U.O. Health Physics, Ospedali Galliera Genova, Genova, Italy.
J Appl Clin Med Phys. 2013 Jan 7;14(1):3678. doi: 10.1120/jacmp.v14i1.3678.
The Novac7 and Liac are linear accelerators (linacs) dedicated to intraoperative radiation therapy (IORT), which produce high energy, very high dose-per-pulse electron beams. The characteristics of the accelerators heads of the Novac7 and Liac are different compared to conventional electron accelerators. The aim of this work was to investigate the specific characteristics of the Novac7 and Liac electron beams using the Monte Carlo method. The Monte Carlo code BEAMnrc has been employed to model the head and simulate the electron beams. The Monte Carlo simulation was preliminarily validated by comparing the simulated dose distributions with those measured by means of EBT radiochromic film. Then, the energy spectra, mean energy profiles, fluence profiles, photon contamination, and angular distributions were obtained from the Monte Carlo simulation. The Spencer-Attix water-to-air mass restricted collision stopping power ratios (sw,air) were also calculated. Moreover, the modifications of the percentage depth dose in water (backscatter effect) due to the presence of an attenuator plate composed of a sandwich of a 2 mm aluminum foil and a 4 mm lead foil, commonly used for breast treatments, were evaluated. The calculated sw,air values are in agreement with those tabulated in the IAEA TRS-398 dosimetric code of practice within 0.2% and 0.4% at zref (reference depth in water) for the Novac7 and Liac, respectively. These differences are negligible for practical dosimetry. The attenuator plate is sufficient to completely absorb the electron beam for each energy of the Novac7 and Liac; moreover, the shape of the dose distribution in water strongly changes with the introduction of the attenuator plate. This variation depends on the energy of the beam, and it can give rise to an increase in the maximum dose in the range of 3%-9%.
Novac7 和 Liac 是用于术中放射治疗（IORT）的直线加速器（linacs），可产生高能、高剂量/脉冲电子束。与传统电子加速器相比，Novac7 和 Liac 加速器头的特性有所不同。本工作旨在采用蒙特卡罗方法研究 Novac7 和 Liac 电子束的特定特性。采用 BEAMnrc 蒙特卡罗程序对头部进行建模并模拟电子束。通过比较电子束的模拟剂量分布与 EBT 光致变色胶片的测量值，对蒙特卡罗模拟进行了初步验证。然后，从蒙特卡罗模拟中获得了能谱、平均能量分布、注量分布、光子污染和角分布。还计算了 Spencer-Attix 水-空气质量限制碰撞阻止本领比（sw,air）。此外，评估了由于存在由 2mm 铝箔和 4mm 铅箔组成的夹心衰减器板（通常用于乳房治疗）引起的水中百分深度剂量（反向散射效应）的变化。计算的 sw,air 值在 0.2%和 0.4%以内与 IAEA TRS-398 剂量学实践规程中列出的值一致，分别用于 Novac7 和 Liac 的 zref（水中参考深度）。对于实际剂量测定，这些差异可以忽略不计。衰减器板足以完全吸收 Novac7 和 Liac 的每个能量的电子束；此外，水中剂量分布的形状随衰减器板的引入而强烈变化。这种变化取决于束的能量，并且可能导致最大剂量在 3%-9%范围内增加。
