Department of Clinical Medicine, Aarhus University, Aarhus, Denmark.
Department of Oncology, Aarhus University Hospital, Aarhus N, Denmark.
Med Phys. 2022 May;49(5):3432-3443. doi: 10.1002/mp.15568. Epub 2022 Mar 3.
Electronic brachytherapy (eBT) is considered a safe treatment with good outcomes. However, eBT lacks standardized and independent dose verification, which could impede future use.
To validate the 3D dose-to-water distribution of an electronic brachytherapy (eBT) source using a small-volume plastic scintillation detector (PSD).
The relative dose distribution of a Papillon 50 (P50) (Ariane Medical Systems, UK) eBT source was measured in water with a PSD consisting of a cylindrical scintillating BCF-12 fiber (length: 0.5 mm, Ø: 1 mm) coupled to a photodetector via an optical fiber. The measurements were performed with the PSD mounted on a motorized stage in a water phantom (MP3) (PTW, Germany). This allowed the sensitive volume of the PSD to be moved to predetermined positions relative to the P50 applicator, which pointed vertically downward while just breaching the water surface. The percentage depth-dose (PDD) was measured from 0 to 50 mm source-to-detector distance (SDD) in 1-3 mm steps. Dose profiles were measured along two perpendicular axes at five different SDDs with step sizes down to 0.5 mm. Characterization of the PSD consisted of determining the energy correction through Monte Carlo (MC) simulation and by measuring the stability and dose rate linearity using a well-type ionization chamber as a reference. The measured PDD and profiles were validated with corresponding MC simulations.
The measured and simulated PDD curves agreed within 2% (except at 0 mm and 43 mm depth) after the PSD measurements were corrected for energy dependency. The absorbed dose decreased by a factor of 2 at 7 mm depth and by a factor of 10 at 26 mm depth. The measured dose profiles showed dose gradients at the profile edges of more than 50%/mm at 5 mm depth and 15%/mm at 50 mm depth. The measured profile widths increased 0.66 mm per 1 mm depth, while the simulated profile widths increased 0.74 mm per 1 mm depth. An azimuthal dependency of > 10% was observed in the dose at 10 mm distance from the beam center. The total uncertainty of the measured relative dose is < 2.5% with a positional uncertainty of 0.4 mm. The measurements for a full 3D dose characterization (PDD and profiles) can be carried out within 8 h, the limiting factor being cooling of the P50.
The PSD and MP3 water phantoms provided a method to independently verify the relative 3D dose distribution in water of an eBT source.
电子近距离放射疗法（eBT）被认为是一种安全有效的治疗方法，具有良好的疗效。然而，eBT 缺乏标准化和独立的剂量验证，这可能会阻碍其未来的应用。
使用小体积塑料闪烁探测器（PSD）验证电子近距离放射疗法（eBT）源的三维水剂量分布。
使用由圆柱形闪烁 BCF-12 光纤（长度：0.5mm，Ø：1mm）组成的 PSD ，通过光纤与光电探测器耦合，在水中测量 Papillon 50（P50）（Ariane Medical Systems，英国）eBT 源的相对剂量分布。测量是在 PTW（德国）的水模体（MP3）中使用安装在电机驱动台上的 PSD 进行的。这使得 PSD 的灵敏体积能够相对于垂直向下且刚好触及水面的 P50 施源器移动到预定位置。在 0 到 50mm 的源到探测器距离（SDD）范围内以 1-3mm 的步长测量百分深度剂量（PDD）。在五个不同的 SDD 下，沿着两个垂直轴测量剂量分布，步长低至 0.5mm。PSD 的特性包括通过蒙特卡罗（MC）模拟确定能量校正，以及使用井型电离室作为参考测量稳定性和剂量率线性度。测量的 PDD 和分布通过相应的 MC 模拟进行验证。
在对 PSD 测量进行能量依赖性校正后，除了在 0mm 和 43mm 深度处，测量的 PDD 曲线与模拟曲线的偏差在 2%以内。在 7mm 深度处，吸收剂量降低了 2 倍，在 26mm 深度处降低了 10 倍。测量的剂量分布在 5mm 深度处的轮廓边缘处显示出超过 50%/mm 的剂量梯度，在 50mm 深度处显示出 15%/mm 的剂量梯度。测量的轮廓宽度随深度增加 0.66mm，而模拟的轮廓宽度随深度增加 0.74mm。在距光束中心 10mm 的距离处，观察到剂量的方位角依赖性超过 10%。测量的相对剂量总不确定度小于 2.5%，位置不确定度为 0.4mm。使用 PSD 和 MP3 水模体进行完整的 3D 剂量特征描述（PDD 和分布）的测量可以在 8 小时内完成，限制因素是 P50 的冷却。
PSD 和 MP3 水模体提供了一种方法，可以独立验证 eBT 源在水中的相对 3D 剂量分布。
