Evolution of Log File-Based Patient-Specific QA across Multiple Linac Replacements 📝

Author: Gweneth Andersen, Colleen M. Foote, David (Bo) McClatchy, Hannah McFarland 👨‍🔬

Affiliation: Massachusetts General Hospital 🌍

Abstract:

Purpose: To quantify the impact of MLC model differences and LINAC age on treatment plan evaluation by log file-based patient-specific QA (PSQA) through the lifetime of the machines.
Methods: 3D gamma passing rates were collected for N=16,194 plans from Plan Checks (PC) and QA Checks (QAC) calculated by Mobius3D over 6 years using TPS DICOMs and LINAC trajectory log files. PC, QAC, and |QAC-PC| gamma values were stratified by machine and treatment site. PC and |QAC-PC| histograms for final 100-plan cohorts of historically low-gamma treatment sites previously planned for Agility (Head&Neck, Sarcoma) were compared to initial 100-plan cohorts planned for TrueBeam. Monthly mean QAC and |QAC-PC| were plotted against months since LINAC installation.
Results: PC gamma increased and |QAC-PC| decreased between final Agility and initial TrueBeam cohorts for Head&Neck and Sarcoma plans. Agility QAC decreased overall at a rate of 0.32%/year (p<0.001), while older TrueBeam QAC decreased by only 0.092%/year (p<0.001), but QAC of plans for TrueBeams replacing Agility LINACS increased by 0.283%/year (p=0.0758). |QAC-PC| of Agility plans increased overall by 0.028%/year (p<0.001) to a mean of ~0.5% upon final decommissioning at 12.5 years since installation, and newer TrueBeam |QAC-PC| increased by 0.032%/year (p=0.0783), but only to a mean of ~0.2% at 2 years since installation. |QAC-PC| changes for Ethos and older TrueBeam plans, and QAC changes for Ethos plans, were negligible over 2-3 years.
Conclusion: Changes in QAC and |QAC-PC| distributions for matched cohorts from the same treatment site suggest a contrast in PSQA agreement between LINACs and log file analysis systems from different vendors. Longitudinal PSQA data demonstrate a small differential decline in accuracy of treatment plan delivery over LINAC lifetimes.

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