Wang Xin, Poenisch Falk, Sahoo Narayan, Zhu Ronald X, Lii MingFwu, Gillin Michael T, Li Jing, Grosshans David
The University of MD Anderson Cancer Center.
J Appl Clin Med Phys. 2016 Sep 8;17(5):366-376. doi: 10.1120/jacmp.v17i5.6327.
This is a real case study to minimize the neutron dose equivalent (H) to a fetus using spot scanning proton beams with favorable beam energies and angles. Minimum neutron dose exposure to the fetus was achieved with iterative planning under the guidance of neutron H measurement. Two highly conformal treatment plans, each with three spot scanning beams, were planned to treat a 25-year-old pregnant female with aggressive recurrent chordoma of the base of skull who elected not to proceed with termination. Each plan was scheduled for delivery every other day for robust target coverage. Neutron H to the fetus was measured using a REM500 neutron survey meter placed at the fetus position of a patient simulating phantom. 4.1 and 44.1 μSv/fraction were measured for the two initial plans. A vertex beam with higher energy and the fetal position closer to its central axis was the cause for the plan that produced an order higher neutron H. Replacing the vertex beam with a lateral beam reduced neutron H to be comparable with the other plan. For a prescription of 70 Gy in 35 fractions, the total neutron H to the fetus was estimated to be 0.35 mSv based on final measurement in single fraction. In comparison, the passive scattering proton plan and photon plan had an estimation of 26 and 70 mSv, respectively, for this case. While radiation therapy in pregnant patients should be avoided if at all possible, our work demonstrated spot scanning beam limited the total neutron H to the fetus an order lower than the suggested 5 mSv regulation threshold. It is far superior than passive scattering beam and careful beam selection with lower energy and keeping fetus further away from beam axis are essential in minimizing the fetus neutron exposure.
这是一个真实的案例研究，旨在使用具有合适束能量和角度的点扫描质子束，将胎儿所接受的中子剂量当量（H）降至最低。在中子剂量当量测量的指导下，通过迭代规划实现了胎儿接受的最低中子剂量暴露。针对一名25岁患有侵袭性复发性颅底脊索瘤且选择不终止妊娠的孕妇，制定了两个高度适形的治疗计划，每个计划均包含三束点扫描束。每个计划每隔一天进行一次治疗，以确保对靶区的充分覆盖。使用放置在模拟患者体模胎儿位置的REM500中子测量仪，对胎儿所接受的中子剂量当量进行测量。两个初始计划测得的剂量分别为4.1和44.1 μSv/分次。产生的中子剂量当量高出一个数量级的计划，其原因是使用了能量更高且胎儿位置更靠近其中心轴的头顶束。用侧束取代头顶束后，中子剂量当量降低至与另一个计划相当的水平。对于35次分割、处方剂量为70 Gy的情况，根据单次分割的最终测量结果，估计胎儿所接受的总中子剂量当量为0.35 mSv。相比之下，对于该病例，被动散射质子计划和光子计划的估计值分别为26 mSv和70 mSv。虽然应尽可能避免对孕妇进行放射治疗，但我们的工作表明，点扫描束将胎儿所接受的总中子剂量当量限制在比建议的5 mSv监管阈值低一个数量级的水平。它远比被动散射束优越，谨慎选择能量较低的束并使胎儿远离束轴对于将胎儿的中子暴露降至最低至关重要。
