Behrens C F
Department of Oncology (R), University Hospital Herlev, Radiofysisk afsnit (54D3), Herlev Ringvej 75, DK2730 Herlev, Denmark.
Phys Med Biol. 2006 Nov 21;51(22):5937-50. doi: 10.1088/0031-9155/51/22/015. Epub 2006 Oct 26.
It has been shown in several studies that the build-up in photon beams behind air cavities (such as in the head and neck) increases with energy. In this study this effect is investigated over a broad range of energies that have been used for treating head and neck tumours. The study addresses the question of whether an energy lower than 6 MV is desirable and is based on measurements and Monte Carlo (MC) simulations. In a PMMA phantom containing an air cavity (3 x 16 x 3 cm3 at 3 cm depth) an ionization chamber (Capintec PS-033) was used to measure the dose build-up behind the cavity for 4, 6 and 8 MV beam qualities for different field sizes (from 3 x 6 cm2 to 8 x 8 cm2). MC simulations were made using the EGSnrc code for the same geometry and energies as well as for Co-60. Measurements and MC simulations agree well when the fixed-separation plane-parallel chamber measurements have been corrected for the expected over-response in the build-up region. This work demonstrates that the build-up effect of 6 MV is 'closer' to the build-up effect of 8 MV than to that of 4 MV. This suggests that if the build-up effect is of concern when the target volume is in the vicinity of air cavities, 4 MV should be preferred over both 6 MV and 8 MV. This work also shows that the build-up effect for Co-60 is significantly smaller than that of 4 MV. Moreover, the build-up effect increases as the field size decreases. With the increasing use of IMRT (and radiosurgery), small fields are used more frequently making these issues even more relevant. This should be taken into consideration when choosing the accelerator energies for a radiotherapy department.
多项研究表明，光子束在空气腔（如头颈部）后方的剂量积累随能量增加。在本研究中，对用于治疗头颈部肿瘤的广泛能量范围进行了该效应的研究。该研究基于测量和蒙特卡罗（MC）模拟，探讨了低于6 MV的能量是否合适的问题。在一个含有空气腔（深度3 cm处为3×16×3 cm³）的PMMA体模中，使用电离室（Capintec PS - 033）测量不同射野尺寸（从3×6 cm²到8×8 cm²）下4、6和8 MV射束质在腔后方的剂量积累。使用EGSnrc代码对相同几何结构、能量以及钴 - 60进行了MC模拟。当对固定间距平行板电离室在剂量积累区域的预期过响应进行校正后，测量结果与MC模拟结果吻合良好。这项工作表明，6 MV的剂量积累效应比4 MV更“接近”8 MV的剂量积累效应。这表明，当靶区体积靠近空气腔时，如果剂量积累效应是一个需要关注的问题，相对于6 MV和8 MV，应优先选择4 MV。这项工作还表明，钴 - 60的剂量积累效应明显小于4 MV。此外，剂量积累效应随射野尺寸减小而增加。随着调强放疗（IMRT）（以及放射外科）的使用增加，小射野使用得更频繁，使得这些问题更加重要。在为放疗科室选择加速器能量时应考虑到这一点。
