A Historical Review of CBCT QA Performance On One Truebeam LINAC
K Episcopia, D Lovelock*, P Klages, H Kuo, S Lim, J Sillanpaa, Memorial Sloan-Kettering Cancer Center, New York, NY
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PO-GePV-I-14 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To assess the stability of image quality of a Linac onboard cone beam imaging system over a period of 12 years.
Methods: Full-fan and half-fan CBCT performance stability was assessed using CBCT images of a Catphan CTP504 phantom from one Truebeam Linac. The scans were acquired over a period of 12 years as part of the annual quality assurance process. Contrast to Noise Ratio (CNR) was determined using the difference of the mean values for the Teflon and polystyrene inserts in the Catphan divided by the standard deviation of bulk phantom region. Other metrics such a high contrast resolution were determined using commercial software (TotalQA, Image Owl).
Results: There are no significant trends shown in the CNR time series data for the Pelvis, Spotlight, and Head scan modes. The CNR values are dominated numerically by the denominator so minor changes in the bulk phantom noise have a noticeable effect on the recorded values, but indicated when procedural changes in calibrations were needed. Image scaling and high contrast (MTF 10%) did not change significantly throughout the 12-year period.
Conclusion: Maintenance of CBCT image quality is a vital aspect of patient safety. The performance of the CBCT imaging system was found to be remarkably stable over time; no degradation in contrast to noise ratio was observed in the pelvis, spotlight, and head scans. In some scans there was evidence of ring artifacts. These artifacts were addressed by increasing the frequency of the dark field, flood field, bad pixel correction, and air norm calibration procedures for each CBCT mode to once every 6 months.
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