MO‐F‐CAMPUS‐T‐04: Implementation of a Standardized Monthly Quality Check for Linac Output Management in a Large Multi‐Site Clinic

Abstract

Purpose:This work is to investigate the feasibility of a standardized monthly quality check (QC) of LINAC output determination in a multi‐site, multi‐LINAC institution. The QC was developed to determine individual LINAC output using the same optimized measurement setup and a constant calibration factor for all machines across the institution.Methods:The QA data over 4 years of 7 Varian machines over four sites, were analyzed. The monthly output constancy checks were performed using a fixed source‐to‐chamber‐distance (SCD), with no couch position adjustment throughout the measurement cycle for all the photon energies: 6 and 18MV, and electron energies: 6, 9, 12, 16 and 20 MeV. The constant monthly output calibration factor (Nconst) was determined by averaging the machines’ output data, acquired with the same monthly ion chamber. If a different monthly ion chamber was used, Nconst was then re‐normalized to consider its different NDW,Co‐60. Here, the possible changes of Nconst over 4 years have been tracked, and the precision of output results based on this standardized monthly QA program relative to the TG‐51 calibration for each machine was calculated. Any outlier of the group was investigated.Results:The possible changes of Nconst varied between 0–0.9% over 4 years. The normalization of absorbed‐dose‐to‐water calibration factors corrects for up to 3.3% variations of different monthly QA chambers. The LINAC output precision based on this standardized monthly QC relative to the TG‐51 output calibration is within 1% for 6MV photon energy and 2% for 18MV and all the electron energies. A human error in one TG‐51 report was found through a close scrutiny of outlier data.Conclusion:This standardized QC allows for a reasonably simplified, precise and robust monthly LINAC output constancy check, with the increased sensitivity needed to detect possible human errors and machine problems.