Kry Stephen F, Howell Rebecca M, Polf Jerimy, Mohan Radhe, Vassiliev Oleg N
Department of Radiation Physics, The University of Texas M.D Anderson Cancer Center, Houston, TX, USA.
Phys Med Biol. 2009 Mar 7;54(5):1265-73. doi: 10.1088/0031-9155/54/5/011. Epub 2009 Feb 3.
The requirements for shielding a treatment vault with a Varian Clinac 2100 medical linear accelerator operated both with and without the flattening filter were assessed. Basic shielding parameters, such as primary beam tenth-value layers (TVLs), patient scatter fractions, and wall scatter fractions, were calculated using Monte Carlo simulations of 6, 10 and 18 MV beams. Relative integral target current requirements were determined from treatment planning studies of several disease sites with, and without, the flattening filter. The flattened beam shielding data were compared to data published in NCRP Report No. 151, and the unflattened beam shielding data were presented relative to the NCRP data. Finally, the shielding requirements for a typical treatment vault were determined for a single-energy (6 MV) linac and a dual-energy (6 MV/18 MV) linac. With the exception of large-angle patient scatter fractions and wall scatter fractions, the vault shielding parameters were reduced when the flattening filter was removed. Much of this reduction was consistent with the reduced average energy of the FFF beams. Primary beam TVLs were reduced by 12%, on average, and small-angle scatter fractions were reduced by up to 30%. Head leakage was markedly reduced because less integral target current was required to deliver the target dose. For the treatment vault examined in the current study, removal of the flattening filter reduced the required thickness of the primary and secondary barriers by 10-20%, corresponding to 18 m(3) less concrete to shield the single-energy linac and 36 m(3) less concrete to shield the dual-energy linac. Thus, a shielding advantage was found when the linac was operated without the flattening filter. This translates into a reduction in occupational exposure and/or the cost and space of shielding.
评估了使用Varian Clinac 2100医用直线加速器在有和没有均整过滤器的情况下对治疗室进行屏蔽的要求。使用6、10和18 MV射束的蒙特卡罗模拟计算了基本屏蔽参数，如原射线十分之一值层（TVL）、患者散射分数和墙体散射分数。通过对有和没有均整过滤器的多个疾病部位的治疗计划研究确定了相对积分靶电流要求。将均整射束的屏蔽数据与NCRP第151号报告中公布的数据进行比较，并将未均整射束的屏蔽数据相对于NCRP数据呈现。最后，确定了单能（6 MV）直线加速器和双能（6 MV/18 MV）直线加速器典型治疗室的屏蔽要求。除了大角度患者散射分数和墙体散射分数外，去除均整过滤器后治疗室的屏蔽参数降低。这种降低在很大程度上与FFF射束平均能量的降低一致。原射线TVL平均降低了12%，小角度散射分数降低了多达30%。头部泄漏明显减少，因为输送靶剂量所需的积分靶电流减少。对于本研究中检查的治疗室，去除均整过滤器使主屏蔽层和次屏蔽层所需厚度减少了10-20%，这相当于屏蔽单能直线加速器所需的混凝土减少18立方米，屏蔽双能直线加速器所需的混凝土减少36立方米。因此，发现直线加速器在没有均整过滤器的情况下运行具有屏蔽优势。这转化为职业照射的减少和/或屏蔽成本及空间的降低。
