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IROC phantoms accurately detect MLC delivery errors
Sharbacha S Edward1, Julianne M Pollard-Larkin1, Peter A Balter1
1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas, USA.
Journal of Applied Clinical Medical Physics
|February 19, 2025
Summary
Multileaf collimator (MLC) errors in radiation therapy can cause dose deviations, with whole-bank shifts being more impactful than random shifts. However, even with smaller fraction sizes, dosimetric accuracy remained clinically acceptable.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Physics
Background:
- Accurate radiation delivery is crucial for effective cancer treatment.
- Multileaf collimators (MLCs) are essential components in modern radiotherapy systems, shaping radiation beams.
- Understanding the impact of MLC delivery errors is vital for ensuring patient safety and treatment efficacy.
Purpose of the Study:
- To evaluate the dosimetric impact of random and whole-bank MLC delivery errors in IROC phantom audits.
- To compare delivery accuracy between IROC phantom prescriptions and clinical fraction sizes.
- To assess the influence of fraction size on MLC position accuracy and dose delivery.
Main Methods:
- IMRT head and neck (H&N) and SBRT spine phantoms were used with simulated MLC errors (random shifts -2 to 2 mm, whole bank shifts 0.5-2 mm).
- Plans were recalculated and delivered on Varian Truebeam and 21EX machines; log files were analyzed.
- IROC prescriptions were scaled to simulate smaller (2 Gy) and larger (27 Gy) fraction sizes for accuracy assessment.
Main Results:
- Random MLC errors caused minor PTV dose deviations (up to -2.8% for H&N, 0.7% for spine).
- Whole-bank MLC shifts led to larger PTV dose deviations (up to 8% for H&N, 7.1% for spine).
- Smaller fraction sizes resulted in higher MLC RMS errors but did not compromise dosimetric accuracy (<1% deviation).
Conclusions:
- MLC delivery errors, both random and whole-bank, produce dose deviations comparable to clinical findings.
- Ion chamber measurements closely matched delivery log file analysis.
- While smaller fractions increase MLC RMS errors, they remain clinically insignificant regarding overall plan accuracy.
