Performance of a newly designed end-to-end phantom compatible with magnetic resonance-guided radiotherapy systems.

Purpose: In this study, we report on our proposed phantom based on the new end-to-end (E2E) methodology and its results. In addition, we verify whether the proposed phantom can replace conventional phantoms.

Methods: The hexagonal-shaped newly designed phantom has pockets on each side for a film dosimeter of size 80 × 90 mm , which is easily removable, considering the Co penumbra. The new phantom comprises water, shell, and auxiliary shell phantoms. The shell and auxiliary shell materials are Solid Water HE. A mock tumor (aluminum oxide) was attached by a single prop in the water phantom and placed at the center of the new phantom. The results of a conventional E2E test were compared with those of the novel E2E test using the newly designed phantom. The irradiated film dosimeter in the novel E2E test was scanned in a flatbed scanner and analyzed using an in-house software developed with MATLAB. The irradiated field center, laser center, and mock tumor center were calculated. In the novel image-matching E2E (IM-E2E) test, image matching is performed by aligning the laser center with ruled lines. In the novel irradiation-field E2E (IF-E2E) test, the displacement of the irradiation-field center was defined as its distance from the laser center. In the composite E2E test, the overall displacement, which included the accuracy of the irradiated field and image matching, was defined as the distance between the irradiated field center and mock tumor center. In addition, using the newly designed phantom, the overall irradiation accuracy of the machine was evaluated by calculating the three-dimensional (3D) center of the irradiated field, phantom, and laser. The composite E2E test could be performed using the newly designed phantom only.

Results: In the IM-E2E test, the results of the conventional and novel IM-E2E tests were significantly different in each direction (left-right direction: p-value < < 0.05, anterior-posterior direction: p-value = 0.002, and superior-inferior direction: p-value = 0.002). The displacement directions were the same in both the conventional and novel IM-E2E tests. In the analysis of the IF-E2E test, no significant difference was evident between the results in each direction. Moreover, the displacement directions were the same in the conventional and novel IF-E2E tests, except for the left-right lateral direction of head three. In addition, the 3D analysis results of the novel IF-E2E test were less than 1 mm in all directions. In the analysis of the composite E2E test, the maximum displacement was 1.4 mm in all directions. In addition, almost all results of 3D analysis for the composite E2E test were less than 1 mm in all directions.

Conclusion: The newly designed E2E phantom simplifies the E2E test for MRIdian, and is a possible alternative to the conventional E2E test. Furthermore, we can perform the previously unfeasible composite E2E tests that include the entire treatment process.