SU-F-T-339: Comparison Between Coplanar and Non-Coplanar RapidArc Approach of Hippocampal-Sparing Whole Brain Radiotherapy

Purpose: To compare the dosimetry of coplanar and non-coplanar RapidArc whole brain plans for hippocampal sparing Methods: We studied the RapidArc plans of patient with brain metastases, with the prescription of 3750 cGy in 15 fractions. The coplanar approach used a full clockwise (CW) arc and a full counterclockwise (CCW) arc, with the couch angle to be 0°. The non-coplanar approach used a full arc with a couch angle of 0°, and a partial arc with a couch angle of 90°. Treatment planning system is Eclipse Ver. 11. Constraints for eyes, lens, brainstem, optical nerves and chiasm are employed in the optimization so that these OARs' dose are below tolerance. Constraints for hippocampus are employed so that they receive dose as low as possible while maintain good coverage to whole brain. The beam delivery machine is Varian 21 IX. T1-weighted MRI images were used for hippocampus contouring. Results: The target coverage index for coplanar and non-coplanar RapidArc plans are 94.9% and 95.4%, respectively, with homogeneity index of 0.223 vs 0.226, which is defined as (D2% – D98%)/Dmean of target volume. V95 and V100 are 99.0% and 94.8% for coplanar plan, vs 99.1% and 95.4% for non-coplanar plan, while the mean dose of hippocampus are 1244.5 cGy for coplanar plan vs 1212.3 cGy for non-coplanar plan. Dose for eyes, lens, optical nerves, optical chiasm and brainstem are all below tolerance. Conclusion: Coplanar RapidArc plan provides good target coverage while achieves good hippocampal sparing, and there is no benefit to use non-coplanar approach.