Purpose: The purpose of this work is to present our investigation of the error detection capabilities of the Delta4 Discover transmission detector.
Methods: DICOM treatment plans were modified through in-house-developed software to mimic various delivery errors that have been observed in the past. Errors included a shift of all MLC leaves in one direction, an opening of the field, where all leaves from both banks retract, a shift in the location of a single leaf, a change in energy from what was planned, and a simulation of what would happen if secondary jaws were stuck and did not track with the leaves as they moved. The baseline plan was delivered with both the Discover device installed on the linac, and the Phantom+ phantom at isocenter, so that a baseline gamma pass-rate could be determined. All modified plans were then delivered using the Discover only and the predicted change in gamma pass-rate, as well as the detected leaf positions were evaluated.
Results: For static and step-and-shoot IMRT plans, the minimum MLC leaf positional error detected was 0.5 mm, while it was 1 mm for sliding window IMRT plans, even for cases where only a single leaf was displaced. The unit also detected the expected drop in gamma pass-rates for cases where jaw-tracking was intentionally disabled. Finally, the gamma pass-rates successfully detected when the energy used for the delivery was switched from the intended one.
Conclusion: With its ability to discover IMRT leaf position errors as small as 0.5 mm, as well as detect errors where the secondary jaws were not in the correct position, or the wrong energy was delivered, the Discover promises to be a valuable independent error detection tool that can help prevent unfortunate errors known to occur during delivery of radiotherapy.
Radiation Detectors, Quality Assurance
TH- External beam- photons: Quality Assurance - Linear accelerator