Does gated beam delivery impact delivery accuracy on an Elekta linac?

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Respiratory motion significantly impacts radiotherapy delivery accuracy.
• Gated radiotherapy aims to mitigate motion artifacts by delivering radiation only during specific phases of the respiratory cycle.
• Evaluating the performance of linear accelerators (linacs) under gated conditions is crucial for ensuring treatment efficacy.

Purpose of the Study:

• To assess the delivery accuracy of an Elekta linac for gated radiotherapy.
• To investigate the impact of short gating windows and long beam hold times on delivery precision.
• To evaluate gated radiotherapy performance for both free-breathing and breath-hold techniques using various treatment plans.

Main Methods:

• Performance evaluation using an Elekta linac with a 20cm x 20cm open field and specific beam-on/beam-off times.
• Delivery accuracy assessed using an IBA MatriXX 2D ion chamber array and gamma passing rates.
• Measurements included free-breathing gating, simulated breath-hold gating, and volumetric modulated arc therapy (VMAT) for stereotactic body radiotherapy (SBRT) plans.

Main Results:

• Close agreement between gated and non-gated deliveries for open fields, with discrepancies emerging at high beam-off to beam-on ratios (5:1).
• Gamma passing rates (± 1%, 2%/1 mm) for SBRT plans ranged from 95% to 100% across various gating windows (25%-83%).
• Free-breathing and breath-hold gating achieved gamma passing rates between 95.7% and 100% under simulated respiratory motion.

Conclusions:

• Elekta linacs demonstrate accurate delivery capabilities for respiratory-gated radiotherapy in both free-breathing and breath-hold scenarios.
• The study highlights potential discrepancies with very narrow gating windows due to low monitor units per beam-on segment.
• Careful consideration of gating window parameters is recommended for optimal treatment delivery accuracy.