SU‐E‐T‐806: IMRT Technique for CSI: Better Dose Distribution, Simplified Setup, No Need to Shift Junctions

Abstract

Purpose: To report an IMRT technique for CSI, which reduces the dose to critical structures in cranial area, improves the dose coverage to PTVs and simplifies the setup. Methods: A supine setup was used for both patient comfort and anesthesia administration in pediatric patients. Patient is immobilized with indexed head rest and face mask, and also with indexed vaclock from the mid‐thigh down to feet. Four half beam (Y1=0) IMRT fields with ISO at the level of the chin to cover the cranial PTV. Cranial PTV is consisted with brain volume plus 5mm margin and portion of the C‐spine superior of the ISO with proper margin. From the first ISO the inferior portion has a PA field (PA1). 2nd PA field (PA2) is added in order to cover the spinal PTV. The spinal PTV is the volume of the spinal canal plus 2cm laterally. The inferior extension of this volume is determined by MRI to cover the thecal nerve. The optimization constrains are applied to maximize the dose coverage of both cranial PTV and spinal PTV, to minimize the dose to parotid glands and lens and to achieve better dose homogeneity. There should be several cm overlap between the two PA fields. During the optimization the leaf sequence forms step wedges in the overlap area and widens the penumbra of these two fields. This makes it more forgiving to the daily patient setup variations. Results: There is 32.8% reduction in mean parotid gland dose, 46.2% reduction for max lens dose without losing dose coverage to brain PTV, compared with the opposite lateral fields setup. The dose homogeneity for the spinal PTV is improved. And the setup is greatly simplified. Conclusions: We have developed a technique which reduces the dose to critical structures and improves the dose coverage and simplifies the setup in CSI.