Technical Note: Real-time image-guided adaptive radiotherapy of a rigid target for a prototype fixed beam...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Fixed beam radiotherapy simplifies linear accelerator design by using couch movement instead of gantry rotation.
• Maintaining clinical accuracy in fixed beam systems requires addressing potential target motion.

Purpose of the Study:

• To investigate the feasibility of delivering accurate fixed beam radiotherapy treatments.
• To compare the accuracy of fixed beam vs. conventional rotating beam radiotherapy with real-time image guidance.
• To assess the impact of rigid target motion on treatment accuracy.

Main Methods:

• A prototype fixed beam system with a vertical linac and rotational stage was developed.
• Kilovoltage Intrafraction Monitoring (KIM) and real-time multi-leaf collimator (MLC) tracking were employed for adaptive radiotherapy.
• Film dosimetry was used to compare dose distributions between fixed and rotating beam geometries with and without real-time adaptation.

Main Results:

• Dose distributions were equivalent between fixed and rotating beam geometries when using KIM and MLC tracking for adaptation.
• Gamma analysis demonstrated >98% agreement (2%/2 mm criteria) with adaptation for static and periodic motion.
• Real-time adaptation maintained accuracy despite rigid target motion.

Conclusions:

• Fixed beam radiotherapy with real-time adaptation is dosimetrically equivalent to conventional rotating beam treatments.
• This approach maintains high clinical accuracy for rigid targets, even with motion.
• Simpler fixed beam geometry is viable for achieving accurate real-time adaptive radiotherapy.