Ependymoma
Radiation Therapy (RT)
Some types of ependymoma may not benefit from postoperative RT. This depends on tumor location and histology.
Variants of ependymoma thought not to require Radiation Therapy:
| Subependymoma | Do well with surgery alone. Perhaps due to different biological nature of these tumors |
| Myxopapillary ependymoma of the spine | Good local control after surgery alone. |
| Supratentorial, low grade tumors | After gross total resection these patients can do well without adjuvant radiotherapy.
In the previous COG ACNS0121 study, after complete resection of a supratentorial low grade ependymoma, patients went on to an observation arm only |
For the great majority of posterior fossa ependymomas, post-operative radiotherapy (RT) is almost always given after surgical resection as RT reduces the risk of local recurrence.
Target Volume
- In the past most children received craniospinal RT, but the great majority of relapses were local and craniospinal RT was associated with significant morbidity.
- The standard of care is now high dose local RT after as complete a resection as possible (as long as any evidence of craniospinal metastatic disease has been excluded prior to therapy).
The previous COG ependymoma protocol was ACNS0121
- After a gross total resection all children (except for those with a gross total resection of a low grade, supratentorial tumor) went on to receive high dose local RT.
- Where it had not been possible to achieve a gross total resection, patients were given combination chemotherapy in an attempt to shrink disease and then had second look surgery to try to excise all gross residual disease.
- Conformal RT was standard practice to increase the dose to the primary site and at the same time decrease the side effects of treatment.
Guidelines from that protocol:
GTV = Gross Tumor Volume (GTV) = pre-operative tumor bed and residual tumor (based on initial MR imaging). Any anatomic changes that have resulted from the surgery must be taken into account.
NB. Sometimes ependymomas extend down through the foramen magnum to lie adjacent to the cervical cord. In these circumstances highly focused techniques such as proton therapy may be appropriate to spare the normal cervical cord.
CTV = the clinical target volume = added margin meant to treat subclinical microscopic disease - an anatomically defined margin of 1.0 cm surrounding the GTV and the planning target volume.
PTV = geometric margin of 0.3 - 0.5 cm to account for to account for uncertainty in immobilization, image registration and daily variability in patient positioning.
Dose - All patients were given a total dose of 59.4 Gy with 180 cGy fraction size per day.
Patients less than 18 months of age with a gross total resection received 54 Gy
Cord dose was limited to a maximum of 54 Gy using techniques such as field size reduction, conformal/stereotactic boost or protons.
Technique
- Immobilization devices are essential for planning. The immobilization technique used depends on the area being targeted. An aquaplast mask and vaculock bag will likely be required to reproduce set up daily.
- Many children will require anesthesia.
- The prone position is often the most effective for 4th ventricle tumors, while the supine position is mostly used for supratentorial tumors.
- Fusion of a preoperative MRI with a planning CT is essential for tumor bed delineation.
- 3D conformal treatment using multiple beams (4 MV or greater energy) should be used to cover the PTV with 95% isodose surface whilst minimizing the dose to critical structures and normal brain.
Proton Therapy can be appropriate especially in:
- Very young children (less than 3 years old) to limit damage to developing normal brain.
- If the disease extends through the foramen magnum to abut the cervical cord (at levels such as C3). In this case 59.4 Gy with conventional photons would be associated with a significant risk of damage to the upper cervical cord.
Sample Plan
This child had a large infratentorial ependymoma treated using 6 beams. 2 beams were posterior superior oblique (Image 1), 2 beams were right and left lateral and 2 were right and left lateral posterior obliques (Image 2). A “cone beam view” depicts the entry angles of the beams (Image 3)
Image 1: Sag. Dose Distribution Profile (note superior posterior oblique beams)
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Image 2 Coronal Dose Distribution Profile (note Lateral posterior oblique beams
and lateral beams)
Image 3: Lateral Cone Beam View (Note 6 fields used in treatment, the lateral fields are not well seen in this view)
