Click here to

Session: Therapy: MR-guided RT Dosimetry and QA [Return to Session]

Evaluation of the Dosimetric Accuracy and Angular Dependency of the Novel SRSMapCheck Design in the Magnetic Field

O Pen*, O Green, A Price, Washington University School of Medicine, St. Louis, MO


SU-IePD-TRACK 6-6 (Sunday, 7/25/2021) 5:30 PM - 6:00 PM [Eastern Time (GMT-4)]

Purpose: SRS MapCheck (SRS-MC) developed by the SunNuclear Corporation is a commercial device used for small field patient-specific dosimetry. However, functionality in the presence of a magnetic field for the purposes of MRI-guided radiation therapy QA comes with certain challenges and requires novel engineering solutions, as well as rigorous testing to confirm the improved performance of the new design.

Methods: Angular dependency of the central axis ion chamber reading was investigated for 2.5x2.5 cm and 5x5 cm fields. An additional set of measurements were taken at both the isocenter and at a 2.5 cm lateral offset for a SRS-MC detector plane parallel and perpendicular to the couch on the MRidian ViewRay Linac, which has constant background magnetic field of 0.35T. Control sets of measurements were taken on a Varian Edge Linac. Measurements were acquired for a traditional SRS-MC design and a proprietary novel engineering design. Several plans of varying complexity were planned in the ViewRay TPS and then measured on the ViewRay Linac, with acquired measurements being compared to the pre-planned and evaluated based on γ-analysis. Absolute dose calibration was performed with readings acquired with A1SL Extradin IC and PTW diamond detector in a solid water phantom.

Results: Angular dependency observed in the traditional design of the SRS-MC was rectified with the new design. CAX readings matched were similar with and without the magnetic field. A γ-rate passing rate of 97.8% (3%/3mm) was achieved for all plans in relative dose mode. Absolute dose mode also reveals good agreement when properly calibrated for the dose at the StereoPhan center.

Conclusion: Novel design of the MRI-compatible SRS-MC addresses previously discovered deficiencies and irregularities in the dose measurement introduced by the presence of the magnetic field and ensures the suitability of the SRS-MC as a tool for patient-specific QA in small field dosimetry.



    Dosimetry, Magnetic Fields, Stereotactic Radiosurgery


    Not Applicable / None Entered.

    Contact Email