Dose Rate Versus Gantry Speed Performance Evaluation for SRT/SBRT Treatment Deliveries Using DICOM RT Plans
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PO-GePV-T-381 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Varian provides a DICOM RT Plan file that users can deliver to the EPID panel to confirm the linear accelerator delivers consistent output for varying dose rates and gantry speeds (DRGS). This work investigates if 1) the vendor-provided DRGS DICOM RT Plan reaches gantry speeds relevant to SRT/SBRT treatments and 2) if output constancy is maintained at those gantry speeds on a TrueBeam.
Methods: I wrote Python code to iterate through all control points in the DICOM RT Plan files for 16 SRT/SBRT patients and calculate the MU per degree values for each control point. A histogram was generated to illustrate how MU per degree was distributed among the control points from the patient plans. Then, the total number of MUs was increased in the vendor-provided DRGS DICOM file to make a “modified DRGS DICOM RT Plan” which surpasses the maximum MU per degree value found in the patient plans, forcing the gantry to travel at slow speeds and deliver more MU per degree over the same arc length (representative of those during SRT/SBRT treatment delivery). The modified DRGS DICOM RT Plan file was then delivered on a TrueBeam to acquire EPID images of the dose distribution. The EPID images were analyzed with Pylinac, a Python library that analyzes DICOM RT Images acquired during routine linac QA.
Results: The vendor-provided DRGS DICOM file does not evaluate the entire clinically-relevant range of MU per degree values, as seen in the histogram data. The Pylinac analysis of the EPID-acquired found a maximum deviation of 0.4% when comparing the results from the modified DRGS DICOM file to the current baselines.
Conclusion: The modified DRGS DICOM file can be used to determine if a TrueBeam linac is operating within specifications even when very low gantry speeds are reached.
Keywords
DICOM-RT, Linear Accelerator, Quality Assurance
Taxonomy
TH- External Beam- Photons: Quality Assurance - IMRT/VMAT
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