Postoperative XRT is almost always given after surgical resection. It may be necessary to delay radiation therapy if a child is less than three years of age to avoid possible long-term neurological sequelae. Postoperative radiotherapy is more effective than surgery alone, as most posterior fossa tumours cannot be completely resected. There may be some histological variants of tumours for which postoperative radiation therapy offers little benefit.

Table 9: Histologic variants thought not to require radiation therapy

Target Volume and Localization

In the past all children received craniospinal XRT, but most failures were local. Now in most centres across North America, if there is no evidence of metastatic disease outside the posterior fossa, local radiation therapy to the tumour bed is used (doses greater than 5000 cGy) to reduce the morbidity of treatment.

The optimal target volume to be used for ependymomas remains somewhat controversial. Traditional approaches define the initial volume for posterior fossa tumours as the entire posterior fossa. This can be later reduced to boost the tumour bed alone.

Alternatively when treating ependymomas one can treat the tumour bed alone. Relatively tight margins on the tumour bed may be used as these tumours rarely invade adjacent brain. More often, they adhere to the floor of the fourth ventricle, the cranial nerves, or surrounding vessels and these regions are difficult to surgically access. These are common areas of recurrence and one must pay particular attention to include these sites of residual disease in the treatment volume.

Three-dimensional CT planning is used to identify the GTV. Fusion of a preoperative MRI with a planning CT may be useful for tumour bed delineation. The GTV is defined by the tumour bed based on preoperative imaging, anatomical changes postoperatively and possible microscopic residual disease. The CTV is defined by a 1 and 1.5cm margin on the GTV. Depending on the immobilization technique, the PTV is expanded by an additional 5-7mm.

Dosage

The dose-response data for ependymomas are incomplete. Most data indicate, however, that there is improved disease with greater than >45 Gy. Doses of 50-55 GY (1.8 Gy per day) are usually used for local control.

The administered dosage should conform to the target volume. Dose volume histograms should be constructed for the temporal lobes, pituitary area, whole brain, and optic chiasm. The dose to these structures should be kept within tolerance.

Technique

Immobilization devices are essential for planning. Most children will require anesthesia. 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. The prone position is most effective for fourth ventricle tumours, while the supine position is mostly used for supratentorial tumours.

Conformal three-dimensional CT planning is now standard practice. IMRT (Intensity Modulated Radiation Therapy) may also be used. The treatment beams should be arranged to cover the PTV with 95% isodose surface whilst minimizing the dose to critical structures and normal brain. Photon beams with energy greater or equal to 4MV would be appropriate. See below for a sample plan.

Sample Plan

This child had a large infratentorial ependymoma treated using six beams. Two beams were posterior superior oblique (Image 1), two beams were right and left lateral and two were right and left lateral posterior obliques (Image 2). A "cone beam view" depicts the entry angles of the beams (Image 3 and Image 4).

Image 1: Saggital Dose Distribution Profile (note superior posterior oblique beams)

Image 2: Coronal Dose Distribution Profile (note Lateral posterior oblique beams and lateral beams)

Image 3: Lateral Cone Beam View (Note six fields used in treatment; the lateral fields are not well seen in this view)

Image 4: Anterior Cone Beam View (Note six fields used for treatment; exit beams are seen inferior to patient)