Room: Exhibit Hall | Forum 4
Purpose: With double-focused MLCs and a small effective leaf width, the MRIdian MR-Linac has dosimetric features that make cranial radiosurgery treatment planning attractive. The adaptive planning workflow and MRI-only planning may enable online SRS workflows, but the lack of collimator and couch rotation capabilities precludes techniques utilizing non-coplanar beam geometries. This work explored the dosimetric feasibility of using the MRIdian for SRS treatments by comparing plans generated for the MRIdian with plans generated for an HDMLC-equipped TrueBeam using BrainLab iPlan.
Methods: For both the MRIdian and the TrueBeam, spherical targets within an anthropomorphic head phantom of diameter 0.6 â€“ 2.5 cm were planned such that 20 Gy covered 99% of the target volume. 3D Plans were generated for each target using 11 coplanar, equally-spaced static beams with an emphasis on prescription dose conformity. The maximum dose was constrained to approximately 25 Gy, corresponding to 80% isodose line normalization. For each target, an additional MRIdian plan was optimized to minimize V12Gy (MRIdian_V12). An additional set of TrueBeam plans was generated which utilized five non-coplanar dynamic conformal arcs, mimicking a typical clinical approach.
Results: For targets with volumes greater than 1 cc, non-coplanar TrueBeam plans had the lowest V12Gy. Coplanar TrueBeam and MRIdian_V12 plans had non-statistically significant differences in V12Gy (p=0.06). MRIdian_V12 plans had a higher average max dose (30.0 Gy) compared to the coplanar TrueBeam plans (24.4 Gy), indicating optimal MRIdian normalization is around 65% for these tests. For coplanar plans constrained to a max dose of 25 Gy, TrueBeam plans had lower V12Gy (p=0.002) compared to MRIdian plans.
Conclusion: MRIdian plans can achieve V12Gy values comparable to those generated for a TrueBeam with HDMLC. The MRIdian can be used to treat lesions with volumes up to approximately 6.75 cc while maintaining V12Gy values of less than 10 cc.