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Failure to Detect and Interlock An Out-Of-Tolerance Photon Beam Symmetry by a Linear Accelerator Monitor Chamber

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N Knutson

N Knutson*, N Nguyen , M Schmidt , M Price , Rhode Island Hospital, Providence, RI


PO-BPC-Exhibit Hall-1 (Saturday, March 5, 2016)  Room: Exhibit Hall

Purpose: The purpose of this work is to describe a scenario where a linear accelerator failed to detect large changes in symmetry and output over the course of the treatment day due to target failure.

Methods: During routine after-hour patient specific quality assurance (QA), a ~10% change in the 6MV cross calibration factor was observed. Review of morning QA results indicated beam parameters were within tolerance. Repeating morning QA measurements indicated symmetry and output values substantially outside of tolerance. The linac continued to operate without interlock and further investigation was warranted using a variety of dosimeters to confirm beam characteristics.

Results: Investigation using a calibrated 2D diode array demonstrated a large change in the 6MV beam’s radial symmetry of ~10%. Additionally, measurement with ADCL-calibrated ion chamber confirmed a ~10% decrease in output since the completion of morning QA. The 10 MV beam was confirmed to be within tolerance, indicating a faulty 6MV target. To confirm, the target’s position was shifted slightly which resulted in the beam symmetry and output returning to within tolerance. Visual inspection of the removed target showed signs of degradation, confirming our suspicions. To function in this state without interlock, it is hypothesized that the deficit in charge from photons coming from the target at the monitor chamber was replaced by an equal charge coming from primary electrons now escaping the target. Thus, the asymmetry of the photon profile measured at depth did not correlate with the charge collected on each side of the radial monitor chamber which should have triggered an interlock.

Conclusion: Large changes in symmetry and output during the treatment day can go undetected by the linear accelerator monitor chambers. This highlights the need for possible increased QA frequency or perhaps a redesign of the monitor chamber system.

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