Quality assurance devices for dynamic conformal radiotherapy

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Conformal dynamic arc treatment (CDAT) and intensity modulated radiation therapy (IMRT) require precise quality control.
• Existing quality control methods may not detect all geometric and dosimetric errors, especially during arbitrary gantry angles.

Purpose of the Study:

• To develop and evaluate novel quality control devices for CDAT and IMRT.
• To assess the geometric precision and dosimetric accuracy of linear accelerator treatment systems (LINACs) in three dimensions.
• To identify potential sources of error, such as gantry sag, mechanical displacement, and gravitational effects on multi-leaf collimator (MLC) positioning.

Main Methods:

• Designed and implemented specialized light-field and radiation-field devices.
• Attached devices to the LINAC couch at the isocenter, allowing rotation parallel to the gantry head.
• Evaluated light and radiation fields at arbitrary gantry angles.
• Investigated the influence of gravitational acceleration on MLC leaf positioning errors.

Main Results:

• The devices successfully evaluated light and radiation fields at any gantry angle.
• Identified geometric field shifts caused by isocenter calibration errors, which conventional methods missed.
• Confirmed a statistically significant influence of gravity on MLC leaf positioning, though dose inaccuracy was small.

Conclusions:

• The developed quality control devices provide an independent, three-dimensional check of LINAC performance for CDAT and IMRT.
• These devices are crucial for detecting errors missed by conventional quality control, enhancing treatment accuracy.
• Gravitational effects on MLCs are measurable but currently result in minimal dose inaccuracy.