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Practical Application of NIPAM KV-CBCT Dosimetry to Determine KV-MV Isocenter Alignment

K Pant1*, M Oldham2, W Giles2, J Adamson2, (1) Medical Physics Graduate Program, Duke University, Durham, NC (2) Duke University Medical Center, Durham, NC

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: We report practical clinical scenarios in which an N-isopropylacrylamide (NIPAM) kV-CBCT dosimetry was used to detect kV-MV isocenter misalignment on a Varian TrueBeam STx.


Methods: Isocenter verification was performed using a NIPAM 3D gel dosimeter, irradiated at eight unique couch/gantry angles (~16Gy). Pre- and post- irradiated CBCTs were acquired. Because a density change was observed in the dosimeter as dose is deposited, the dose signal was extracted as the difference between the pre- and post-irradiation CBCTs. Matlab code developed in house detects beam geometry within the CBCTs and quantifies the isocenter uncertainty and coincidence with the imaging system. Accuracy of couch and gantry angles were also quantified. We demonstrate two cases in which this method was able to determine kV-MV isocenter misalignment on a TrueBeam STx linear accelerator. First, a misalignment was simulated by introducing a 0.5mm shift applied in the direction of leaf travel for a 10mm diameter MLC field. Second, the test was carried out before and after a scheduled isocenter adjustment made by the manufacturer.


Results: Setup, imaging, and irradiation of the dosimeter could be completed in ~45 minutes; automatic Matlab analysis in ~15 minutes. When a 0.5mm MLC shift was applied, the smallest radius to encompass all beams increased from 0.38mm to 0.90mm. Prior to maintenance, the TrueBeam STx isocenter diameter measured at 0.976mm (ISO Lock measurement by vendor). This value decreased to 0.519mm after repair. Our results showed the radial distance from the radiation isocenter to the CBCT origin before and after maintenance was 0.45mm and 0.31mm, respectively, with smallest radius to intersect all beams decreasing from 0.46mm to 0.43mm.


Conclusion: The NIPAM kV-CBCT dosimetry isocenter verification offers a unique comprehensive method to quantify and visualize kV-MV isocenter uncertainties, which is especially useful in clinical settings where the isocenter is expected to change.

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