Radiation therapy dosimetry using magnetic resonance imaging of polymer gels

Further progress in the development of polymer gel dosimetry using MRI is reported, together with examples of its application to verify treatment plans for stereotactic radiosurgery and high dose rate brachytherapy. The dose distribution image produced in the tissue‐equivalent gel by radiation‐induced polymerization, and encoded in the spatial distribution of the NMR transverse relaxation rates (R₂) of the water protons in the gel, is permanent. Maps of R₂ are constructed from magnetic resonance imaging data and serve as a template for dose maps, which can be used to verify complex dose distributions from external sources or brachytherapy applicators. The integrating, three‐dimensional, tissue‐equivalent characteristics of polymer gels make it possible to obtain dose distributions not readily measured by conventional methods. An improved gel formulation (BANG‐2) has a linear dose response that is independent of energy and dose rate for the situations studied to date. There is excellent agreement between the dose distributions predicted using treatment planning calculations and those measured using the gel method, and the clinical practical utility of MRI‐based polymer gel dosimetry is thereby demonstrated.