安井 啓祐

ABSTRACT 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: 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.

BACKGROUND: Proton pencil beam scanning (PBS) is susceptible to dose degradation because of interplay effects on moving targets. For cases of unacceptable motion, respiratory-gated (RG) irradiation is an effective alternative to free breathing (FB) irradiation. However, the introduction of RG irradiation with larger gate widths (GW) is hindered by interplay effects, which are analogous to those observed with FB irradiation. Accurate estimation of interplay effects can be performed by recording spot timestamps. However, our machine lacks this feature, making it imperative to find an alternative approach. Thus, we developed an RG 4-dimensional dynamic dose (RG-4DDD) system without spot timestamps. PURPOSE: This study aimed to investigate the accuracy of calculated doses from the RG-4DDD system for PBS plans with varying breathing curves, amplitudes, and periods for 10%-50% GW. METHODS: RG-4DDDs were reconstructed using in-house developed software that assigned timestamps to individual spots, integrated start times for spills with breathing curves, and utilized deformable registrations for dose accumulation. Three cubic verification plans were created using a heterogeneous phantom. Additionally, typical liver and lung cases were employed for patient plan validation. Single- and multi-field-optimized (SFO and IMPT) plans (ten beams in total) were created for the liver and lung cases in a homogeneous phantom. Lateral profile measurements were obtained under both motion and no-motion conditions using a 2D ionization chamber array (2D-array) and EBT3 Gafchromic films on the CIRS dynamic platform. Breathing curves from the cubic plans were used to assess nine patterns of sine curves, with amplitudes of 5.0-10.0 mm (10.0-20.0 mm target motions) and periods of 3-6 sec. Patient field verifications were conducted using a representative patient curve with an average amplitude of 6.4 mm and period of 3.2 sec. Additional simulations were performed assuming a ± 10% change in assigned timestamps for the dose rate (DR), spot spill (0.08-s), and gate time delay (0.1-s) to evaluate the effect of parameter selection on our 4DDD models. The 4DDDs were compared with measured values using the 2D gamma index and absolute doses over that required for dosing 95% of the target. RESULTS: The 2D-array measurements showed that average gamma scores for the reference (no motion) and 4DDD plans for all GWs were at least 99.9 ± 0.2% and 98.2 ± 2.4% at 3%/3 mm, respectively. The gamma scores of the 4DDDs in film measurements exceeded 95.4% and 92.9% at 2%/2 mm for the cubic and patient plans, respectively. The 4DDD calculations were acceptable under DR changes of ±10% and both spill and gate time delays of ±0.18 sec. For the 4DDD plan using all GWs for all measurement points, the absolute point differences for all validation plans were within ±5.0% for 99.1% of the points. CONCLUSIONS: The RG-4DDD calculations (less than 50% GW) of the heterogeneous and actual patient plans showed good agreement with measurements for various breathing curves in the amplitudes and periods described above. The proposed system allows us to evaluate actual RG irradiation without requiring the ability to record spot timestamps.