Çatli Serap
Gazi University.
J Appl Clin Med Phys. 2015 Sep 8;16(5):46–52. doi: 10.1120/jacmp.v16i5.5612.
High atomic number and density of dental implants leads to major problems at providing an accurate dose distribution in radiotherapy and contouring tumors and organs caused by the artifact in head and neck tumors. The limits and deficiencies of the algorithms using in the treatment planning systems can lead to large errors in dose calculation, and this may adversely affect the patient's treatment. In the present study, four commercial dental implants were used: pure titanium, titanium alloy (Ti-6Al-4V), amalgam, and crown. The effects of dental implants on dose distribution are determined with two methods: pencil beam convolution (PBC) algorithm and Monte Carlo code for 6 MV photon beam. The central axis depth doses were calculated on the phantom for a source-skin distance (SSD) of 100 cm and a 10 × 10 cm2 field using both of algorithms. The results of Monte Carlo method and Eclipse TPS were compared to each other and to those previously reported. In the present study, dose increases in tissue at a distance of 2 mm in front of the dental implants were seen due to the backscatter of electrons for dental implants at 6 MV using the Monte Carlo method. The Eclipse treatment planning system (TPS) couldn't precisely account for the backscatter radiation caused by the dental prostheses. TPS underestimated the back scatter dose and overestimated the dose after the dental implants. The large errors found for TPS in this study are due to the limits and deficiencies of the algorithms. The accuracy of the PBC algorithm of Eclipse TPS was evaluated in comparison to Monte Carlo calculations in consideration of the recommendations of the American Association of Physicists in Medicine Radiation Therapy Committee Task Group 65. From the comparisons of the TPS and Monte Carlo calculations, it is verified that the Monte Carlo simulation is a good approach to derive the dose distribution in heterogeneous media.
牙科植入物的高原子序数和密度在对头颈部肿瘤进行放射治疗时，会在提供精确剂量分布以及勾勒肿瘤和器官轮廓方面引发重大问题，这是由头部和颈部肿瘤中的伪影导致的。治疗计划系统中所使用算法的局限性和不足之处，可能会在剂量计算中导致较大误差，进而可能对患者的治疗产生不利影响。在本研究中，使用了四种商用牙科植入物：纯钛、钛合金（Ti-6Al-4V）、汞合金和牙冠。采用两种方法确定牙科植入物对剂量分布的影响：笔形束卷积（PBC）算法和针对6兆伏光子束的蒙特卡罗代码。使用这两种算法，在模体上针对源皮距（SSD）为100厘米且射野为10×10平方厘米的情况计算中心轴深度剂量。将蒙特卡罗方法和Eclipse治疗计划系统（TPS）的结果相互比较，并与先前报道的结果进行比较。在本研究中，使用蒙特卡罗方法发现，对于6兆伏的牙科植入物，由于电子的反向散射，在牙科植入物前方2毫米处的组织中出现了剂量增加。Eclipse治疗计划系统（TPS）无法精确考虑牙科假体引起的反向散射辐射。TPS低估了反向散射剂量，高估了牙科植入物后方的剂量。本研究中在TPS中发现的较大误差是由于算法的局限性和不足之处。根据美国医学物理学家协会放射治疗委员会任务组65的建议，将Eclipse TPS的PBC算法的准确性与蒙特卡罗计算结果进行了比较。通过TPS和蒙特卡罗计算结果的比较，证实了蒙特卡罗模拟是得出非均匀介质中剂量分布的一种良好方法。
