Abstract
Purpose:To determine the segment reduction and dose resolution necessary for machine log‐files to effectively replace current phantom‐based patient‐specific quality assurance, while minimizing computational cost.Methods:Elekta's Log File Convertor R3.2 records linac delivery parameters (dose rate, gantry angle, leaf position) every 40ms. Five VMAT plans [4 H&N, 1 Pulsed Brain] comprised of 2 arcs each were delivered on the ArcCHECK phantom. Log‐files were reconstructed in Pinnacle on the phantom geometry using 1/2/3/4° control point spacing and 2/3/4mm dose grid resolution. Reconstruction effectiveness was quantified by comparing 2%/2mm gamma passing rates of the original and log‐file plans. Modulation complexity scores (MCS) were calculated for each beam to correlate reconstruction accuracy and beam modulation. Percent error in absolute dose for each plan‐pair combination (log‐file vs. ArcCHECK, original vs. ArcCHECK, log‐file vs. original) was calculated for each arc and every diode greater than 10% of the maximum measured dose (per beam). Comparing standard deviations of the three plan‐pair distributions, relative noise of the ArcCHECK and log‐file systems was elucidated.Results:The original plans exhibit a mean passing rate of 95.1±1.3%. The eight more modulated H&N arcs [MCS=0.088±0.014] and two less modulated brain arcs [MCS=0.291±0.004] yielded log‐file pass rates most similar to the original plan when using 1°/2mm [0.05%±1.3% lower] and 2°/3mm [0.35±0.64% higher] log‐file reconstructions respectively. Log‐file and original plans displayed percent diode dose errors 4.29±6.27% and 3.61±6.57% higher than measurement. Excluding the phantom eliminates diode miscalibration and setup errors; log‐file dose errors were 0.72±3.06% higher than the original plans – significantly less noisy.Conclusion:For log‐file reconstructed VMAT arcs, 1° control point spacing and 2mm dose resolution is recommended, however, less modulated arcs may allow less stringent reconstructions. Following the aforementioned reconstruction recommendations, the log‐file technique is capable of detecting delivery errors with equivalent accuracy and less noise than ArcCHECK QA.I am funded by an Elekta Research Grant
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