Hybrid Forward and Inverse Planned IMRT for Cranio-spinal Axis Irradiation

Abstract

To describe a technique for cranio-spinal irradiation (CSI) of patients in the supine position that is quicker to treat and more robust than conventional matching of static fields. CSI in the supine position improves patient comfort and simplifies pediatric anesthesia. In conventional CSI, lateral brain fields are matched to a posterior spine field. The couch and collimator settings of the brain fields are rotated to match the divergence of these fields to the upper spine field. When required, an additional lower spine field is matched at depth to the upper spine field. Patients are treated prone, as daily field alignment is performed by marking field borders on patients' skin. Radiation doses near match lines are extremely sensitive to misalignments, so junctions are moved several times through the course of treatment. In the hybrid technique, patients are treated supine. The inferior edges of the lateral brain fields and the superior edge of the lower posterior spine field are manually designed with long, smooth dose gradients by forward planned IMRT field-in-field (FIF) techniques. Additional FIF segments may be added to compensate for dose heterogeneities arising from spine curvature or lateral separation. The upper posterior spine field is inverse planned by the IMRT optimization algorithm of our planning system (Varian Eclipse) to match the pre-calculated gradients of the brain and lower spine fields. The resulting 4-field plan delivers very uniform dose to the cranio-spinal axis with no abrupt field junctions. kV imaging is used to localize the patient for treatment each day. Therapists enter the couch coordinates for the brain fields into a custom spreadsheet which calculates couch shifts for each of the spine fields, based on planned isocenters. Intra-fractional patient motion has been simulated by translating isocenters in the planning system, and dosimetric implications were compared to similar errors for conventionally planned static deliveries. Treatment of the hybrid IMRT plan is significantly simpler than conventional techniques, as geometric isocenter shifts are calculated automatically each day and couch and collimator rotations for brain fields are not required. Treatment planning is also simplified, as smooth field transitions obviate the need for match line shifts. In addition, delivery of hybrid IMRT plans is less susceptible to geometric inaccuracies. 1 mm misalignments of matched static fields can lead to dose errors of up to 40%. Because of the gradual transition between fields with the hybrid technique, 3 mm errors lead to dose errors of only 15%. The technique described here allows for quick, safe, and robust supine CSI.