paediatrics Brussels 17

Childs Nerv Syst

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

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. 2 ).

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

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