Development of a high-resolution two-dimensional detector-based dose verification system for tumor-tracking irradiation in the CyberKnife system

We aim to evaluate the basic characteristics of SRS MapCHECK (SRSMC) for CyberKnife (CK) and establish a dose verification system using SRSMC for the tumor-tracking irradiation for CK. The field size and angular dependence of SRSMC were evaluated for basic characterization. The output factors (OPFs) and absolute doses measured by SRSMC were compared with those measured using microDiamond and microchamber detectors and those calculated by the treatment planning system (TPS). The angular dependence was evaluated by comparing the SRSMC with a microchamber. The tumor-tracking dose verification system consists of SRSMC and a moving platform. The doses measured using SRSMC were compared with the doses measured using a microchamber and radiochromic film. The OPFs and absolute doses of SRSMC were within ±3.0% error for almost all field sizes, and the angular dependence was within ±2.0% for all incidence angles. The absolute dose errors between SRSMC and TPS tended to increase when the field size was smaller than 10 mm. The absolute doses of the tumor-tracking irradiation measured using SRSMC and those measured using a microchamber agreed within 1.0%, and the gamma pass rates of SRSMC in comparison with those of the radiochromic film were greater than 95%. The basic characteristics of SRSMC for CK presented acceptable results for clinical use. The results of the tumor-tracking dose verification system realized using SRSMC were equivalent to those of conventional methods, and this system is expected to contribute toward improving the efficiency of quality control in many facilities

Observation of Cerenkov-ring in water during irradiation of high-energy electron or X-ray beam

It is commonly believed that when electron beams with energy of more than MeV scale are irradiated to water, a Cerenkov-ring is formed and observed in the forward direction of the beam. We tried to observe the Cerenkov-ring in water during irradiation of high-energy electrons or X-ray beams. We irradiated 1-cm-diameter electron beams to water with energy of 6 to 12 MeV from a linear accelerator (LINAC), and then we imaged the produced light with a translucent plate in water at the forward position. We also irradiated 0.5 x 0.5-cm 6-MV and 10-MV X-rays to water, and imaging was conducted with the same set-up. For all electron energies, the formed images did not have any ring shape but only a round shape with highest intensity at the center of the circles. Monte Carlo simulation also showed that no ring formed and that the distributions were similar to those of the experimental results. For X-ray irradiation, high intensity spots were observed at the center of the image, and no ring shape was observed. Our results indicate that electrons or X-ray beams do not produce any Cerenkov-ring in the forward direction with the beam energies we used for the experiments. This is because the scattered electrons produced Cerenkov-light not only in the forward direction but also in many directions, which made it impossible to observe the ring shape, in addition to the effect of the water’s luminescence having lower energy than the Cerenkov-light threshold