Deformable 3D dosimetry of MRI-tracking radiotherapy

During breathing, anatomical structures within the torso move which can result in inaccurate radiation dose delivery. With the introduction of MRI-linear accelerators (MRI-linacs), the ability to simultaneously acquire high-contrast soft tissue images during irradiation allows for compensation of intrafraction motion and deformation by gating or tracking the target volume. The implementation of MRI-guided tracking radiotherapy is not without risks as any lag or organ deformation may not be accounted for. Polymer gel dosimetry has the potential to measure the integral dose delivered to deforming and moving targets as the gels are flexible and provide a high-resolution 3D dose profile. Spherical silicone casts filled with polymer gel and silica beads were compressed to measure the deformation of gel phantoms. The effect of compression on the dose response was studied by irradiating and scanning the spherical phantoms in various states of compression. End-to-end gel dosimetry experiments with MRI-guided tracking radiotherapy were conducted on a prototype MRI-linac (Australian MRI-Linac) using a moving, non-deformable, rectangular-shaped gel dosimeter phantom and an MRI-safe, pneumatically actuated, anthropomorphic, breathing phantom containing a liver-shaped gel dosimeter. Radiochromic film dosimeters within the phantoms were used as secondary validations of the dose profile. The tracking performance of the MRI-linac was assessed by comparing measured dose distributions in the phantoms in static and actuated experiments. There was no significant impact of compression on the dose response in the irradiated spheres. The gel-measured dose profiles in the phantoms matched closely with the film dosimeters for tracked and non-tracked scenarios. The end-to-end gel dosimeter experiments illustrate the improvement in dose conformality with MRI-guided tracking. Deformable 3D gel dosimeters in an anthropomorphic body phantom can be used for assessing 3D geometric accuracy of tracked treatments on MRI-linacs, but care should be taken to account for the oxygen inhibition at the edges of the dosimeter.