Wiezorek Tilo, Voigt Alexander, Metzger Noreen, Georg Dietmar, Schwedas Michael, Salz Henning, Wendt Thomas G
Department of Radiotherapy, Friedrich Schiller University of Jena, Germany.
Strahlenther Onkol. 2008 Feb;184(2):73-9. doi: 10.1007/s00066-008-1743-4.
To quantify the relative peripheral doses (PD) to healthy tissues outside the treated region for different intensity-modulated radiotherapy (IMRT) technologies.
On a linear accelerator (linac) Oncor Impression (Siemens OCS) with two photon energies (6 MV, 15 MV), point dose measurements were performed at different depths in a solid phantom at 29 cm off-axis distance inplane. PD associated with artificial fluence distributions were compared with open beam contributions, where intensity-modulated (IM) beams were generated by segmented multileaf-modulated (sMLM) IMRT, by tin+wax compensators (TWComp), and by lead-containing cerrobend compensators (CComp). The field size of the open field and the maximum area (isocenter distance) exposed with the primary beam for the IMRT fields was 20 x 22 cm2. Measurements were performed with two kinds of thermoluminescence dosimeters to quantify photon and neutron components separately. Furthermore, experiments were done with and without phantom material in the direct beam to separate different scatter dose components.
The results for the photon components and the neutron components are reverse. For the open field, the photon components increase with decreasing photon energy. In comparison with the open field, the photon components are further (factor 1.2-1.8 depending on energy and depth) increased when delivering IMRT with sMLM. When using CComp or TWComp, this factor is even higher and reaches a maximum of 2.4. At depths beyond 20 mm, photon component values slightly decrease with increasing photon energy for all types of IMRT techniques. Near the surface (10 mm depth), photon component values are distinctly higher than those at larger depth, and they increase with increasing photon energy. As expected, neutron components could be detected only for 15 MV. For sMLM and compensators, neutron components increased by factors 4 and 1.5 relative to the open field. The experiments with different scatter conditions show that about 50-70% of the photon components and all neutron components NPD are caused by radiation emanating from the linac head.
PD in IMRT can be minimized by proper selection of treatment delivery method and photon beam energy. When selecting the IMRT technique in centers where compensator IMRT and MLC IMRT is available, PD burden should be taken into account. The large amount of photon components and neutron components caused by leakage radiation from the treatment head leads to the recommendation that radiation protection aspects for patients undergoing IMRT should be considered in linac design. For further clarification, additional experiments have to be carried out on other types of linacs.
量化不同调强放射治疗（IMRT）技术对治疗区域外健康组织的相对周边剂量（PD）。
在具有两种光子能量（6兆伏、15兆伏）的直线加速器（linac）Oncor Impression（西门子OCS）上，在固体模体中离轴距离29厘米处的不同深度进行点剂量测量。将与人工注量分布相关的PD与开放野贡献进行比较，其中调强（IM）射束由分段多叶调制（sMLM）IMRT、锡+蜡补偿器（TWComp）和含铅的铈基合金补偿器（CComp）产生。开放野的射野尺寸以及IMRT射野主射束照射的最大面积（等中心距离）为20×22平方厘米。使用两种热释光剂量计分别对光子和中子成分进行量化测量。此外，在直接射束中有模体材料和无模体材料的情况下进行实验，以区分不同的散射剂量成分。
光子成分和中子成分的结果相反。对于开放野，光子成分随光子能量降低而增加。与开放野相比，使用sMLM进行IMRT时，光子成分进一步增加（根据能量和深度，增加因子为1.2 - 1.8）。使用CComp或TWComp时，该因子甚至更高，最高可达2.4。对于所有类型的IMRT技术，在深度超过20毫米时，光子成分值随光子能量增加略有下降。在表面附近（10毫米深度），光子成分值明显高于较大深度处的值，并且随光子能量增加而增加。正如预期的那样，仅在15兆伏时能检测到中子成分。对于sMLM和补偿器，中子成分相对于开放野分别增加了4倍和1.5倍。不同散射条件下的实验表明，约50 - 70%的光子成分和所有中子成分NPD是由直线加速器机头发出的辐射引起的。
通过适当选择治疗投照方法和光子束能量，可将IMRT中的PD降至最低。在有补偿器IMRT和多叶准直器IMRT的中心选择IMRT技术时，应考虑PD负担。治疗机头泄漏辐射导致大量光子成分和中子成分，这建议在直线加速器设计中应考虑接受IMRT患者的辐射防护问题。为进一步阐明，必须在其他类型的直线加速器上进行额外实验。
