Novel Theoretical Concepts for Supporting Reference Dosimetry of Mrgrt Beams
Abstract
Purpose
This study aims at addressing key concepts of radiation dosimetry in the presence of a magnetic field as a complement to the AAPM TG-351 reference dosimetry protocol for radiotherapy guided by magnetic resonance imaging (MRgRT).
Methods
Important concepts related to ion-chamber dosimetry are studied and linked to fundamental principles to support MRgRT clinical dosimetry. The breakdown of cavity theory in MRgRT is theoretically linked to Fano’s theorem in external magnetic fields and its impact is further illustrated with Monte Carlo simulations. Cavity fluence perturbations are predicted with analytical track length calculations and supported by cavity dose response simulations. Ion chambers perturbation factors are simulated with various field sizes to illustrate the impact of Fano’s theorem breakdown, using no magnetic field, a constant magnetic field, and a density-scaled magnetic field. The impact of airgaps on rebuildup distances is investigated as a function of gap size.
Results
The violation of Fano’s theorem caused by a magnetic field is illustrated with Monte Carlo simulations, showing reestablished equilibrium in the presence of a magnetic field distribution scaled with the mass density in depth dose curves and dose profiles. Track-length calculations exhibit three regimes that can be identified on electron fluence perturbations obtains with Monte Carlo: 1) non-ERE; 2) ERE with larger track length; 3) ERE with smaller track length. When scaling the magnetic field, perturbation factors are reduced compared to a constant field, but still exist. Air gaps are shown to impact the electron fluence in distances that are much longer than anticipated.
Conclusion
The present study provides valuable theoretical concepts that confirms the challenges in clinical dosimetry of MRgRT beams. The findings are helpful to the community to support AAPM’s TG-351 dosimetry protocol for reference dosimetry of MRgRT beams.