3.4      Case Study – Oligodendroglioma of the brain

Oligodendrogliomas comprise a class of glial tumours in which the oligodendroglial cell is the predominant cell type (see appendix 1 for brain tumour classification).  Normally, oligodendroglial cells form myelin, the fatty substance surrounding the axons of the nerve cells, providing insulation making the nerve cell electrical transmission faster and more efficient.  In oligodendroglioma the mitotic rate of the oligodendroglial cells has exceeded the mitotic rate of other glial cells (e.g. astrocytes), hence the former become the most numerous forming the tumour.

Oligodendroglioma may be benign, where the cells do not spread from the site of origin, or malignant (cancerous), where the cells invade and destroy surrounding tissue and may spread to other parts of the brain and occasionally outside the brain.  Oligodendrogliomas is normally found in the cerebral white matter, particularly in the frontal and temporal lobes of the brain (CancerBACUP, 2001; Hariharan, 2002).

CancerBACUP (2001) state that oligodendroglioma may be divided into two types, the well-differentiated tumour, which grows slowly, or the faster growing anaplastic oligodendroglioma.  Like other gliomas, oligodendrogliomas may be graded between 1 and 4 depending on their malignancy and rate of growth.  Grade 1 is the least malignant and the grade 4 is the most.   Grade 1 and 2 oligodendrogliomas may be referred to as low-grade tumours and grade 3 and 4 as high-grade tumours. (see appendix 1 for other tumours).

Like most brain tumours the cause of oligodendrogliomas is unknown.  It is most common in adults however, it may occur in children.  For unknown reasons in it is more common in men than in women (CancerBACUP, 2001).

Grossly oligodendrogliomas are heterogeneous masses regardless of their grade.  In addition to the solid cellular portions of the tumour, areas of myxoid accumulation may make the tumour appear cystic.  Oligodendrogliomas may as present with multiple haemorrhages.  The primary imaging modalities in investigating bran tumours include CT, MRI and cerebral Angiography.  Calcification within a neoplasm is a hallmark of oligodendrogliomas.  On skull radiographs, 47% of oligodendrogliomas may be depicted through these calcifications.  However, plain CT is the most sensitive modality in detecting these calcifications (Rees, Lee, Smiriotopoulus, 1999).

In contrast to low-grade asrtocytomas, which are usually homogenous masses on imaging, oligodendrogliomas are seen as heterogeneous.  On plain CT they are seen as mixture of hypodensity, isodensity, calcification and occasionally haemorrhage.  Calcifications can be detected on MRI, but difficult to identify if it is stippled or diffuse.  CT is also sensitive in the detection of erosion of the inner table of the calvarium, which is another feature of oligodendrogliomas. 

On MRI, oligodendrogliomas present as a mass, which may appear heterogeneous on all sequences.  It is typically located in the frontal lobe and may extend to the cortical surface.  T1-weighted images are sensitive in identifying areas of previous haemorrhage, which appear hyperintense.  Oedema around the tumour is clearly identified on all T2-waeighted images.

3.5.1 Patient

This 50-year old gentleman presented with headaches, nausea with changes in mood and personality.  This patient was diagnosed of having a left parietal oligodendroglioma (Grade 2) in September 2000.  The tumour was surrounded by oedema and steroids were administered to reduce the intracranial pressure.  This was immediately followed by a craniotomy and the tumour was totally resected.  In November 2001 the patient was put on external radiotherapy to destroy any remaining malignant cells after the surgery.  Unfortunately, in November 2001 this gentleman had a relapse of the oligodendroglioma, discovered after having an MRI scan of his brain.  The patient was then put on chemotherapy showing signs of improvement.  The following MRI examination was performed to assess the patient’s current condition.

3.5.2 Materials and methods

The patient was scanned using a Signa General Electric 1.5Tesla MRI scanner.  The patient was screened before the examination for any metal objects and was asked to change into a gown.  The patient was asked to lie supine on the examination couch with their head within the head coil.  The patient’s head was positioned straight with the interpupillary line parallel with the couch.  The longitudinal alignment light was in the midline and the centring light passed through the nasion (Westbrook, 1999).  Foam pads were used for immobilisation.  Copies of the previous films of this gentleman were not available since the previous examinations had been carried out in different centres.

3.5.3        Protocols and pulse sequence parameters

The pulse sequences with the respective parameters used are listed in the table 1.

Table 3 Pulse sequences and parameters used in brain MRI

The above sequences used include a routine brain study.  The extra sequences include the post contrast sequences.  This protocol is similar to the protocols recommended by Brown (1999).

Our sequences differ from the protocol of the Massachusetts General Hospital ((MGH), 2002).  For brain neoplasia, MGH (2002) also perform routine brain with contrast.  However, the axial PD/T2 sequence, coronal T1 and sagittal T2 pre-contrast are not included.  Moreover, post contrast imaging includes T1-weighted images in the three planes.  Patients with neoplasia get MR spectroscopy (MRS) at MGH.  This aids in the differentiation of tumour from other benign lesions and to define the aggressiveness of the tumour.  Unfortunately, our MR unit does not offer MRS.

Rees et al (1999), recommend the use GRE technique in brain tumour MRI.  This sequence is very sensitive in the identification of small or diffuse calcifications in the brain.  It can also identify erosions in the calvarium and detect acute and chronic haemorrhages.  3D Spoiled GRE is often requested in our centre, with pre and post contrast sequences.  Since 3D SPGR are heavily T1-weighted and contrast enhancement images fit very nicely with this sequence.  However, it must be mentioned that GRE have increased magnetic susceptibility effects.  However, GRE sequences may prove particularly useful in the detection of haemorrhage when compared to SE/FSE techniques.  Moreover, since 3D reformations are possible the exact origin and extent of tumour may be identified after the examination.

Figure 4 Sagittal T2-weighted section   

Figure 5 Axial PD-weighted image showing oligodendroglioma

Figure 6 Axial T2-weighted image showing oligodendroglioma

3.5.4 Results

On scanning the patient with the above sequences and parameters a large irregular enhancing lesion can be visualised in the left parietal lobe surrounded be oedema.  This oedema is causing compression of the left lateral ventricle and displacement of the midline structures.  According to the radiologist report this is consistent with recurrence of the oligodendroglioma.

3.5.5 Evaluation of the examination

This gentleman in this examination was co-operative but needed constant communication during the examination due to anxiety.  Images achieved were acceptable.  On the sagittal T2 FSE sequence (Fig 4) motion artefacts due to patient involuntary eye-ball/eye lid movement can be identified.  Lee and Bogdan (1999) recommend the swapping of phase and frequency-encoding directions in cranial imaging which will rotate the ghost artefact from orbital motion from anterior-posterior direction to the left-right direction. 

MRI with contrast should be the modality of choice in the evaluation of neoplastic disease of the brain since many more lesions can be demonstrated by MRI as compared to CT (Price, 1992).  When a lesion is solitary as this oligodendroglioma, the selection of appropriate surgical therapy is possible through MRI.  Accurate planning of the surgical approach, determination of radiation therapy ports and recurrence all contribute to optimal patient care and improved clinical outcome.    

8mls of Gadolinium-GTPA were administered in this case and scanned in T1-weighted images as is always recommended in cases of tumour evaluation (Price, 1992).  Contrast enhancement in not usually intense and not a striking feature in oligodendrogliomas as observed in Figures 7 and 8 (Jager and Rich, 2001).  The oligodendroglioma yielded a high signal on the T2 weighted images.  This was important in showing the extent of oedema around the tumour and demyelinating processes, however the tumour itself was obscured hence justifying the use of contrast enhancement.  Contrast-enhanced images showed the exact size and location of the tumour

Seizures are usually frequently associated with intracranial neoplasia and should be imaged initially with MRI to exclude the presence of tumours.  Areas of temporal atrophy may be the only finding which correlates to the seizure focus are difficult to assess by CT.  Nevertheless, CT is still the modality of choice in the following:

·        Faster acquisition time (acutely ill or injured patient)

·        Sensitivity to bone calcifications

·        Absence of a magnetic field

·        Improved definition of orbital abnormalities

(Price, 1992)

Jager and Rich (2001) state that up to 90% of visible oligodendrogliomas contain visible calcification, which is best, depicted by CT.  On MRI, areas of calcification may be difficult to appreciate within a heterogeneous mass of predominantly increased signal intensity on proton density and T2 weighted images (See figs 5 and 6).   This is due to the variable appearance of calcification on MRI, which depends on the calcium concentration, and the amount of bound water.  Areas of low signal intensity on T2-weighted images in the tumour may indicate areas of haemorrhage, occurring relatively

frequent in oligodendrogliomas.  In figure 6 areas of low signal intensity can be seen which according to the radiologist may indicate areas of calcification or haemorrhage.

Figure 7 Post contrast T1-weighted coronal section

Figure 8 Post contrast T1-weighted axial section

3.5.6 Treatment and prognosis

The patient as already stated has been already operated.  Both radiotherapy and chemotherapy have been administered.  On discussion with the patient’s oncologist another course of chemotherapy will be administered after this recurrence.  Nevertheless, Hariharan (2002) stets that the patient should be closely observed resulting from continuing treatment of radiotherapy and chemotherapy such as radiation necrosis or neuropathy.  The patient will be also monitored with regular follow-up care and MRI scans will be performed every 3 months initially and then every 6 months to 1 year.

A number of variables determine the prognosis for an individual patient, including the age of the patient at diagnosis, the location and extent of surgical resection, postoperative performance status, histologic features, and the use of adjuvant therapies.  Patients with nonanaplastic oligodendrogliomas are expected to survive for 5-10 years from the time of diagnosis.  The survival rate in anaplastic tumours is likely to be 3-4 years  (Hariharan, 2002).

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