Comprehensive Radiation and Imaging Isocenter Verification Using NIPAM kV-CBCT Dosimetry

Purpose: To develop a comprehensive method to measure the radiation uncertainty and coincidence with the kV-CBCT imaging coordinate system using NIPAM kV-CBCT dosimetry.

Methods: An N-isopropylacrylamide (NIPAM) dosimeter is irradiated at eight gantry/couch combinations which enter the dosimeter at unique orientations such that the beams do not overlap except at the isocenter. 1-3 CBCT images are acquired before and immediately after irradiation, radiation profile is detected per beam, and the displacement from the imaging isocenter is quantified. This test has been performed on SRS cone sizes ranging from 4 mm to 15 mm diameter and a 5 mm diameter MLC field, delivering approximately 16 Gy per beam. Matlab code was developed in house to detect each beam’s geometry and to quantify relevant parameters, including radiation isocenter and coincidence with the CBCT origin and the actual gantry and couch angles per beam. The dose profile of each beam was detected in the CBCT using the contrast-to-noise ratio (CNR) of the irradiated high dose regions relative to the surrounding background signal of the dosimeter. Reproducibility was demonstrated by repeating the test on two separate NIPAM dosimeters using the 6 mm cone. To determine the robustness of our test, our results were compared to the results of the traditional Winston-Lutz test, film based “star shots,” and the Varian Machine Performance Check (MPC). The ability of our Matlab code to detect alignment errors was demonstrated by applying a 0.5 mm shift to the MLCs in the direction of leaf travel.

Results: Setup, irradiation, and imaging can be completed in under 40 minutes. The minimum radius to encompass all beams calculated by automated analysis for the MLCs, 4 mm cone, 6 mm cone, 7.5 mm cone, 12.5 mm cone, and 15 mm cone was 0.38 mm, 0.44 mm, 0.53 mm, 0.48 mm, 0.75 mm, 0.5 mm, and 0.57 mm, respectively. When determined manually, these values slightly decreased to 0.28 mm, 0.40 mm, 0.33 mm, 0.41 mm, 0.61 mm, 0.48, and 0.34 mm, respectively. The isocenter verification test was repeated using the 6 mm cone; in both tests, the smallest radius to encompass all beams was found to be 0.53 mm, indicating that the test is reproducible. For comparison, the 3D isocenter radius was 0.24 mm, 0.25 mm, and 0.28 mm for the traditional Winston-Lutz test with MLCs, the Varian MPC, and a “star shot” QA sample. Lastly, when a 0.5 mm shift was applied to the MLCs, the smallest radius to encompass all beams increased from 0.38 mm to 0.90 mm.

Conclusion: The results of this project demonstrate the feasibility of a comprehensive isocenter verification test using NIPAM kV-CBCT dosimetry which incorporates the evaluation of radiation coincidence with the imaging coordinate system, and is capable of producing sub-mm results. This test is applicable to all SRS cone sizes as well as MLCs and can be performed in a typical QA time slot.