Room: AAPM ePoster Library
Purpose: evaluate the Integral Quality Monitor (IQM) large-area ionization chamber transmission detector as a tool for optimizing multi-leaf collimator (MLC) model specific parameters: dosimetric leaf gap (DLG) and transmission factor (TF).
Methods: standard dynamic chair static IMRT pattern was generated and a basis fluence was calculated for commissioned values of DLG and TF. Dose was calculated for a spectrum of values for TF and DLG, while holding constant the dynamic MLC pattern, optimal fluence, and monitor units. The optimum values of the model parameters were evaluated against beam delivery on a 2-D ion chamber array (MatriXX). Gamma analysis, a common metric for patient-specific quality assurance, was used to evaluate optimum parameter settings. Parameters of gamma evaluation (2%/2 mm, 95% threshold, local) were chosen to have a high sensitivity to small parameter changes. The IQM monitor tool was used to measure cumulative beam delivery signal for 11 patient treatment fields yielding an assessment of the reproducibility (n=3) of the system.
Results: values from MatriXX measurements established a baseline trend localized on commissioned DLG and TF values. Standard deviations for MatriXX measurements (n=3) were large (2.9% - 11.8%) over the range considered, precluding the appointment of absolute optimized MLC parameters. Conversely, measurements with the IQM resulted in smaller deviations (0.08% - 0.32%), enabling the optimization of TF and DLG at higher resolutions.
Conclusion: of the DLG and TF, are frequently used to increase sensitivity of patient-specific quality assurance measurements. Common tools for optimization (MatriXX, film, EPID, etc.) collect spatial data that is collapsed to a scalar to facilitate comparisons. The IQM uses integral data collection that is more sensitive to small dose differences. It has the potential to increase the resolution of DLG and TF optimization measurements.
Transmission Measurements, Quality Assurance, MLC
TH- External Beam- Photons: Quality Assurance - Linear accelerator