Technical note: Cryostat transmission characterization for MR linac - temporal stability, clinical impact and change...

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• The Unity MR linac requires characterization of beam attenuation through its cryostat and coil support.
• Accurate beam modeling in the treatment planning system (TPS) is crucial for output calibration and gantry angle variations.

Purpose of the Study:

• To assess the dosimetric impact of cryostat transmission characterization (CTC) curve changes due to helium level variations.
• To report on the clinical experience of updating the CTC curve in the TPS.

Main Methods:

• Re-calculated 20 stereotactic body radiotherapy (SBRT) treatment plans (10 prostate, 10 oligo-metastatic) using an updated beam model with a post-helium top-up CTC curve.
• Commissioned the new model considering treatment plan adaptation and offline/online TPS components, aligning with reference conditions.

Main Results:

• Observed average CTV mean dose reduction of 0.45% in prostate cases.
• Observed average GTV mean dose reduction of 0.22% in oligo-metastatic cases.
• Validated the updated model, showing good agreement between measurements and TPS calculations.

Conclusions:

• CTC measurements confirmed temporal constancy and method robustness.
• Helium level changes impact CTC, causing small but systematic dose calculation inaccuracies.
• Model validation and end-to-end testing demonstrated minimal impact from transitioning to the new beam model and reference conditions.