Purpose: Head-and-neck (H&N) cancers are challenging to treat with non-coil bridge MR-guided radiotherapy (MRgRT) linacs due to non-integration/compatibility of the receiver coils into common radiotherapy H&N immobilization systems. We developed, characterized, and clinically implemented the first commercial H&N immobilization system for a non-coil bridge MRgRT linac.
Methods: The H&N immobilization system includes an indexable custom baseplate that affixes to a head cradle, an open-face thermoplastic mask, and head-mold cushion. The baseplate is composed of clear acrylic and white Delrin and is constructed as the negative imprint of the H&N receiver coil. We quantified the baseplate’s radiographic integrity, dosimetric attenuation, dosimetric impact of daily IGRT localization positioning shifts relative to the baseplate, and image quality effects due to its presence in a 0.35T magnetic field.
Results: Mean baseplate CT # was -54.7±212.5 HU. For a single posterior beam, attenuation measured was 4.1% versus TPS calculated of 2.8%--difference of 1.3%. For a 13-field IMRT plan delivered to a 3D-IMRT QA array positioned anterior to the baseplate, the 3D gamma 3%/2mm passing rate was 98.8%. The dosimetric impact of IGRT localization was negligible (<0.1%) for couch shifts of 1, 2, 5, 10, and 50mm in AP/LR/CC directions as measured by shifting the baseplate ROI and comparing 3D gamma 3%/2mm passing rate between shifted and non-shifted calculated dose distributions. Image quality metrics in presence of baseplate and without baseplate (n=6 months) was 0.95 versus 0.89 (max geometric fidelity diameter 350mm, p=0.05), 37.5 versus 35.9 (SNR, p=0.35), 224 versus 435 (uniformity diameter 300mm, p<0.01), 2.12 versus 1.91 (high-contrast spatial resolution FWHM PSF, p=0.01), respectively.
Conclusion: The H&N immobilization system presented here provides a clinically viable immobilization solution for H&N patients on an MRgRT linac. The immobilization system does not appear to induce significant dosimetric perturbations and does not significantly impact the MR image quality.
Funding Support, Disclosures, and Conflict of Interest: Dr. Mittauer reports personal fees from ViewRay Inc, other from MR Guidance LLC, grants from ViewRay Inc.
MRI, Immobilization, Commissioning
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance