Establishment of local diagnostic reference levels for planning CT in adult and pediatric radiotherapy patients in 18 Japanese university hospitals.

Planning computed tomography (CT) for radiotherapy is essential for accurate treatment delivery but also contributes to patient radiation exposure. Therefore, diagnostic reference levels (DRLs) are recommended to optimize imaging doses. However, national DRLs (NDRLs) for planning CT in Japan have only recently been introduced. The aim of this study was to establish local diagnostic reference levels (LDRLs) for planning CT scans at 18 Japanese university hospitals for both adult and pediatric anatomical patients. Data were retrospectively collected from 2008 to 2022 for six adult anatomical regions and three pediatric regions. CT dose indices (volume CT dose index [CTDIvol] and dose length product [DLP]) were analyzed, and the 50th and 75th percentiles were calculated as the achievable and reference levels, respectively. In adults, the 75th percentile CTDIvol values were 86, 34, 24, 26, 24, and 38 mGy for the head, head and neck, thorax, upper abdomen, pelvis, and female pelvis, respectively. These values are generally higher than DRLs in several other countries but comparable to Japan’s DRLs in 2025. For pediatric head scans, the CTDIvol ranged from 59 to 74 mGy (75th percentile) across the age groups. The corresponding DLP ranged from 1469 to 2175 mGy·cm. The 75th percentiles of the CTDIvol were 25 and 18 mGy for the upper abdomen and whole body, respectively. This is the first multicenter study in Japan to propose LDRLs for planning CT scans, including pediatric cases. These findings support national dose optimization initiatives and provide a foundation for the establishment of NDRLs for radiotherapy.