Tsiakalos Miltiadis F, Stathakis Sotirios, Plataniotis George A, Kappas Constantin, Theodorou Kiki
Medical Physics Department, Medical School, University of Thessalia, Larissa, Hellas, Greece.
Radiother Oncol. 2006 Apr;79(1):131-8. doi: 10.1016/j.radonc.2006.02.012. Epub 2006 Apr 17.
Low megavoltage photon beams are often the treatment choice in radiotherapy when low density heterogeneities are involved, because higher energies show some undesirable dosimetric effects. This work is aimed at investigating the effects of different energy selection for low density tissues.
BEAMnrc was used to simulate simple treatment set-ups in a simple and a CT reconstructed lung phantom and an air-channel phantom. The dose distribution of 6, 15 and 20 MV photon beams was studied using single, AP/PA and three-field arrangements.
Our results showed no significant changes in the penumbra width in lung when a pair of opposed fields were used. The underdosage at the anterior/posterior tumor edge caused by the dose build-up at the lung-tumor interface reached 7% for a 5 x 5 cm AP/PA set-up. Shrinkage of the 90% isodose volume was noticed for the same set-up, which could be rectified by adding a lateral field. For the CT reconstructed phantom, the AP/PA set-up offered better tumor coverage when lower energies were used but for the three field set-up, higher energies resulted to better sparing of the lung tissue. For the air-channel set-up, adding an opposed field reduced the penumbra width. Using higher energies resulted in a 7% cold spot around the air-tissue interface for a 5 x 5 cm field.
The choice of energy for treatment in the low density areas is not a straightforward decision but depends on a number of parameters such as the beam set-up and the dosimetric criteria. Updated calculation algorithms should be used in order to be confident for the choice of energy of treatment.
当涉及低密度不均匀性时，低兆伏光子束通常是放射治疗的首选，因为较高能量会显示出一些不良的剂量学效应。这项工作旨在研究不同能量选择对低密度组织的影响。
使用BEAMnrc在简单的和CT重建的肺部体模以及空气通道体模中模拟简单的治疗设置。使用单野、前后对穿野和三野设置研究了6、15和20兆伏光子束的剂量分布。
我们的结果表明，当使用一对对穿野时，肺部半值层宽度没有显著变化。对于5×5厘米的前后对穿野设置，肺-肿瘤界面处的剂量积累导致肿瘤前后边缘的剂量不足达到7%。对于相同的设置，注意到90%等剂量体积缩小，这可以通过添加侧野来纠正。对于CT重建体模，使用较低能量时，前后对穿野设置能提供更好的肿瘤覆盖，但对于三野设置，较高能量能更好地保护肺组织。对于空气通道设置，添加对穿野可减小半值层宽度。对于一个5×5厘米的射野，使用较高能量会在空气-组织界面周围产生7%的冷点。
在低密度区域选择治疗能量并非易事，而是取决于许多参数，如射野设置和剂量学标准。应使用更新的计算算法，以便对治疗能量的选择有信心。
