(Sunday, 7/14/2019) 5:00 PM - 6:00 PM
Room: 301
Purpose: We demonstrate a fast & comprehensive method to (1) verify radiation isocenter wander over various gantry and couch angles similar to Winston-Lutz (WL) test, and (2) directly measure coincidence with the imaging coordinate system.
Methods: An N-isopropylacrylamide (NIPAM) 3D dosimeter for which dose is observed as increased electron density in kV-CBCT is irradiated at eight couch/gantry combinations which enter the dosimeter at unique orientations. A CBCT is immediately acquired, radiation profile is detected per beam, and displacement from imaging isocenter is quantified. We performed this test using both 7.5mm and 4mm cones, delivering approximately 16Gy per beam. CBCT settings were 4050 mAs, 80 kVs, smooth filter, 1mm slice thickness. The 2D displacement of each beam from the imaging isocenter was measured within the planning system. Detectability of the dose profile in the CBCT was quantified as the contrast-to-noise ratio (CNR) of the irradiated high dose regions relative to the surrounding background signal.
Results: Setup, irradiation, & readout could be carried out within 38 minutes. The 2D vector displacement of each beam from the imaging isocenter was 0.06±0.03cm (mean ± standard deviation), with a range of [0.02cm 0.11cm] for the 7.5mm cone and 0.04±0.01cm [0.04cm 0.05cm] for the 4mm cone. In comparison, the traditional WL was 0.04±0.01cm [0.03cm 0.06cm]. The CNR of the high dose regions in the CBCT was 3.1 and 1.6 for the 7.5mm and 4mm cones, respectively. For the 4mm cone we subtracted the background signal from the pre-CBCT, which increased the CNR to 4.0.
Conclusion: This work demonstrated the feasibility of a comprehensive isocenter verification using a NIPAM dosimeter which (1) eliminates potential false positives from user setup error, (2) incorporates evaluation of coincidence with imaging coordinate system, and (3) may be applicable to any SRS cone, as well as MLCs for isocentric and multi-target SRS.
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