Objectives: We present a method to directly measure radiation isocenter uncertainty and coincidence with the kV-CBCT imaging coordinate system, and utilize it to quantify this uncertainty for various SRS collimation systems (both MLCs and cones).

Methods: The isocenter verification is carried out using an N-isopropylacrylamide (NIPAM) 3D dosimeter, irradiated to 16Gy at eight unique couch/gantry combinations. Pre and post-irradiation CBCTs are acquired.  Because the density of the dosimeter increases with radiation dose, the dose signal is extracted as the difference between the pre- and post-irradiation CBCTs.  Code developed in house was used to detect the radiation beam geometry and quantify the isocenter uncertainty and coincidence with the kV-CBCT imaging system. We performed this test using the 4mm cone, 7.5mm cone, and a 10mm MLC field to evaluate the variation in radiation isocenter uncertainty between collimation systems.  We also compared the results to the results of a traditional Winston-Lutz test, film based "star shots," and the Varian MPC.

Results: The minimum radius that encompassed all beams for the 4mm cone, 7.5mm cone, and 10mm MLC field was 0.45mm, 0.48mm, and 0.40mm. In comparison, the traditional Winston-Lutz test with MLCs measured the 3D isocenter diameter to be 0.48mm, and the Varian MPC measured the "isocenter size" to be +0.24mm which also corresponds to a 0.48mm diameter.  The time required to perform the isocenter verification test on the treatment machine for a single collimation system was <40 minutes (including pre-CBCT, irradiation, and post-CBCT).

Conclusions: The isocenter uncertainty was found to be consistent between the three collimation systems tests.  This work also demonstrates the feasibility of applying a comprehensive isocenter verification test using NIPAM-kV-CBCT dosimetry in a practical clinical setting.