清水 秀年

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.

BACKGROUND: Despite advances in treatment for unresectable locally advanced non-small cell lung cancer (LA-NSCLC), overall survival (OS) remains poor. The effects of coronary artery calcification (CAC) and heart radiation doses on OS in LA-NSCLC patients, especially their combined impact, have not been thoroughly investigated. This study aimed to examine the individual and combined effects of CAC and heart dose on OS in LA-NSCLC patients treated with radiotherapy over a 3-year follow-up period. PATIENTS AND METHODS: The study included 140 patients who received definitive radiotherapy for LA-NSCLC (stage III, 92.1%) from 2015 to 2021. The endpoint was OS, with each patient followed for a fixed 3-year period. RESULTS: Univariate Cox regression analysis identified mean heart dose (MHD; hazard ratio [HR], 4.0 [2.2-7.3]; P < .001) and CAC in multiple vessels (HR, 2.6 [1.5-4.8]; P = .001) as significant predictors of worse OS, both serving as independent predictors of poorer outcomes in multivariate analysis. Kaplan-Meier analysis revealed that combining MHD and CAC in any coronary artery, each specific artery, and multivessels provided enhanced risk stratification for OS (P < .001 for all combinations). Among patients with higher MHD, those with calcification in the left main trunk (LMT) had the highest annual event rate (28.2%), showing a significant difference (P < .001) compared to patients with lower MHD (4.4%). CONCLUSION: Combination of CAC and heart dose enhanced risk stratification for 3-year OS in LA-NSCLC patients treated with radiotherapy. Importantly, patients with calcification in the LMT derived the greatest benefit from reducing heart doses.

BACKGROUND: Accurate absolute dosimetry is essential for achieving high-precision proton beam therapy. Consequently, a comprehensive characterization of the ionization chamber's response properties is necessary. PURPOSE: This study aimed to evaluate the average f Q using Monte Carlo (MC) code PHITS to assess uncertainties among different MC simulation tools. Additionally, P Q values for PTW 30013, NACP-02, and PTW 31013 ionization chambers are calculated using PHITS to provide new reference data for P Q . Furthermore, a new k Q factor for PTW 31013 chamber is established using MC method, contributing to advancements in proton beam dosimetry protocols. METHODS: Monoenergetic proton beams were employed to calculate f Q , k Q , and P Q for Farmer, Semiflex, and plane-parallel chambers. The absorbed dose deposited within the sensitive volume of each chamber was determined via simulations employing PHITS, thereby providing the basis for the estimation of these factors. Computed f Q values were compared with previous reports, while k Q and P Q were benchmarked against literature and Technical Reports Series No. 398 (TRS-398) Rev.1 guideline. RESULTS: Incorporating PHITS-derived f Q values reduced the uncertainty of f ¯ Q P H I T S compared to previous findings. The k Q factor for PTW 31013 followed trends observed in cylindrical chambers with varying sensitive volumes; notably, this study represents the first MC estimation of k Q for this chamber. P Q values for values deviated by up to 1.7% from unity. CONCLUSION: The data generated in this study provide important insights for refining proton beam dosimetry, contributing to the improvement of treatment precision.