What Is an Oligodendroglioma and Why Is Genetic Diagnosis Essential?
An oligodendroglioma develops from the oligodendrocytes that produce the myelin sheathing nerve fibres, and it usually settles in the frontal lobes of the brain. The modern definition of this tumor is entirely molecular: to speak of a true oligodendroglioma, both an IDH gene mutation and the combined loss (codeletion) of the short arm of chromosome 1 (1p) and the long arm of chromosome 19 (19q) must be shown. Without these two markers, a tumor that looks like an oligodendroglioma under the microscope may in fact be a different glioma. Tumors with 1p/19q codeletion respond better to treatment and have a more favourable overall course; this is why completing the pathological examination with molecular tests is mandatory. Diagnosis therefore does not end with saying 'there is a glioma' but is completed by revealing the tumor's genetic identity.
Symptoms and Diagnosis
The most common first symptom of an oligodendroglioma is an epileptic seizure; a significant proportion of patients are diagnosed after a first-ever seizure while having no other complaint. Because of the frontal location, a headache, mild loss of strength, difficulty speaking, and changes in personality and behaviour may be added over time. Contrast-enhanced brain MRI is the basic tool in diagnosis; an oligodendroglioma often appears as a mass with relatively distinct borders containing calcification, which is clearly seen on computed tomography. However, imaging alone cannot make the definitive diagnosis; the tumor's true type and 1p/19q status become clear only by pathological and genetic examination of tissue taken at surgery or biopsy. One aim of surgery is therefore to obtain enough quality tissue.
The Place of Surgery and Seizure Control
The first step in treating an oligodendroglioma is usually surgery. The aim is twofold: to remove the tumor as widely as possible while preserving functional brain tissue ('maximal safe resection'), and to provide quality tissue for a correct genetic diagnosis. The volume of tumor removed favourably affects both the course of the disease and the control of the seizures common in these tumors. Because the tumor often sits in the frontal lobe near speech and movement areas, a safe margin is defined using awake craniotomy, neuronavigation and functional mapping when needed. Seizure control has a particular importance in oligodendroglioma; appropriate antiepileptic treatment after surgery and regular follow-up directly affect the patient's quality of life. For deep-seated tumors not suited to surgery, a diagnostic biopsy is performed.
After Surgery: Radiotherapy and PCV Chemotherapy
Treatment after surgery is determined by the tumor's grade (usually grade 2 or 3) and risk status. In some young, low-risk patients with completely resectable tumors, close MRI follow-up may be considered; in higher-risk or incompletely resected tumors, radiotherapy followed by chemotherapy is added. In 1p/19q-codeleted oligodendroglioma, the chemotherapy combination known as PCV (procarbazine, lomustine, vincristine) following radiotherapy is known to contribute to survival; in some situations temozolomide may also be used. The treatment plan is always set in a multidisciplinary board where the neurosurgeon, radiation and medical oncologists evaluate together. Throughout treatment, seizure control, follow-up imaging and neurological monitoring are maintained regularly.
Realistic Expectations
Oligodendroglioma is, among gliomas, a group that generally responds better to treatment and has a relatively more favourable course; in 1p/19q-codeleted tumors a good quality of life over many years may be possible with appropriate treatment. But 'more favourable' does not mean 'guaranteed cure': these tumors too can recur or rise in grade over time, so follow-up is lifelong. The course varies from person to person according to the tumor's grade, genetic profile, the proportion removed at surgery and the response to treatment. In no case do we promise a guaranteed outcome; expectations are shared openly with the patient and family once the tumor's true grade and 1p/19q status are known. Correct and honest information is an inseparable part of treatment.