On the measurement of radiation isocenter for medical accelerators using 3D polymer gel dosimetry. Introduction, application, and good practices

This work reviews the issue of determining the radiation isocenter for quality assurance tests of medical accelerators. High resolution 3D polymer gel dosimetry is employed, which is composed of a polymer gel dosimeter, iterative cone-beam computed tomography and a dedicated software package. Among the approaches for the determination of radiation isocenter, the centroid does not always produce a good result and a gradientbased optimization method is the slowest, however produces the same results as an analytical solution for the smallest intersecting circle. A protocol of using the 3D dosimetry for the radiation isocenter determination is presented, which includes the preparation and handling of the 3D dosimeter, star-shot irradiation with 10,000 MU per each of four beams of a medical accelerator following by immediate 3D scanning (Pelvis mode) and data processing (averaging signal, background subtraction). Uncertainty budget points to 1.3 mm spatial uncertainty. Potential problems when determining the radiation isocenter are discussed.