation to the brain and spinal cord when treated on the Head Start III regimen. Although these results are early, preliminary conclusions are promising and suggestive that this using high doses of chemotherapy to avoid radiation is safe and effective for very young patients.
Other novel agents, including inhibitors of the Sonic hedgehog signaling pathwayare being studied and may be promising less toxic agents for the treatment of medulloblastoma.
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