INTRODUCTION
Medulloblastoma is the most common childhood malignant central nervous system (CNS) tumor [1]. Peak incidence occurs at age 7 with slightly greater incidence in males [1,2]. A large proportion of medulloblastoma patients have craniospinal fluid (CSF) spread at the time of diagnosis. The standard of care for medulloblastomas involves surgical resection, craniospinal irradiation (CSI), and chemotherapy [3]. For average-risk patients the 5-year survival rate is over 80% while high-risk patients have a 5-year survival rate of less than 50% [2,4].
CSI presents challenges due to its large target volume which extends beyond the 40 cm x 40 cm field size limitation of a commonly used C-arm linear accelerator collimator opening [5, 6]. Therefore, the use of multiple plan isocenters provides a solution to this limitation by dividing the target volume into three fields—the whole brain, the upper spine, and the lower spine. CSI is commonly performed using 3D conformal radiation therapy (3D-CRT) technique which is prone to errors due to the complexity of the planning and the treatment delivery setup [7-12]. This technique results in dose inhomogeneity and non-conformality which yields significant dose to the anterior of the spine target volume. 3D CSI also requires feathering the junctions resulting in multiple plan pairs, gap calculation and couch rotations, making the planning and treatment procedures complex and cumbersome and prolonging treatment times.
Overall, Volumetric Modulated Arc Therapy (VMAT) CSI has been observed to create plans with better dose conformality, better dose homogeneity, greater normal tissue sparing, low sensitivity to positioning errors, and shorter treatment time compared to 3D-CRT CSI [13-17]. While VMAT can produce clinically favorable plans even with setup errors of up to 3 mm margin, accurate patient alignment with minimal setup remains important. A multicenter study conducted by Gram et al. showed daily image guidance with 6-DoF couch corrections was found optimal in significantly reducing positioning errors and uncertainties for pediatric CSI patients [11].
While daily image guidance and 6-degrees of freedom couch corrections can assist in optimizing patient setup, the inherent risks for positioning errors and uncertainties cannot be eliminated for VMAT CSI due to the use of multiple isocenters and field matching. Helical-delivery radiation treatment techniques such as TomotherapyTM can reduce these risks associated with multi-center CSI treatments by using a ring-based gantry to deliver a single field CSI treatment as the patient moves into the treatment ring [18-20]. A study by Lee et al. reported Tomotherapy CSI to have acceptable inter-fractional and intra-fractional errors, and setup verification based on the measurements and evaluations of treatment setup for 83 patients [19]. In addition, Tomotherapy CSI techniques have been demonstrated to produce highly conformal and homogeneous treatment plans compared to 3D CSI [21,22].
RefleXionTM (RefleXion Medical Inc., Hayward, CA) is a novel Positron Emission Tomography (PET) treatment modality that similarly utilizes a ring-based gantry for axial step-and-shoot IMRT delivery. The first clinical installation of RefleXion X1 was recently conducted at our institution [23,24]. The RefleXion X1 design provides potential advantages to CSI treatments using a single isocenter which can potentially decrease complexity of planning, image guidance and delivery reducing risk of shift and localization errors. This study aims to test the feasibility of treatment planning of X1 CSI and compare the plan quality and beam-on time to the current standard of care at our institution – VMAT CSI planned in Eclipse and delivered using Trilogy linear accelerator (Varian Medical Systems, Palo Alto, CA).