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planning. The Houndsfield unit represents tissue density for
radiation dose calculation. The CT defines the coordinate
system for radiation therapy which can be verified using the
kilo- or megavoltage imaging systems which are part of the
modern linear accelerator. Early on, target volumes and
normal tissue contours were drawn directly on CT data using
dedicated treatment planning computers. More recently,
planning systems are capable of incorporating and register-
ing multiple sets of pre- and postoperative magnetic
resonance (MR) imaging and other data including PET.
The use of MR imaging to define target volumes and
normal tissue contours refined the treatment of ependy-
moma for most patients have posterior fossa tumors where
the performance of CT is limited. The superior contrast of
MR has allowed investigators to define and differentiate
residual tumor from postoperative change and has increased
the accuracy of the definition of functional subunits of the
brain. The use of MR imaging in treatment planning has
revealed that changes in the position of normal tissue
volumes may occur as a function of time after surgery and
that MR imaging should coincide with radiation therapy
planning. Further, the position of the patient for the MR
study is critical, especially for patients with posterior fossa
tumors, where differences in flexion and extension of the
head may impact the position of the spinal cord. Regardless
of whether the patient is treated in the prone or supine
position, state-of-the-art radiation therapy planning there-
fore requires MR imaging to be performed in the same
position as the treatment planning CT study (Fig. ).
Fig. 2 Sagittal digitally reconstructed treatment planning CT data
with spinal cord contour (blue arrow) and contour of diagnostic MR
spinal cord contour (red arrow) to demonstrate potential registration
errors when MR is not performed in the position of treatment delivery
Fig. 1 Axial preoperative ( left)
and postoperative treatment
planning (right) postcontrast T1-
weighted MR and treatment
planning (center) CT images of
a patient with posterior fossa
ependymoma. Gross target vol-
ume (blue), clinical target vol-
ume (magenta ), and planning
target volume (red ) contours are
shown on all images
Childs Nerv Syst