[Reduction of Lens Absorbed Dose through Optimization of the X-ray Tube Position during Localizer Radiograph in Radiation Therapy Planning CT].

Purpose: The International Commission on Radiological Protection Publication 118 reduced the threshold dose for cataract formation to 0.5 Gy and requires dose optimization in computed tomography (CT) for radiation therapy planning. This study investigated the operational characteristics of the CT automatic exposure control (CT-AEC) system using a carbon fiber couch top and head immobilization devices, as well as the X-ray tube position during the localizer radiograph orientation (hereafter referred to as the localizer orientation). It also influenced the lens absorbed dose and reduced the lens absorbed dose through optimization of the localizer orientation.

Methods: Measured the image noise and the lens absorbed dose based on the localizer orientation. The reference localizer orientation was set to the right-lateral to left-lateral (RL) orientation, and the image quality setting value (noise index: NI) of the CT-AEC was set to 6, which corresponds to the predefined threshold.

Results: The average standard deviation in the anterior-posterior (AP) orientation was 4.1±0.1, which was larger than the 3.9±0.1 for the RL orientation (P<0.05). The absorbed dose to the lens absorbed dose was 68.5±1.7 mGy for the AP orientation and 73.3±2.3 mGy for the reference RL orientation, representing an 8% reduction (P<0.01).

Conclusion: This study suggests that imaging in the AP orientation potentially reduces the lens absorbed dose.