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Automation of Monthly Linac QA Using Ion Chamber Arrays and Automation Scripts - a Tenfold Speed Increase

L Skinner*, Y Yang , A Yu , T Niedermayr , Stanford University Cancer Center, Stanford, CA


(Tuesday, 7/31/2018) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 4

Purpose: This work, which can be applied to any C-arm linac, combines automation scripts and a commercially available ion chamber array detector to minimize error pathways and expedite monthly QA by a factor ten.

Methods: An ion chamber array and automation scripts were used to perform the majority of the TG-142 recommended dosimetry and mechanical monthly QA tests. The scripts provide two benefits: elimination of user error pathways, and expedience of data collection and analysis. In addition to the standard metrics, novel automated workflows for collimator and couch walkouts, leaf gap constancy, and asymmetric jaw positions are introduced. The sensitivity of each test is investigated by varying the data inputs. Several key dependencies of the ion chamber array were found, including field size, energy, backscatter , software options, and triggering. Solutions for minimizing the impact of these dependencies are provided. To establish stability, safety, and sensitivity, the new methods were performed in parallel with previous methods across 4 linacs over 6 months.

Results: The new methods allow 20 of the 26 TG-142 monthly QA dosimetry and mechanical tests to be performed and analyzed in 30mins, a task that previously took 5 hours. Output differences between farmer chamber in plastic water measurements over 24 sets of monthly QA measurements were found to have standard deviations of 0.3% for photons and 0.5% for electrons. Measurement fidelity was found to be improved for jaw positions, energy constancy, field size, couch and collimator walkout tests, compared to our previous methods.

Conclusion: The equipment, procedure, and scripts used here significantly reduce the time burden of routine QA tests, especially for older generation linacs where integrated automated QA solutions are limited. One compromise is that uncertainties in the output constancy are slightly increased compared to farmer chamber measurements. For most tests measurement precision is improved.


Quality Assurance, Ionization Chamber, Data Acquisition


TH- Radiation dose measurement devices: Multi-dimensional diode/chamber arrays

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