What is Oligodendroglioma?
Oligodendroglioma (OG) is a kind of brain tumor that makes up about 5% of all primary brain tumors. It often affects the outer layer of the brain, known as the cortical gray matter, and is usually found in the frontal lobes of the brain. In the past, doctors diagnosed OG based on how the tumor appeared under a microscope. But in 2016, the World Health Organization (WHO) changed the guidelines on how brain tumors like OG are classified. Now, doctors also take into account the genetic makeup of the tumor.
OGs are typically low-grade tumors, meaning they are slow-growing and do not spread aggressively. These tumors, classified as WHO grade II, tend to respond well to treatment compared to other types of brain tumors. On the other hand, grade III anaplastic OG is a more aggressive form of the tumor with a poorer prognosis. This can occur by itself or can develop from a lower grade OG.
What Causes Oligodendroglioma?
The term “OGs” was first used by Bailey and Bucy back in 1929. They chose this name due to the similarity these cells bear to oligodendrocytes under a microscope. Oligodendrocytes are types of cells that help improve the function of nerve cells by producing a substance called myelin. However, it’s not entirely clear whether OGs come from these oligodendrocytes.
Instead, current thinking suggests that these tumors, or abnormal growths, seem to come from early nerve cells (neuroprogenitor cells) with markers of glial cells. Glial cells are the support system for the nerves in the brain. These early nerve cells then change into OGs, but they don’t have the ability to produce myelin like oligodendrocytes.
Supporting this idea, a common genetic mutation called isocitrate dehydrogenase (IDH) has been found in various types of brain tumors including OGs. This hints that similar processes underlie these different tumor types.
Risk Factors and Frequency for Oligodendroglioma
OGs, or oligodendrogliomas, are not very common, affecting only about 0.2 out of 100,000 people. They are the third most common type of brain tumor, following glioblastoma and diffuse astrocytoma. They make up around 5% of all primary tumors in the central nervous system. Males are slightly more likely to have OGs than females.
These tumors are more commonly found in adults, with most cases appearing in the fourth and fifth decades of life. However, there’s another smaller peak in cases among children aged 6 to 12 years old. The OGs in children tend to show different molecular markers when compared to those found in adults, leading some to question whether they are the same type of tumor.
Signs and Symptoms of Oligodendroglioma
Patients with OG, a medical condition, most often have unspecific symptoms like headaches. They also frequently experience seizures which, according to various studies, affect about 35% to 85% or even as high as 91% of patients. Some individuals may exhibit specific neurological issues depending on where the tumor is located. It’s important that anyone with new seizures or distinct neurological problems get a CNS imaging procedure done.
Testing for Oligodendroglioma
If you’re experiencing unclear neurological symptoms, a non-contrast-enhanced computed tomography (NECT) is often the first imaging tool your doctors may use. This is done to check for conditions such as bleeding in the brain, stroke or other potential urgent issues. On this type of CT scan, certain tumors, like OGs (oligodendrogliomas), usually show up as a less dense (hypodense) or sometimes equally dense (isodense) growth located at the outer edges of the brain. These tumors can cause changes in the skull due to their slow growth, and often show coarse calcification (hardening due to deposits of calcium), which is seen in up to 90% of cases.
Magnetic resonance imaging (MRI) provides better details of the tumor than a CT scan and it helps to determine how far the OG has spread. The tumor usually appears brighter on T2 weighted images and generally locates on the outer layers of the brain near the cortex. The tumor might show mixed signals, indicating areas of cysts, small areas of bleeding, and calcium deposit dropouts. T1 weighted images, another type of MRI imaging, give us information on tumor enhancement. The enhancement reflects the blood supply of the tumor and it does not necessarily indicate the grade of the tumor. Furthermore, OG tumors typically show an increase in water diffusion, which can be seen on a type of imaging called an apparent diffusion coefficient (ADC) map.
Other advanced MRI techniques such as spectroscopy (a technique that can provide information about the metabolic changes in a tumor) and perfusion MRI (which gives insight on blood flow within the tumor) can be complementary to anatomical imaging but are not always needed since most OGs are low-grade (grow slowly).
MRI diffusion tensor imaging (DTI) sequences and functional MRI (fMRI) are used preoperatively to assess important white matter tracts and eloquent cortical areas before surgery.
However, just using MRI imaging on its own isn’t always enough to distinguish an OG from other brain tumors. Practitioners also rely on a biopsy, involving the removal of some tumor cells for testing, for accurate diagnosis. Genomic testing is carried out on the biopsy sample to check for specific genetic mutations and deletions, which is an important part of making a clear diagnosis and deciding on the best treatment path.
So, while imaging is a key part in identifying a potential OG, the complete diagnosis process combines imaging, biopsy and genotypic analysis, for the most accurate results.
Treatment Options for Oligodendroglioma
Treating oligodendroglioma, often referred to as OG, involves a combination of surgery, chemotherapy, and radiation therapy.
Surgery is the first line of treatment and its approach depends on a number of factors, such as the tumor’s location, the patient’s overall health, and any factors that might increase surgical risks. The ultimate aim is to remove the entire tumor if possible, as this increases the overall survival time and can potentially cure the patient. If the entire tumor cannot be removed, what we call “debulking” procedures are conducted. This involves removing as much of the tumor as possible to help manage symptoms and improve survival time.
The techniques used for surgical removal of the tumor depend largely on its location, as our goal is to avoid damaging important parts of the brain. Sometimes, surgery may be performed while the patient is awake in order to map out the surrounding brain tissue and ensure as much of the tumor as possible can be safely removed. During surgery, imaging techniques such as MRI or ultrasound can be utilized for guidance. Still, these techniques can extend the surgical time and require special tools and training. Another tool that can be used is a fluorescence dye that lights up cancerous tissue, helping ensure a more complete removal of tumors.
Radiation therapy is another treatment method often utilized following surgery. It employs focused radiation to destroy cancer cells and it can delay tumor growth and help control seizures following surgery. The radiation dose and schedule are tailored to each individual case, considering factors such as the patient’s prognosis and the tumor grade.
Chemotherapy, which involves using drugs to kill cancer cells, is another common large part of treatment, often used alongside radiation therapy. The types of chemotherapy drugs used largely depends on their effectiveness and the side effects they cause. Although a combination of three drugs, procarbazine, lomustine, and vincristine, can help treat OG, their side effects such as leukopenia (reduction of white blood cells) and thrombocytopenia (reduction of platelets) sometimes lead to early discontinuation of the treatment. As a result, temozolomide (TMZ) is often preferred because it tends to be better tolerated by patients.
Bevacizumab, a drug that blocks blood vessel growth in tumors, is another non-chemotherapy treatment approved in several countries for recurrent brain cancer. It can also potentially benefit certain OG patients. It works by reducing neovascularization, the creation of new blood vessels, thus reducing swelling around the tumor and improving symptoms.
The treatment of OG is continually evolving, with ongoing studies working to better understand these therapies, their effectiveness, and the best approach for each individual patient.
What else can Oligodendroglioma be?
When trying to diagnose certain types of brain tumors, doctors often have to consider other conditions that may have similar characteristics. These include:
- Diffuse Astrocytoma: This is often indistinguishable from Oligodendroglioma (OG) in medical images. However, genetic markers can help distinguish between the two.
- Glioblastoma (GBM): These tumors often have a worse prognosis and are more common. They often show distinctive features on medical scans, such as more aggressive features, like larger growth and blood vessel formation, and lack certain genetic markers.
- Dysembryoplastic Neuroepithelial Tumors (DNET): These tumors can look similar to OG but tend to have a more “bubbly” appearance on medical scans and are often found in children and young adults.
- Ganglioglioma: This type of tumor is more often found in children and young adults and can be a condition that causes seizures. They usually appear as an enhancing nodule on a cyst in medical scans and are more commonly seen in the temporal lobe as opposed to OG.
- Multinodular and Vacuolating Neuronal Tumor (MVNT): This lesion could be mistaken for an OG on a scan as they could appear similar. It usually shows up on medical scans as a cluster of small, bright cysts that don’t typically show enhancement. It’s often found incidentally and may be a development problem rather than a true tumor.
These conditions can be confusing because of their similarities, but a detailed medical investigation would help doctors determine the exact condition.
What to expect with Oligodendroglioma
Low-grade oligodendroglioma, also known as OG and categorized as WHO Grade II, typically have a better outlook compared to other types of astrocytomas which don’t carry two specific genetic markers – 1p/19q deletion and IDHmt. Astrocytoma is a type of brain tumor and WHO Grade II means it’s a slow-growing tumor, 1p/19q deletion and IDHmt are genetic changes that can happen within tumor cells. With these genetic markers, OGs typically have a median survival time of 10-12 years and chances of remaining disease-free for 5 years ranges between 51% to 83%.
Young patients, those without additional health conditions, and those undergoing a more complete tumor removal tend to have better outcomes. However, the chance of survival decreases with higher-grade anaplastic OGs which are aggressive tumors, with an average survival time of 3.5 years for those classified as WHO Grade III tumors.
Possible Complications When Diagnosed with Oligodendroglioma
Complications following an operation to treat oligodendroglioma (OG), a type of brain tumor, can vary. Some people might experience seizures, difficulties related to the surgery, or blood clots. Chemotherapy may also cause complications, like limiting the body’s ability to produce new blood cells, which is called myelosuppression, as well as other common side effects like nausea, vomiting, and others related to the stomach and intestines. Also, the long-term consequences of radiation treatment can include cognitive problems, abnormal ways of walking, or radiation necrosis, which is tissue damage caused by radiation. Lastly, some patients might face the return or persistence of OG, which may, over time, turn into a more aggressive type of tumor.
Possible complications and side effects include:
- Seizures
- Difficulties following surgery
- Blood clots
- Myelosuppression from chemotherapy
- Nausea and vomiting
- Gastrointestinal symptoms
- Cognitive impairments from radiation treatment
- Abnormal ways of walking
- Radiation necrosis
- Recurrent or residual OG
- Malignant degeneration over time