Highly Conformal Hybrid Forward and Inverse Planned Intensity Modulated Radiation Therapy for Supine Craniospinal Irradiation (CSI): Long-term Results and Implications for Pencil Beam Proton Planning

Abstract

Proton pencil beam scanning allows for highly conformal radiation therapy making target volume definition and treatment localization increasingly important. Since 2009 we have utilized a highly conformal, supine, hybrid forward and inverse IMRT technique for photon craniospinal irradiation (CSI). We report clinical outcomes utilizing our CSI technique and implications for pencil beam proton planning. Charts of 35 patients who received supine, hybrid IMRT CSI from 2009-2014 were retrospectively reviewed. Craniospinal CTV was defined as intracranial contents on bone windows with special attention to cribriform plate, without intentional inclusion of optic nerves or globes; and thecal sac defined as spinal canal including nerve roots terminating at the lateral border of the vertebral body. PTV was expanded 2 mm at cribriform plate/orbit, 5 mm for remaining intracranial CTV, 3-5 mm ant/post, and 5-8 mm right/left/inferior for thecal sac depending on age and size of patient. Dose was prescribed such that >95% of PTV received > 95% of prescription dose and >99% of CTV received >99% of prescription with no attempt to include vertebral bodies. Lateral fields were utilized at the cranium and upper cervical spine; inferior spine fields were either single posterior, 2-3 obliques, or VMAT depending on patient age and depth to CTV. Plans were generated with a hybrid of forward and inverse planned IMRT. Inferior borders of the brain fields and the superior border of the lower spine field were designed with 6-10 cm long, gradual dose gradients by sequential closing of MLC leaves using forward planned multiple static segment IMRT delivery. The sliding window upper spine IMRT field was created by the inverse planning system to match gradients of the brain and lower spine fields. Onboard kV imaging of cranial fields and a single posterior spine image confirmed daily patient position. Subsequent shifts were calculated based on couch coordinates and planned isocenter shifts. Histologies included 19 medulloblastomas, 9 primitive neuroectodermal tumors, 4 germ cell tumors, 1 pineoblastoma, 1 anaplastic ganglioglioma, and 1 atypical teratoid/rhabdoid tumor. Median CSI dose was 36 Gy (range: 18-39.6 Gy) and median boost dose was 55.8 Gy (range: 30.6-60 Gy). At a median follow up of 51 months (range: 3.2-89.2 months), there were no isolated recurrences or spinal myelopathies at field gradients or CTV margins. Fourteen patients had recurrence, all of which were intracranial. Our hybrid forward and inverse planned IMRT supine CSI technique did not result in any isolated recurrences or myelopathies at field gradients. Furthermore, the safety and efficacy of this highly conformal technique suggests our target volumes and blended gradients are appropriate for incorporation into spot scanning proton therapy plans.