(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: this work we characterize the influence of detector design and geometry in a low magnetic field on the results for patient specific quality assurance (QA)
Methods: commercial single plane ionization array (Octavius 1500, non 4D) and two planes orthogonal diode array (Delta4) were calibrated and implemented in a clinical MR-guided radiotherapy (MRgRT) program. Fourteen plans to treat different anatomical sites were selected to show the results of the study. The plans were developed using the ViewRay MRIdian planning system asking for a wide range of gantry angles and beams segments size. Patient specific QA plans were generated for the Octavius 1500 and for the Delta4. For the Gamma Index Analysis, 2%/2mm criteria were used with a threshold of 90%
Results: Octavius 1500 showed an angular dependence. We found that a region of interest (ROI) excluding the peripheral detectors needs to be defined. That means, the area of analysis has to be cropped in order to perform in comparison with the Delta4. For the cropped area, 12 of 14 results were comparable. No restrictions in the couch position when the Delta4 was used was found. In some cases the Octavius 1500 could not be used because the couch should be positioned outside the allowable range. For the Delta4 the pass rate was above 92% and all QA plans passed the test. For the Octavius, without cropping, only 2 of 14 plans passed. With cropping, 2 plans of 14 failed
Conclusion: Delta4 performed better than the Octavius 1500. No crop of the detectors area was necessary when the Delta4 was used, allowing the analysis in the low dose, low gradient region. The Delta4 allows the analysis in two orthogonal planes with higher spatial resolution (0.5mm).
Funding Support, Disclosures, and Conflict of Interest: Dr Romaguera reports honorarium and travel expenses by ScandiDos
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