Yoshihito Aikawa

A new irradiation unit constructed of self-moving gantry-CT and linac.

PURPOSE To improve reproducibility in stereotactic irradiation (STI) without using noninvasive immobilization devices or body frames, we have developed an integrated computed tomography (CT)-linac irradiation system connecting CT scanner and linac via a common treatment couch. METHODS AND MATERIALS This system consists of a linac, a CT scanner, and a common treatment couch. The linac and the CT gantry are positioned on opposite ends of the couch so that, by rotating the treatment couch, linac radiotherapy or CT scanning can be performed. The rotational axis of the linac gantry is coaxial with that of the CT gantry, and the position of the linac isocenter on the couch matches the origin of the coordinate system for CT scanning when the couch is rotated 180 degrees toward the CT side. Instead of the couch moving into the gantry, as in conventional CT, in this case the table is fixed and scanning is accomplished by moving the gantry. We evaluated the rotational accuracy of the common couch and the scan-position accuracy of the self-moving gantry CT. RESULTS The positional accuracy of the common couch was 0.20, 0.18, and 0.39 mm in the lateral, longitudinal, and vertical directions, respectively. The scan-position accuracy of the CT gantry was less than 0.4 mm in the lateral, longitudinal, and vertical directions. CONCLUSION This irradiation system has a high accuracy and is useful for noninvasive STI and for verification of the position of a target in three-dimensional conformal radiotherapy.

Determination of a standard deviation that could minimize radiation exposure in an automatic exposure control for pulmonary thin-section computed tomography

PurposeThe aim of this study was to determine a standard deviation (SD) that most reduces the radiation dose without sacrificing the diagnostic accuracy of thinsection computed tomography (CT) for clinical use.Materials and methodsA total of 120 patients were examined by multidetector CT. They were assigned to one of four SD groups: 8, 9, 11, and 12. Each SD group consisted of 30 patients. The CT images of the same patients with SD10 that had formerly been examined were used for comparison. Two radiologists independently evaluated the degrees of image noise and diagnostic acceptability of the pulmonary diseases using a point score grading system. We compared the scores between each SD and the SD10 group.ResultsGenerally, image noise was significantly more prominent in the higher-SD groups. The mean score of diagnostic acceptability was significantly lower in the SD12 group (4.2 ± 1.6) than in the SD10 group (4.6 ± 1.1) group (P < 0.001), whereas no difference was present between the SD8 (4.9 ± 0.7), SD9 (4.8 ± 1.0), and SD11 (4.4 ± 1.5) groups and the SD10 group (4.7 ± 1.1, 4.6 ± 1.4, 4.6 ± 1.1, respectively).ConclusionThin-section CT with SD12 is not acceptable. SD11 seems to be the setting with the lowest radiation dose while providing acceptable imaging quality for pulmonary thin-section CT.