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Feasibility of Online SRS with MRIdian: Dosimetric Comparison of MRIdian to TrueBeam with HDMLC

M Malin*, E Lobb , M Stevens , G Redler , K Yenice , B Aydogan , The University of Chicago Medical Center, Chicago, IL


(Monday, 7/30/2018) 1:15 PM - 1:45 PM

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 – 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.


Stereotactic Radiosurgery, Treatment Planning


TH- External beam- photons: intracranial stereotactic/SBRT

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