Development of a new concept automatic frequency controller for an ultrasmall C-band linear accelerator guide

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Accelerator Engineering

Background:

• Four-dimensional image-guided radiotherapy requires precise control of X-ray beams from agile, gimbaled heads.
• Small C-band accelerator guides in these systems generate significant heat, causing resonance frequency shifts.
• Conventional automatic frequency controllers (AFCs) struggle with the rapid frequency variations inherent in step-and-shoot intensity modulated radiotherapy (IMRT).

Purpose of the Study:

• To develop and validate a novel digital AFC for a four-dimensional radiotherapy system.
• To compensate for thermal-induced resonance frequency variations in a C-band accelerator guide.
• To ensure stable and optimized X-ray delivery for IMRT applications.

Main Methods:

• Designed and implemented a new digital AFC based on a novel concept.
• Tested the AFC prototype on an existing accelerator guide at 500 cGy/min and 300 Hz pulse-repetition frequency.
• Evaluated AFC performance, focusing on capture-frequency range and response time.

Main Results:

• The new digital AFC demonstrated a capture-frequency range of 24 MHz, significantly exceeding the 0.9 MHz of conventional analog AFCs.
• The AFC successfully acquired the accelerator guide's resonance frequency within 0.15 seconds after beam activation.
• Stable tracking and adjustment of the microwave source frequency were achieved, with planned improvements targeting 33 ms acquisition time.

Conclusions:

• The developed digital AFC effectively compensates for thermal characteristics of the C-band accelerator guide.
• The system's wide capture-frequency range and fast response are crucial for precise, stable IMRT delivery.
• This innovation enhances the reliability and performance of advanced image-guided radiotherapy systems.