Commissioning a secondary dose calculation software for a 0.35 T MR-linac

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• Secondary dose calculations are essential for safety in radiation oncology.
• Magnetic Resonance Image-Guided Radiation Therapy (MRgRT) systems introduce magnetic fields that may affect dose calculations.
• Accurate monitor unit (MU) calculations are critical for patient safety and treatment efficacy.

Purpose of the Study:

• To evaluate the accuracy of RadCalc software for secondary dose calculations on a 0.35 T MRgRT system.
• To assess the impact of the 0.35 T magnetic field on point dose and monitor unit calculations.
• To validate RadCalc's performance against treatment planning system (TPS) values for simple and complex geometries, including patient-specific plans.

Main Methods:

• Monitor unit (MU) calculations were performed using RadCalc with and without magnetic field considerations (wMag/noMag) for the ViewRay MRIdian system.
• The magnetic field's effect was indirectly accounted for using asymmetric profiles and a single transmission value for double-stacked multi-leaf collimator (MLC) leaves.
• Calculations were conducted for simple and complex geometries, including phantoms and 25 patient-specific treatment plans, comparing results to TPS values.

Main Results:

• Simple geometry calculations using RadCalc were within 2% of TPS values for both wMag and noMag methods.
• Complex MLC patterns and phantom geometry calculations were within 5% of TPS values.
• Patient-specific plan calculations showed minimal average percentage differences (0.09% ± 2.5% for noMag, 0.08% ± 2.5% for wMag), with all wMag results within 5%.

Conclusions:

• Commercially available RadCalc software demonstrates accuracy in performing secondary MU calculations for a 0.35 T MRgRT system.
• The 0.35 T magnetic field has a minimal impact on monitor unit dose calculations performed by RadCalc.
• This study is the first to show successful dose calculations using RadCalc on the low-field 0.35 T ViewRay MRIdian system, supporting its use in clinical practice.