Reevaluating Backup Monitor Chamber Tolerances for Stereotactic Body Radiotherapy Treatments using...

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Backup monitor chambers in linear accelerators ensure radiation dose accuracy and safety.
• Current 2% constancy tolerance may be insufficient for stereotactic body radiotherapy (SBRT) due to high monitor units (MUs).

Purpose of the Study:

• To evaluate backup monitor chamber constancy in SBRT VMAT treatments on an Elekta Infinity accelerator.
• To determine if current tolerances ensure treatment efficacy and safety.

Main Methods:

• Retrospective analysis of five SBRT patients treated with VMAT.
• Deviations between primary (MU1) and backup (MU2) MUs were recorded at various tolerance levels (1.5% to -1.5%).
• Impact of gantry angle and field size on MU2 readings was assessed.

Main Results:

• All SBRT plans had errors at 1.5% tolerance; errors persisted at 1.0% and 0.5% for high-MU cases.
• No errors were reported at 0.0% and -1.5% tolerance settings.
• Minor gantry angle variations slightly affected MU2 readings; field size had no significant impact.

Conclusions:

• The 2% constancy tolerance is likely suboptimal for SBRT on the Elekta Infinity.
• A tolerance closer to 0.0% is recommended to minimize delivery errors and enhance patient safety.
• Further validation across different accelerators and patient groups is necessary.