清水 秀年

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.

PURPOSE: 177Lu-DOTATATE-targeted radionuclide therapy (TRT) is effective for patients with somatostatin receptor (SSTR)-positive neuroendocrine tumors; however, radiation safety regulations often necessitate hospitalization, particularly in countries with stringent discharge criteria. This study aimed to identify pretreatment factors predicting outpatient eligibility. METHODS: We retrospectively analyzed 26 patients who underwent their first cycle of 177Lu-DOTATATE TRT with complete data for analysis. The external dose rate at 1 m (EDR-1 m) was measured 6 h after administration. Patients were divided into two groups: EDR-1 m greater than or equal to 18 μSv/h and less than 18 μSv/h. Characteristics, including age, sex, BMI, body surface area, estimated glomerular filtration rate, administered dose, and tumor site, were compared. In addition, the whole-body washout rate from pretreatment SSTR imaging was evaluated as a potential predictor. Logistic regression and receiver operating characteristic (ROC) analyses were conducted. RESULTS: Fourteen of the 26 (53.8%) patients met the discharge criterion at 6 h. No significant differences were observed in demographic or clinical characteristics between groups. The median washout rate was significantly higher in those meeting the criterion (57.6 vs. 35.0%; P < 0.001). The area under the ROC curve for the washout rate was 0.929, indicating excellent predictive ability. An optimal cut-off value of 53.5% predicted same-day discharge with a sensitivity of 92.9% and specificity of 91.7%. CONCLUSION: The whole-body washout rate derived from pretreatment SSTR imaging is a strong, practical predictor for outpatient eligibility following 177Lu-DOTATATE TRT. Incorporating this simple, noninvasive marker into clinical workflow could support individualized discharge planning and improve patient access under strict radiation safety regulations.

To quantify radiation dose reduction in radiotherapy treatment-planning CT (RTCT) using a deep learning-based reconstruction (DLR; AiCE) algorithm compared with adaptive iterative dose reduction (IR; AIDR). To evaluate its potential to inform RTCT-specific diagnostic reference levels (DRLs). In this single-institution retrospective study, 4-part RTCT scans (head, head and neck, lung, and pelvis) were acquired on a large-bore CT. Scans reconstructed with IR (n = 820) and DLR (n = 854) were compared. The 75th-percentile CTDIvol and DLP (CTDIIR, DLPIR vs. CTDIDLR, DLPDLR) were determined per site. Dose reduction rates were calculated as (CTDIDLR - CTDIIR)/CTDIIR × 100% and similarly for DLP. Statistical significance was assessed by the Mann-Whitney U-test. DLR yielded CTDIvol reductions of 30.4-75.4% and DLP reductions of 23.1-73.5% across sites (p < 0.001), with the greatest reductions in head and neck RTCT (CTDIvol: 75.4%; DLP: 73.5%). Variability also narrowed. Compared with published national DRLs, DLR achieved 34.8 mGy and 18.8 mGy lower CTDIvol for head and neck versus UK-DRLs and Japanese multi-institutional data, respectively. DLR substantially lowers RTCT dose indices, providing quantitative data to guide RTCT-specific DRLs and optimize clinical workflows.

This study evaluates current practices and challenges associated with computed tomography number-to-mass density (CT-MD) conversion tables in helical tomotherapy across Japan and explores directions for standardization and quality improvement amid the increasing adoption of adaptive radiotherapy (ART). A nationwide web-based survey was conducted across 34 institutions utilizing the Radixact system. Data were collected on CT acquisition protocols, calibration phantoms, density plugs, reconstruction algorithms, table registration timing and quality assurance (QA) frequency. Registered CT-MD tables were categorized by CT modality: Simulation CT (SimCT), ClearRT and CTrue. ClearRT tables were analyzed by phantom setup (full vs half), and CTrue tables by reconstruction method [filtered back projection (FPB) vs iterative reconstruction (IR)]. Inter-institutional variations in CT numbers and the number of data points were assessed. SimCT tables exhibited the widest variation in the number of data points (median = 10) and high-density CT numbers. ClearRT tables (median = 8) showed variations of up to 300 Hounsfield units (HU) in cortical bone; the half-phantom setup reduced inter-institutional variability. CTrue tables (median = 8) demonstrated high consistency, with negligible differences between IR and FPB. All plug CT numbers of CTrue remained within the tolerance defined by the American Association of Physicists in Medicine Task Group 148. However, CT numbers for air plugs varied by ~±30 HU, indicating inconsistent handling of air reference values. Additionally, 43% of institutions did not perform routine QA. Standardizing phantom geometry, air CT number handling and QA protocols-particularly using half-phantom calibration-may improve CT-MD table consistency and dose accuracy in ART.

The patient setup using the surface-guided radiation therapy (SGRT) system differs from conventional surface marker procedures. Owing to the abundance of three-dimensional information, there may be operator variability in where to focus during the patient setup. This study aimed to clarify the differences between expert and novice operators in SGRT positioning for head and neck cases by tracking their eye movements, thereby providing data for developing efficient patient setup procedures. Six radiation therapists set up a simulated patient on the SGRT system while recording eye movements on the screen using the QG-PLUS eye-tracking system. The positioning time and number of gaze fixations on the screen were analysed, and the relationship between years of experience with SGRT, positioning time and number of gaze fixations was evaluated. No significant correlation was found between SGRT experience and positioning time (r = -0.67, p = 0.15). However, more experienced radiation therapists exhibited fewer gaze fixations per positioning session (r = -0.81, p < 0.05), indicating that they efficiently identified key positioning points. Additionally, experienced radiation therapists focused more intently on a specific screen during the latter half of positioning, suggesting a refined approach for final patient alignment verification. More experienced radiation therapists showed fewer gaze fixations and demonstrated increased attention to a specific screen during the latter half of the patient setup process, suggesting that eye-tracking technology may provide useful data for standardising patient setup procedures in SGRT patient setups.