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Indicator for Target Degradation in Linac Using QA Data of Enhanced Dynamic Wedge

J Li*, M Chan , Memorial Sloan Kettering Cancer Center, Basking Ridge, NJ


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: The performance of linear accelerator depends on the integrity of its target. The catastrophic failure of its target not only breaks down the linear accelerator but also could contribute significant disruptions of patient care. This work is to present a QA method that can be used to identify the risk of target failure before it occurs and interrupts the clinical operation.

Methods: A failure modes and effects analysis (FMEA) was conducted after a recent target failure that resulted in its replacement in one of our linear accelerators. The dosimetric data from the daily QA device were collected and their trends were investigated. After retrieving the broken target, its physical characteristics were analyzed and compared with its original condition. The correlation of its changed physical property and the dramatic change of dosimetric parameters revealed in QA data was studied.

Results: The first indicator for target malfunction was the reduction of beam output (>2%). After output adjustment, beam profiles from both open field and dynamic wedge deliveries of the affected photon energy demonstrated a clear trend of deviation from normal baseline (~3%), starting about a week before the target failure. The daily axial symmetry of a 45-DEG-IN dynamic wedge has an upward trend gradually drifted to almost 3%. The observed phenomena from measured data can be qualitatively explained by the asymmetric pattern of target damage: the melted part of target created a “directional trench� below its surface, resulted in a reduced attenuation and increased photon fluence in the same direction, contributing a significant change in the beam profile.

Conclusion: The daily beam profiles of dynamic wedge delivery can be used to monitor the performance of target and predict its failure. Simultaneous reduction of beam output and changes of beam symmetry profiles should be taken as an alert for potential target failures.


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