# Long‐term stability analysis of beam shape in a robotic radiosurgery system

## TL;DR

This study examines the long-term stability of a robotic radiosurgery system's beam shape to improve quality assurance practices.

## Contribution

The study validates the 2% profile constancy check for early magnetron failure detection in CyberKnife systems.

## Key findings

- The 2% profile constancy check detected magnetron deterioration two months before failure.
- Symmetry and flatness remained stable over 100 million monitor units, but penumbra showed more variability.
- Gradual profile shifts were observed, emphasizing the need for multifaceted QA strategies.

## Abstract

This study aimed to investigate the long‐term stability of CyberKnife beam profile parameters and assess their compliance with existing quality assurance (QA) guidelines. We evaluated beam profiles in both standard and diagonal planes over 3.5 years post‐installation to detect potential issues and ensure consistent beam quality. The findings will contribute to validating and refining current QA practices for CyberKnife systems.

Beam profile measurements were performed monthly using an Octavius 1000SRS detector array. The profiles were evaluated in terms of the beam shape constancy within 2%, and the penumbra, symmetry, and flatness were analyzed using statistical process control methods. Temporal changes in the dose profiles were visualized using dose difference heat maps. The relationship between the beam parameters and accumulated monitor units was also investigated.

The 2% profile constancy check accurately detected magnetron deterioration 2 months before failure, confirming its high sensitivity for beam stability monitoring. While symmetry and flatness remained within 0.7% throughout over 100 × 106 monitor units of operation, penumbra exhibited greater responsiveness to magnetron‐induced changes but did not consistently flag all orientations. Additionally, statistical analyses and heat maps revealed gradual profile shifts independent of acute component failures, highlighting the importance of multifaceted QA strategies.

These findings reinforce the effectiveness of the 2% profile constancy check for early detection of magnetron failure and support its adoption in current CyberKnife guidelines. At the same time, symmetry, flatness, and penumbra parameters remain valuable for characterizing gradual profile variations. Collectively, this study underscores the need for comprehensive beam monitoring and regular maintenance to sustain optimal CyberKnife system performance.