Imaging dose in image-guided radiotherapy for localized prostate intensity-modulated radiotherapy: a nationwide survey in Japan

This study aimed to establish the diagnostic reference levels (DRLs) of imaging doses for image-guided radiotherapy (IGRT) used in intensity-modulated radiotherapy for prostate cancer in Japan. A nationwide survey was conducted to gather data on image acquisition conditions, parameters, and frequencies across 193 radiation therapy institutions using intensity-modulated radiotherapy. IGRT modalities, such as kilovoltage and megavoltage cone-beam computed tomography (CBCT), two-dimensional imaging, and in-room computed tomography (CT), were targeted. Data analysis focused on image acquisition parameters displayed by the devices, such as tube voltage, current, and imaging dose, along with the CT dose index volume (CTDIvol) and dose-length product (DLP), were collected from 222 radiotherapy devices. The results showed that kV-CT/CBCT was the most frequently used modality, used in 94% of the institutions. Imaging dose-reduction techniques were adopted by over half of the institutions, with 56% optimizing imaging parameters and 45% reducing the imaging field size or scan length. The 75th percentile for CTDIvol was 16.0 mGy, while that for DLP was 263 mGy·cm, with considerable variation among devices and institutions. This study provides the first large-scale reference data for IGRT imaging doses used for prostate cancer treatment in Japan. These results are critical for improving patient safety by optimizing imaging protocols and establishing DRLs tailored to IGRT. These findings will serve as a basis for further refinement of radiological protection practices in Japan.