Facure A, da Silva A X, da Rosa L A R, Cardoso S C, Rezende G F S
Comissão Nacional de Energia Nuclear, R. Gal. Severiano 90, sala 409, 22294-900 Rio de Janeiro, RJ, Brazil.
Med Phys. 2008 Jul;35(7):3285-92. doi: 10.1118/1.2940192.
When space limitations are primary constraints, laminated barriers with metals can be an option to provide sufficient shielding for a radiotherapy treatment room. However, if a photon clinical beam with end point energy of 10 MeV or higher interacts with the metal inside the barriers neutrons are ejected and can result in an exposure problem inside and outside the vault. The empirical formulae existing in the literature to estimate neutron dose equivalents beyond laminated barriers do not take into account neutron production for spectra below 15 MV. In this work, the Monte Carlo code MCNP was used to simulate the production and transport of photoneutrons across primary barriers of 10 MV accelerator treatment rooms containing lead or steel, in order to obtain the ambient dose equivalents produced by these particles outside the room and in the patient plane. It was found that the neutron doses produced are insignificant when steel is present in the primary barriers of 10 MV medical accelerators. On the other hand, the results show that, in all cases where lead sheets are positioned in the primary barriers, the neutron ambient dose equivalents outside the room generally exceed the shielding design goal of 20 microSv/week for uncontrolled areas, even when the lead sheets are positioned inside the treatment room. Moreover, for laminated barriers, the photoneutrons produced in the metals are summed with the particles generated in the accelerator head shielding and can represent a significant component of additional dose to the patients. In this work, it was found that once lead sheets are positioned inside the room, the neutron ambient dose equivalents can reach the value of 75 microSv per Gray of photon absorbed dose at the isocenter. However, for all simulated cases, a tendency in the reduction of neutron doses with increasing lead thickness can be observed. This trend can imply in higher neutron ambient dose equivalents outside the room for thinner lead sheets. Therefore, when a medical accelerator treatment room is designed with laminated barriers to receive equipment with an end point energy equal to or higher than 10 MeV, not only the required shielding thickness for photon radiation attenuation should be considered, but also the dose due to photoneutrons produced in the metal, which may involve an increase of the lead thickness or even the use of neutron shielding.
当空间限制成为主要制约因素时，带有金属的层压屏蔽可以作为一种为放射治疗室提供足够屏蔽的选择。然而，如果端点能量为10 MeV或更高的光子临床射束与屏蔽层内的金属相互作用，中子会被射出，这可能导致治疗室内部和外部出现辐射暴露问题。文献中用于估算层压屏蔽外中子剂量当量的经验公式没有考虑15 MV以下能谱的中子产生情况。在这项工作中，使用蒙特卡罗代码MCNP来模拟10 MV加速器治疗室含铅或钢的主屏蔽层中光中子的产生和传输，以便获得这些粒子在治疗室外和患者平面产生的周围剂量当量。研究发现，当10 MV医用加速器的主屏蔽层中存在钢时，产生的中子剂量微不足道。另一方面，结果表明，在所有主屏蔽层中放置铅板的情况下，即使铅板位于治疗室内，治疗室外的中子周围剂量当量通常也会超过非控制区域20微希沃特/周的屏蔽设计目标。此外，对于层压屏蔽，金属中产生的光中子与加速器头部屏蔽中产生的粒子相加，可能会成为患者额外剂量的重要组成部分。在这项工作中发现，一旦在室内放置铅板，中子周围剂量当量在等中心处每吸收1格雷光子剂量可达到75微希沃特的值。然而，对于所有模拟情况，可以观察到随着铅厚度增加中子剂量减少的趋势。这种趋势可能意味着较薄铅板在治疗室外会产生更高的中子周围剂量当量。因此，当设计一个带有层压屏蔽的医用加速器治疗室以容纳端点能量等于或高于10 MeV的设备时，不仅要考虑光子辐射衰减所需的屏蔽厚度，还要考虑金属中产生的光中子剂量，这可能需要增加铅厚度甚至使用中子屏蔽。
