Department of Oncology, Kuopio University Hospital, Box 1777, FIN-70211, Kuopio, Finland.
Phys Med Biol. 2009 Dec 7;54(23):7211-26. doi: 10.1088/0031-9155/54/23/012. Epub 2009 Nov 17.
Previously, we have found that the build-up dose from abutting narrow electron beams formed with unfocussed electron multi-leaf collimator (eMLC) steal leaves was higher than with the respective open field. To investigate more closely the effect of leaf material and shape on dose in the build-up region, straight, round (radius 1.5 cm) and leaf ends with a different front face angle of alpha (leaf front face pointing towards the beam axis at an angle of 90 - alpha) made of steel, brass and tungsten were modelled using the BEAMnrc code. Based on a treatment head simulation of a Varian 2100 C/D linac, depth-dose curves and profiles in water were calculated for narrow 6, 12 and 20 MeV eMLC beams (width 1.0 cm, length 10 cm) at source-to-surface distances (SSD) of 102 and 105 cm. The effects of leaf material and front face angle were evaluated based on electron fluence, angle and energy spectra. With a leaf front face angle of 15 degrees, the dose in the build-up region of the 6 MeV field varied between 91 and 100%, while for straight and round leaf shapes the dose varied between 89 and 100%. The variation was between 94 and 100% for 12 and 20 MeV. For abutting narrow 6 MeV fields with total field size 5 x 10 cm(2), the build-up doses at 5 mm depth for the face angle 15 degrees and straight and round leaf shapes were 96% and 86% (SSD 102 cm) and 89% and 85% (SSD 105 cm). With higher energies, the effect of eMLC leaf shape on dose at 5 mm was slight (3-4% units with 12 MeV) and marginal with 20 MeV. The fluence, energy and angle spectra for total and leaf scattered electrons were practically the same for different leaf materials with 6 MeV. With high energies, the spectra for tungsten were more peaked due to lower leaf transmission. Compared with straight leaf ends, the face angle of 15 degrees and round leaf ends led to a 1 mm (for 6 MeV) and between 1 and 5 mm (12 and 20 MeV at a SSD of 105 cm) decrease of therapeutic range and increase of the field size, respectively. However, profile flatness was better for abutting 6 MeV beams with round (2.5%) and face angle 15 degrees leaves (3.0%) compared to straight leaf shape (5.2%). The eMLC leaves with a face angle of 15 degrees resulted in a marked increase in the build-up dose for the single narrow eMLC beam and thus in the dose in the build-up region from matched abutting fields.
先前，我们已经发现由未聚焦电子多叶准直器（eMLC）形成的相邻窄电子射束的累积剂量比相应的开放野更高。为了更深入地研究叶片材料和形状对累积区域剂量的影响，使用 BEAMnrc 代码对直的、圆的（半径 1.5 厘米）和叶片末端具有不同前角α（叶片前缘相对于射束轴的角度为 90-α）的钢、黄铜和钨叶片进行建模。基于瓦里安 2100 C/D 直线加速器的治疗头模拟，在源皮距（SSD）为 102 和 105 厘米处，计算了窄 6、12 和 20 MeV eMLC 射束（宽度 1.0 厘米，长度 10 厘米）在水中的深度剂量曲线和剖面图。根据电子通量、角度和能谱评估叶片材料和前角的影响。在前角为 15 度时，6 MeV 射束的累积区域剂量在 91%至 100%之间变化，而直叶片和圆叶片的剂量在 89%至 100%之间变化。12 和 20 MeV 之间的变化在 94%至 100%之间。对于总射野尺寸为 5x10cm(2)的相邻窄 6 MeV 射野，在前角为 15 度和直叶片以及圆叶片形状下，5mm 深度处的累积剂量为 96%和 86%（SSD 为 102cm）和 89%和 85%（SSD 为 105cm）。随着能量的增加，eMLC 叶片形状对 5mm 处剂量的影响很小（12MeV 时为 3-4%），而 20MeV 时则微不足道。对于 6 MeV，总电子和叶片散射电子的通量、能量和角度谱对于不同叶片材料几乎相同。由于叶片透射率较低，对于高能量，钨的谱更陡峭。与直叶片末端相比，前角为 15 度和圆叶片末端导致治疗范围分别减少 1mm（6MeV）和 1-5mm（SSD 为 105cm 时的 12 和 20MeV），并且射野尺寸增加。然而，对于相邻的 6 MeV 射束，圆形（2.5%）和前角 15 度叶片（SSD 为 105cm 时的 3.0%）比直叶片形状（5.2%）的平坦度更好。前角为 15 度的 eMLC 叶片导致单个窄 eMLC 射束的累积剂量显著增加，从而导致匹配相邻射野的累积区域剂量增加。
