(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: of the most significant issues of 3D measurement-based IMRT QA is in the 3D registration between the measured dose and planned dose. Through the extended use of manual registration, a ‘best match’ could be found to achieve the highest gamma passing rate. However, a rigorous and fair dose distribution comparison cannot be established when exclusive manual registration is used to align the dose distributions. This study aims to resolve this concern by proposing a new methodology using fiducial registration.
Methods: CT skin markers were placed on the PRESAGE dosimeter with two purposes: setting up the dosimeter for the CT simulation and irradiation, and registration between the measured dose distribution and calculated dose distribution. The dosimeter was placed on the couch with the axial plane perpendicular to the couch surface to measure a VMAT plan of cerebellar metastasis. After irradiation, the 3D dose distribution of the irradiated dosimeter was readout by an in-house optical-CT scanner (OCT). An automatic fiducial localization algorithm was developed in MATLAB to register the markers in the OCT dose images to the CT simulation images. SlicerRT, an extension of 3D Slicer was employed to visualize the measured dose distribution, structures and perform DVHs, 3D gamma index calculation. The expected square target registration error (TRE) was evaluated using approximate expressions derived by Fitzpatrick et al.
Results: expected square TRE at a given point in the 3D space is smaller than 0.5 mm. With the registered coordinates, the proposed methodology can provide structure by structure information (DVHs comparison, Location of gamma failing points) for the patients’ plan, and thus improve the current 2D IMRT QA methods that summarize the gamma test in aggregate.
Conclusion: accurate point-based registration between measured and calculated image spaces using CT fiducial markers, a precise spatial correlation between the two can be found.
Funding Support, Disclosures, and Conflict of Interest: The dosimeters were provided by Heuris Inc. free of charge.
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