Development of 3D printed immobilization for the MR-linac: Toward high precision brain treatments.
Glide-Hurst C, Myers E, Du D, Kim J, Wen N, Movsas B, Siddiqui S, Walbert T, Kalkanis S, and Chetty I. Development of 3D printed immobilization for the MR-linac: Toward high precision brain treatments. Med Phys 2018; 45(6):e481-e482.
Purpose: The recent introduction of the MR-linac offers strong potential for high precision MR-guided radiation therapy (MRgRT). However, MRgRT in the brain has not been widely explored. One limitation is the lack of MRcompatible immobilization devices that accommodate MRI coils to enable reproducible set-up. This work describes the development and evaluation of a novel 3D printed head rest to support cranial MRgRT. Methods: A 3D scanner imaged the posterior 5-element head and neck phase array coil using structural light scanning to output a . STL file for use in CAD software. A clinical foam head rest was CT scanned with a 1 mm slice thickness to provide external dimensions. The two . STL files were merged to generate final dimensions for the 3D printed head rest that accommodates the MRI coil. The final 3D printed head rest was fabricated using a MakerBot Replicator with a standard 3D printer filament (Acrylonitrile-Butadiene-Styrene Copolymer, 5% infill, 3 shells) using fused deposition modeling. Coil fit, attenuation measurements with an MR-compatible ion chamber, and image quality assessments were performed. Repeat MRIs were obtained on a volunteer and rigid MR-MR registration was performed to characterize setup uncertainty. Results: CT numbers of ∼-450 HU for the casing and -790 HU were noted for the lattice infill, with slight speckling observed in a 0.35 T True- FISP sequence. Attenuation through the head rest was 1.6%, 3.4%, and 5.6% in the center, neck, and thickest portions, respectively, suggesting the need to accommodate for the device in planning. Interfraction setup uncertainty was <2 mm and 2° across 5 trials. Conclusion: We have developed a 3D printed head rest to support conventionally fractionated brain MRgRT. A clinical trial to study immobilization device efficacy in a patient cohort is underway. Acquiring serial on-board MRIs in the brain will facilitate treatment response assessment and adaptive radiation therapy.