Abstract
Data visualization technique was applied to analyze the daily QA results of photon and electron beams. Special attention was paid to any trend the beams might display. A Varian Trilogy Linac equipped with dual photon energies and five electron energies was commissioned in early 2010. Daily Linac QA tests including the output constancy, beam flatness and symmetry (radial and transverse directions) were performed with an ionization chamber array device (QA BeamChecker Plus, Standard Imaging). The data of five years were collected and analyzed. For each energy, the measured data were exported and processed for visual trending using an in-house Matlab program. These daily data were cross-correlated with the monthly QA and annual QA results, as well as the preventive maintenance records. Majority of the output were within 1% of variation, with a consistent positive/upward drift for all seven energies (~+0.25% per month). The baseline of daily device is reset annually right after the TG-51 calibration. This results in a sudden drop of the output. On the other hand, the large amount of data using the same baseline exhibits a sinusoidal behavior (cycle = 12 months; amplitude = 0.8%, 0.5% for photons, electrons, respectively) on symmetry and flatness when normalization of baselines is accounted for. The well known phenomenon of new Linac output drift was clearly displayed. This output drift was a result of the air leakage of the over-pressurized sealed monitor chambers for the specific vendor. Data visualization is a new trend in the era of big data in radiation oncology research. It allows the data to be displayed visually and therefore more intuitive. Based on the visual display from the past, the physicist might predict the trend of the Linac and take actions proactively. It also makes comparisons, alerts failures, and potentially identifies causalities.
Highlights
Pre-treatment daily dosimetry monitoring including X-ray and electron output constancy together with the functional safety checks is recommended for all medical accelerators [1]-[3]
The objective of this study is to utilize data visualization to trend the behavior of photon and electron beams of a new linear accelerator (Linac) in our radiotherapy center
The chambers used in their 2100 C/D Linac resulted in an increase in measured output over time for the first 4 years of use, which is nearly the same model as our Trilogy Linac
Summary
Pre-treatment daily dosimetry monitoring including X-ray and electron output constancy together with the functional safety checks is recommended for all medical accelerators [1]-[3]. The daily tests include parameters that can affect dose to the patient, such as dosimetric output constancy, geometric accuracy of lasers, optical distance indicator, and field size, as well as other safety tests. These daily tests are typically carried out by the morning warm-up radiation therapist using a cross-calibrated dosimetry system. The absolute dose output and other dosimetry tests are performed in the monthly QA test with higher level equipment at tighter tolerance criteria This provides confidence in the daily device and will identify trends that may otherwise go undetected over the course of a long period of time
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