Ayala Rafael, Soza Álvaro, García María Jesús, García Rocío, Udías José Manuel, Ibáñez Paula
Servicio de Dosimetría y Radioprotección, Hospital General Universitario Gregorio Marañón, Madrid, Spain.
Nuclear Physics Group and IPARCOS, Department of Structure of Matter, Thermal Physics and Electronics, CEI Moncloa, Universidad Complutense de Madrid, Madrid, Spain.
Med Phys. 2025 Aug;52(8):e18040. doi: 10.1002/mp.18040.
Intraoperative electron radiation therapy (IOERT) relies on accurate and precise dose delivery to the tumor or tumor bed using mobile accelerators and interchangeable applicators, while critical organs are typically displaced or shielded during surgery. Treatment planning and linac commissioning are often based on water measurements, Monte Carlo (MC) simulations of the accelerator head and applicator system provide detailed insights into dose distributions and beam characteristics, offering additional support for clinical evaluation.
This study develops an MC model of the Liac HWL mobile accelerator using a hypothetical linac head geometry, due to the limited availability of detailed information on its internal components resulting from manufacturer disclosure policies. The model is optimized by adjusting three geometric parameters of the linac head and the initial beam energy spectrum to match experimental data. Additionally, it provides a set of Phase Space Files (PSFs) to support research and clinical applications.
The MC code PENELOPE, integrated with the penEasy framework, was used to simulate the Liac HWL. The hypothetical head geometry was defined by parameters such as the inner diameter of the head, the thickness of the scattering foil, and the thickness of the exit window. Output factors (OFs), percentage depth doses (PDDs), and off-axis ratios (OARs) were calculated in a virtual water phantom for different applicator sizes, bevel angles, and energies. Gamma analysis was employed to validate the model by comparing calculated and measured dose distributions. PSFs were made available in the IAEA PHSP format at four energies (6, 8, 10, and 12 MeV).
The model matched measured OFs within 2.5%. PDDs and OARs met the gamma analysis criteria (2% dose difference and 1 mm distance-to-agreement) in more than 93% of the studied cases, with the worst-case scenario occurring for the smallest applicator (3 cm diameter) with a 45° bevel angle at 6 MeV, resulting in OAR gamma passing rates of 85.7% at and 86.1% at .
Despite the use of a hypothetical geometry, the model offers accurate dosimetric data and practical guidance for IOERT commissioning and treatment planning. It highlights potential dosimetric issues, particularly the lack of homogeneity in OARs for large-diameter applicators, and allows fine-tuning based on real-world data. Additionally, the PSFs generated in this study provide a reliable resource for simulating IORT dose distributions and analyzing the characteristics of IOERT beams.
术中电子放射治疗（IOERT）依靠使用移动加速器和可互换施源器将准确且精确的剂量输送至肿瘤或肿瘤床，而关键器官在手术过程中通常会被移位或屏蔽。治疗计划和直线加速器调试通常基于水模测量，加速器机头和施源器系统的蒙特卡罗（MC）模拟可深入了解剂量分布和射束特性，为临床评估提供额外支持。
由于制造商的披露政策导致关于其内部组件的详细信息有限，本研究使用假设的直线加速器机头几何结构开发了Liac HWL移动加速器的MC模型。通过调整直线加速器机头的三个几何参数和初始射束能谱来优化该模型，以使其与实验数据匹配。此外，它还提供了一组相空间文件（PSF）以支持研究和临床应用。
使用与penEasy框架集成的MC代码PENELOPE来模拟Liac HWL。通过机头内径、散射箔厚度和出射窗厚度等参数定义假设的机头几何结构。在虚拟水模中针对不同的施源器尺寸、斜角和能量计算输出因子（OF）、百分深度剂量（PDD）和离轴比（OAR）。采用伽马分析通过比较计算和测量的剂量分布来验证模型。以IAEA PHSP格式在四种能量（6、8、10和12 MeV）下提供PSF。
该模型与测量的OF的匹配度在2.5%以内。在超过93%的研究案例中，PDD和OAR符合伽马分析标准（剂量差异2%和距离一致性1 mm），最坏情况发生在直径为3 cm的最小施源器、45°斜角且能量为6 MeV时，导致OAR伽马通过率在 时为85.7%，在 时为86.1%。
尽管使用了假设的几何结构，但该模型为IOERT调试和治疗计划提供了准确的剂量学数据和实用指导。它突出了潜在的剂量学问题，特别是大直径施源器的OAR缺乏均匀性，并允许根据实际数据进行微调。此外，本研究中生成的PSF为模拟IORT剂量分布和分析IOERT射束特性提供了可靠资源。
