Purpose To develop and demonstrate a comprehensive method to directly measure radiation isocenter uncertainty and coincidence with the cone‐beam computed tomography (kV‐CBCT) imaging coordinate system that can be carried out within a typical quality assurance (QA) time slot. Methods An N‐isopropylacrylamide (NIPAM) three‐dimensional (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. One to three CBCTs are immediately acquired, radiation profile is detected per beam, and displacement from imaging isocenter is quantified. We performed this test using a 5 mm diameter MLC field, and 7.5 and 4 mm diameter cones, delivering approximately 16 Gy per beam. CBCT settings were 1035–4050 mAs, 80–125 kVs, smooth filter, 1 mm slice thickness. The two‐dimensional (2D) displacement of each beam from the imaging isocenter was measured within the planning system, and Matlab code developed in house was used to quantify relevant parameters based on the actual beam geometry. 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. Our results were compared to results determined by the traditional Winston‐Lutz test, film‐based “star shots,” and the vendor provided machine performance check (MPC). The ability to detect alignment errors was demonstrated by repeating the test after applying a 0.5 mm shift to the MLCs in the direction of leaf travel. In addition to radiation isocenter and coincidence with CBCT origin, the analysis also calculated the actual gantry and couch angles per beam. Results Setup, MV irradiation, and CBCT readout were carried out within 38 min. After subtracting the background signal from the pre‐CBCT, the CNR of the dosimeter signal from the irradiation with the MLCs (125 kVp, 1035 mAs, n = 3), 7.5 mm cone (125 kVp, 1035 mAs, n = 3), and 4 mm cone (80 kVp, 4050 mAs, n = 1) was 5.4, 5.9, and 2.9, respectively. The minimum radius that encompassed all beams calculated using the automated analysis was 0.38, 0.48, and 0.44 mm for the MLCs, 7.5 mm cone, and 4 mm cone, respectively. When determined manually, these values were slightly decreased at 0.28, 0.41, and 0.40 mm. For comparison, traditional Winston‐Lutz test with MLCs and MPC measured the 3D isocenter radius to be 0.24 mm. Lastly, when a 0.5 mm shift to the MLCs was applied, the smallest radius that intersected all beams increased from 0.38 to 0.90 mm. The mean difference from expected value for gantry angle was 0.19 ± 0.29°, 0.17 ± 0.23°, and 0.12 ± 0.14° for the MLCs, 7.5 mm cone, and 4 mm cone, respectively. The mean difference from expected for couch angle was −0.07 ± 0.28°, −0.08 ± 0.66°, and 0.04 ± 0.25°. Conclusions This work demonstrated the feasibility of a comprehensive isocenter verification using a NIPAM dosimeter with sub‐mm accuracy which incorporates evaluation of coincidence with imaging coordin...

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We report our progress towards developing a clinical application of NIPAM kV-CBCT dosimetry. The goal is to develop a practical kV-MV isocenter verification test for which the measurement and analysis can be carried out quickly (within an hour), and that eliminates the need for separate readout (other than on board kV-CBCT) or extra analysis steps such as image registration. Isocenter verification is performed using a NIPAM 3D gel dosimeter which is irradiated with a small field to ~16Gy at eight unique couch/gantry angles. Pre- and post-irradiation kV-CBCT images are acquired and dose is manifest as the intensity difference between pre- and post-CBCTs due to radiation induced changes in density. Code was developed to detect the geometry of each beam in the kV-CBCT and quantify relevant parameters. We applied this technique to verify the isocenter for MLCs as well as for SRS cones. The measured radius to encompass all beams for 4mm, 6mm, 7.5mm, 12.5mm, and 15mm cones was 0.55±0.11mm. The efficiency, robustness to setup errors, and unique ability to visualize spatial uncertainties in the kV-CBCT coordinate system make the NIPAM kV-CBCT test a practical and unique tool for kV-MV isocenter verification.

the prostate, esophagus, lung and liver. A large number of pediatric patients with central nervous system (CNS) tumors also benefit from PBT (Gondi et al., 2016). Protons interact with matter in three different ways: interactions with atomic electrons, interactions with the atomic nucleus, and interactions with the atom as a whole (Verhey et al., 1998). Protons that interact with the nucleus may produce Bremsstrahlung radiation, but this occurs so infrequently that its effects are negligible. There is also the possibility that protons will collide with an atom and produce secondary protons, neutrons, or excited nuclei, although these interactions are also rare. Protons primarily lose kinetic energy as they traverse matter via inelastic Coulombic interactions with atomic orbital electrons, which also deflect the proton trajectory (Newhauser and Zhang, 2015). The deflection due to a single interaction is generally quite small as the mass of a proton is much larger than that of an electron. However, the cumulative effect of many such interactions can be significant. The most complete theory of multiple Coulombic scattering was proposed by Molière (1947). Many simplifications of this theory have been proposed, although this simplicity often reduces the accuracy in modeling Coulombic scattering at large angles. Gottschalk et al. (1993) approximated Molière's theory to take the form of a Gaussian function, assuming the small angle approximation in which sin(θ)≈θ: (1)