Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University, Okayama, Japan.
Osaka Proton Therapy Clinic, Department of Radiotherapy, Medical Co. Hakuhokai, Osaka, Osaka, Japan.
Med Phys. 2024 Jan;51(1):566-578. doi: 10.1002/mp.16725. Epub 2023 Sep 6.
We developed a 4-dimensional dynamic dose (4DDD) calculation model for proton pencil beam scanning (PBS). This model incorporates the spill start time for all energies and uses the remaining irradiated spot time model instead of irradiated spot time logs. This study aimed to validate the calculation accuracy of a log file-based 4DDD model by comparing it with dose measurements performed under free-breathing conditions, thereby serving as an alternative approach to the conventional log file-based system.
Three cubic verification plans were created using a heterogeneous block phantom; these plans were created using 10 phase 4D-CT datasets of the phantom. The CIRS dynamic platform was used to simulate motion with amplitudes of 2.5, 3.75, and 5.0 mm. These plans consisted of eight- and two-layered rescanning techniques. The lateral profiles were measured using a 2D ionization chamber array (2D-array) and EBT3 Gafchromic films at four starting phases, including three sinusoidal curves (periods of 3, 4, and 6 s) and a representative patient curve during actual treatment. 4DDDs were calculated using in-house scripting that assigned a time stamp to each spot and performed dose accumulation using deformable image registration. Furthermore, to evaluate the impact of parameter selection on our 4DDD model calculations, simulations were performed assuming a ±10% change in irradiation time stamp (0.8 ± 0.08 s) and spot scan speed. We evaluated the 2D gamma index and the absolute point doses between the calculated values and the measurements.
The 2D-array measurements revealed that the gamma scores for the static plans (no motion) and 4DDD plans exceeded 97.5% and 93.9% at 3%/3 mm, respectively. The average gamma score of the 4DDD plans was at least 96.1%. When using EBT3 films, the gamma scores of the 4DDD model exceeded 92.4% and 98.7% at 2%/2 mm and 3%/3 mm, respectively. Regarding the 4DDD point dose differences, more than 95% of the dose regions exhibited discrepancies within ±5.0% for 97.7% of the total points across all plans. The spot time assignment accuracy of our 4DDD model was acceptable even with ±10% sensitivity. However, the accuracy of the scan speed, when varied within ±10% sensitivity, was not acceptable (minimum gamma scores of 82.6% and maximum dose difference of 12.9%).
Our 4DDD calculations under free-breathing conditions using amplitudes of less than 5.0 mm were in good agreement with the measurements regardless of the starting phases, breathing curve patterns (between 3 and 6 s periods), and varying numbers of layered rescanning. The proposed system allows us to evaluate actual irradiated doses in various breathing periods, amplitudes, and starting phases, even on PBS machines without the ability to record spot logs.
我们开发了一种用于质子铅笔束扫描（PBS）的四维动态剂量（4DDD）计算模型。该模型纳入了所有能量的起始射野时间，并使用剩余照射野时间模型，而不是照射野时间日志。本研究旨在通过与自由呼吸条件下的剂量测量值进行比较，验证基于日志文件的 4DDD 模型的计算准确性，从而为传统的基于日志文件的系统提供替代方法。
使用不均匀块状体模创建了三个立方验证计划；这些计划是使用该体模的 10 个相位 4D-CT 数据集创建的。使用 CIRS 动态平台模拟了 2.5、3.75 和 5.0mm 三种幅度的运动。这些计划包括八层和两层重扫技术。使用二维电离室阵列（2D-array）和 EBT3 Gafchromic 胶片在四个起始相位测量横向轮廓，其中包括三个正弦曲线（周期为 3、4 和 6s）和实际治疗期间的代表性患者曲线。使用内部脚本为每个照射野分配时间戳，并使用可变形图像配准进行剂量累积，从而计算 4DDD。此外，为了评估参数选择对我们的 4DDD 模型计算的影响，模拟了照射野时间戳（0.8±0.08s）和照射野扫描速度的±10%变化。我们评估了计算值与测量值之间的 2D 伽马指数和绝对点剂量。
2D-array 测量结果表明，静态计划（无运动）和 4DDD 计划的伽马评分在 3%/3mm 时分别超过 97.5%和 93.9%。4DDD 计划的平均伽马评分至少为 96.1%。使用 EBT3 胶片时，4DDD 模型的伽马评分在 2%/2mm 和 3%/3mm 时分别超过 92.4%和 98.7%。关于 4DDD 点剂量差异，在所有计划中，超过 95%的剂量区域的差异在±5.0%以内，占总点数的 97.7%。即使敏感性变化±10%，我们的 4DDD 模型的照射野时间分配准确性也可以接受。然而，当扫描速度变化±10%时，准确性则无法接受（最小伽马评分 82.6%，最大剂量差异 12.9%）。
在 5.0mm 以下的振幅下，我们在自由呼吸条件下进行的 4DDD 计算与测量值吻合良好，无论起始相位、呼吸曲线模式（3 至 6s 周期）和重扫层数如何变化。该系统允许我们在各种呼吸周期、振幅和起始相位下评估实际照射剂量，即使在没有记录照射野日志功能的 PBS 机器上也可以进行。
