清水 秀年

PURPOSE: In linear accelerators, deviations in the x-ray focal spot position significantly affect the accuracy of radiation therapy. However, as the focal spot position in bore-type linac systems such as the Radixact system, cannot be assessed using conventional methods, a new evaluation method is required. This study aimed to develop a novel method to measure the focal spot position of Radixact and evaluate any deviations from the ideal x-ray focal spot position. METHODS: A structurally simplified measurement system was developed to evaluate the focal spot position of the Radixact system. This system consisted of a vertically aligned metal bar and an ionization chamber, which was moved stepwise to acquire the beam profiles. The focal spot position deviation was calculated based on the center differences of the profiles obtained from two different upstream and downstream locations of the metal bar. RESULTS: The measurement results indicated that the focal spot position shift was 0.42 mm and -0.36 mm at the target height in the IEC-X and -Y directions, respectively. The measurement uncertainty was 0.187 mm, confirming a slight deviation from the ideal focal position. CONCLUSIONS: This study developed a novel method to accurately evaluate the x-ray focal spot position of the Radixact system, which can potentially be applied to other conventional linear accelerators and bore-type systems, such as Halcyon, to improve the accuracy of radiotherapy. However, its generalizability and applicability to different radiotherapy machines must be explored further.

Abstract Background Accurate dosimetry is important in radiotherapy, and all equipment used for radiotherapy shoud be audited by an independent external dose audit. Radiophotoluminescence glass dosimeter (RPLD) has excellent characteristics and is widely used for postal dose audit; however, postal dose audit for proton therapy using RPLD has not been established. Purpose This study aims to develop a postal dose audit procedure for scanning proton beams using RPLD, estimate uncertainties, and conduct a multicenter pilot study to validate the methodology. Methods A postal toolkit was developed and a postal dose audit procedure for RPLD measurements of scanning proton beams was established in cooperation with several facilities that employ various accelerators, irradiation equipment, and treatment planning systems (TPS) for clinical use. Based on basic and previous studies, an uncertainty budget was developed for estimating relative uncertainty and pilot studies were conducted at each site. A method for postal dose audits was developed in a multicenter collaboration to develop an approach suitable for implementation across multiple facilities. Results The relative response of 60 RPLDs for scanning proton beam examined in this study was 1.00 ± 1.28% mean ± standard deviation. The combined relative standard uncertainty of postal dosimetry for scanning proton beams using the RPLD was 2.97% (k = 1). Under the reference condition, the maximum differences between the ionization chamber measurement (IC) and TPS, RPLD and TPS, and RPLD and IC were 0.97, 1.88, and 2.12%, respectively. The maximum differences between the RPLD and ionization chamber for plateau measurements at 3 cm depth using single‐energy and non‐reference conditions were 11.31 and 4.02%, respectively. Conclusion We established a procedure for the postal dose audits of proton beams using RPLD and presented the results of a multicenter pilot study. By standardizing the reference conditions, the dosimetry uncertainty was estimated at 2.92%. The results demonstrated the feasibility of performing an independent third‐party dose audit of scanning proton beams using RPLD, and for such postal dose audits for proton beams, the irradiation conditions should be standardized to reduce uncertainties. These results are expected to contribute to the development of proton beams.

Abstract This study aimed to evaluate the recent trends in single-fraction conventional radiotherapy (CRT) as palliative treatment in Japan, using data from the National Database published by the Ministry of Health, Labor, and Welfare. Data from fiscal year (FY) 2014 to FY2022, specifically related to the utilization of single-fraction CRT, were analyzed. Multi-fraction CRT, stereotactic body radiotherapy (SBRT), intensity-modulated radiotherapy (IMRT), and brachytherapy were excluded. The primary outcome was the cumulative and annual number of single-fraction CRT courses. Additionally, quarterly course data from FY2019 to FY2022, the period for which monthly data were available, were assessed to evaluate the impact of the coronavirus disease 2019 (COVID-19) pandemic on single-fraction CRT utilization. Of the total 2 315 607 radiotherapy courses, we identified 33 221 single-fraction CRT courses after excluding multi-fraction CRT (n = 1 835 650), SBRT (n = 33 935), IMRT (n = 332 827), and brachytherapy (n = 113 195). The annual number of single-fraction CRT courses increased from 1730 in FY2014 to 5642 in FY2022, with an average annual growth rate of 0.28 (range: −0.07 to 0.65). Outpatient courses significantly increased, particularly from FY2019 onward, surpassing inpatient courses in FY2022 (2914 vs 2728). The highest annual increase was observed in FY2020, particularly from April to December, although this upward trend did not persist in 2021. In conclusion, single-fraction CRT has exhibited a consistent upward trend, highlighting its expanding role in palliative radiotherapy. Although the COVID-19 pandemic temporarily accelerated this trend, its impact has already subsided, with growth rates returning to pre-pandemic levels.

PURPOSE: A novel classification system has been proposed to stratify patients undergoing palliative radiation therapy based on their pain response and time to progression. This study used prospective observational data to quantify quality of life (QoL) changes associated with pain response and the classification system. METHODS AND MATERIALS: Between August 2021 and September 2022, 366 painful lesions with a numerical rating scale of 2 or more from the 261 eligible patients underwent palliative radiation therapy. Patients were followed-up prospectively at 2, 4, 12, 24, 36, and 52 weeks postradiation therapy, with EORTC QLQ-C15-PAL and QLQ-BM22 questionnaires obtained simultaneously with pain response assessments. The primary endpoint was defined as the global health status/QoL improvements at 12 weeks based on minimally clinically important differences and compared by the pain response (responders vs. nonresponders) and by class 1 (no opioids, no reirradiation, n = 89), 2 (neither class 1 nor 3, n = 211), and 3 (opioids and reirradiation, n = 66). RESULTS: With a median follow-up time of 21 weeks for pain response and 13 weeks for QoL assessment, 1773 pairs of QLQ-C15-PAL and QLQ-BM22 questionnaires were collected. The QoL assessment at baseline was covered with 97% (355/366) of lesions and 67% (183/273) at 12 weeks: this compliance was lower in nonresponders than in responders (57% vs. 72%, P = .004) and highest in class 1, followed by classes 2 and 3 (70% vs. 44% vs. 39%, P = .001). The improvement rate was significantly different by class, with class 3 having the lowest in all subscales except nausea and psychosocial aspects: the improvement rate of global health status/QoL was 33% in class 1, 31% in class 2, and 20% in class 3, P = .001). CONCLUSIONS: The QoL changes associated with pain response and the classification system were identified, suggesting that the classification system may help identify populations more or less likely to improve QoL, in addition to separating pain response rates.

PURPOSE: Total body irradiation before bone marrow transplantation for hematological malignancies using Radixact, a high-precision radiotherapy machine, can potentially reduce side effects and the risk of secondary malignancies. However, stable control of couch speed is critical, and direct assessment methods outlined in quality assurance guidelines are lacking. This study aims to develop a real-time couch speed verification system for the Radixact. METHODS: The developed system used a linear encoder to measure couch speed directly. Accuracy was verified via a linear stage, comparing measurements with a laser distance sensor. After placing a phantom simulating the human body on the Radixact couch, the couch speed was verified using predefined speed plans. RESULTS: Operating the linear stage at 0.1, 0.5, and 1.0 mm/s revealed that the maximum position error of the developed verification system compared to the laser distance sensor was nearly equivalent to the distance resolution of the system (0.05 mm/pulse), with negligible average speed error. When the Radixact couch operated at 0.1, 0.5, and 1.0 mm/s, the values obtained by the verification system agreed with the theoretical values within the sampling period (0.01 s) and distance resolution (0.05 mm). CONCLUSION: The verification system developed provides real-time monitoring of the speed of the Radixact table, ensuring treatment effectiveness and patient safety. It would guarantee the couch speed's soundness and contribute to the "visualization" of safety.