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Related Experiment Video

Updated: May 23, 2026

Dynamic Lung Tumor Tracking for Stereotactic Ablative Body Radiation Therapy

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Published on: June 7, 2015

Quantifying the gantry sag on linear accelerators and introducing an MLC-based compensation strategy.

Weiliang Du¹, Song Gao, Xiaochun Wang

¹Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA. wdu@mdanderson.org

Medical Physics

|April 10, 2012

Summary

Gantry sag, a linear accelerator imperfection, can be measured and compensated using multileaf collimator adjustments. This method significantly reduces radiation field inaccuracies, improving dose delivery precision.

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Area of Science:

Medical Physics
Radiation Oncology
Radiotherapy Technology

Background:

Gantry sag is a known mechanical error in linear accelerators (linacs) affecting radiation therapy accuracy.
Spatial accuracy in radiation dose delivery is critical for effective cancer treatment.

Purpose of the Study:

Quantify gantry sag across multiple linacs.
Investigate a multileaf collimator (MLC)-based compensation strategy for gantry sag.
Verify gantry sag and its compensation using film measurements.

Main Methods:

Utilized the Winston-Lutz method to measure gantry sag on three Varian linacs.
Imaged a ball bearing phantom at 10° gantry angle intervals.
Analyzed electronic portal imaging device (EPID) images to derive radiation isocenter and gantry sag, then applied MLC leaf position corrections.

Main Results:

Gantry sag was reproducible, with maximums ranging from 0.7 to 1.0 mm.
Radiation field center shifted inferiorly as the gantry rotated from 0° to 180°.
MLC compensation at a 90° collimator angle reduced maximum gantry sag to <0.2 mm, verified by film.

Conclusions:

Gantry sag can be quantitatively measured using simple phantoms and EPID.
MLC leaf position correction offers a feasible method to reduce gantry sag and enhance spatial accuracy in radiotherapy.