Tominaga Yuki, Wakisaka Yushi, Kato Takahiro, Ichihara Masaya, Yasui Keisuke, Sasaki Motoharu, Oita Masataka, Nishio Teiji
Department of Radiotherapy, Medical Co. Hakuhokai, Osaka Proton Therapy Clinic, Osaka, Osaka, Japan.
Medical Physics Laboratory, Division of Health Science, Graduate School of Medicine, The University of Osaka, Suita-Shi, Osaka, Japan.
Med Phys. 2025 Aug;52(8):e18026. doi: 10.1002/mp.18026.
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.
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.
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.
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.
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.
由于质子笔形束扫描（PBS）对移动靶区存在相互作用效应，易导致剂量退化。对于无法接受的运动情况，呼吸门控（RG）照射是自由呼吸（FB）照射的有效替代方法。然而，较大门宽（GW）的RG照射因相互作用效应而受到阻碍，这种效应与FB照射中观察到的类似。通过记录束斑时间戳可准确估计相互作用效应。但我们的机器缺乏此功能，因此必须找到替代方法。于是，我们开发了一种无束斑时间戳的RG四维动态剂量（RG-4DDD）系统。
本研究旨在探讨RG-4DDD系统针对不同呼吸曲线、幅度和周期（10%-50%GW）的PBS计划所计算剂量的准确性。
使用自行开发的软件重建RG-4DDD，该软件为单个束斑分配时间戳，将射野开始时间与呼吸曲线整合，并利用可变形配准进行剂量累积。使用非均匀体模创建三个立方验证计划。此外，采用典型肝脏和肺部病例进行患者计划验证。在均匀体模中为肝脏和肺部病例创建单野和多野优化（SFO和IMPT）计划（共十条射束）。在CIRS动态平台上，使用二维电离室阵列（2D阵列）和EBT3 Gafchromic胶片在运动和静止条件下获取横向剂量分布测量值。立方计划的呼吸曲线用于评估九种正弦曲线模式，幅度为5.0-10.0毫米（靶区运动10.0-20.0毫米），周期为3-6秒。使用平均幅度为6.4毫米、周期为3.2秒的代表性患者曲线进行患者射野验证。假设剂量率（DR）、束斑射野（0.08秒）和门控时间延迟（0.1秒）的分配时间戳有±10%的变化，进行额外模拟以评估参数选择对我们的4DDD模型的影响。使用二维伽马指数以及超过靶区95%剂量所需的绝对剂量，将4DDD与测量值进行比较。
二维阵列测量显示，在3%/3毫米条件下，所有GW的参考（无运动）计划和4DDD计划的平均伽马分数分别至少为99.9±0.2%和98.2±2.4%。在2%/2毫米条件下，立方计划和患者计划的胶片测量中4DDD的伽马分数分别超过95.4%和92.9%。在DR变化±10%以及射野和门控时间延迟±0.18秒的情况下，4DDD计算结果可接受。对于所有测量点使用所有GW的4DDD计划，所有验证计划的绝对点差异在99.1%的点上 within±5.0%。
非均匀和实际患者计划的RG-4DDD计算（GW小于50%）与上述幅度和周期内各种呼吸曲线的测量结果显示出良好的一致性。所提出的系统使我们能够在无需记录束斑时间戳的情况下评估实际RG照射。
