Department of Radiation Oncology, University of Washington School of Medicine, 1959 NE Pacific Street, Box 356043, Seattle, WA 98195, United States of America. Nuclear Engineering Program, University of Utah, 50 South Central Drive, 1206 MEB, Salt Lake City, UT 84112, United States of America.
Phys Med Biol. 2018 May 15;63(10):105008. doi: 10.1088/1361-6560/aabd52.
The University of Washington (UW) Clinical Neutron Therapy System (CNTS), which generates high linear energy transfer fast neutrons through interactions of 50.5 MeV protons incident on a Be target, has depth-dose characteristics similar to 6 MV x-rays. In contrast to the fixed beam angles and primitive blocking used in early clinical trials of neutron therapy, the CNTS has a gantry with a full 360° of rotation, internal wedges, and a multi-leaf collimator (MLC). Since October of 1984, over 3178 patients have received conformal neutron therapy treatments using the UW CNTS. In this work, the physical and dosimetric characteristics of the CNTS are documented through comparisons of measurements and Monte Carlo simulations. A high resolution computed tomography scan of the model 17 ionization chamber (IC-17) has also been used to improve the accuracy of simulations of the absolute calibration geometry. The response of the IC-17 approximates well the kinetic energy released per unit mass (KERMA) in water for neutrons and photons for energies from a few tens of keV up to about 20 MeV. Above 20 MeV, the simulated model 17 ion chamber response is 20%-30% higher than the neutron KERMA in water. For CNTS neutrons, simulated on- and off-axis output factors in water match measured values within ~2% ± 2% for rectangular and irregularly shaped field with equivalent square areas ranging in a side dimension from 2.8 cm to 30.7 cm. Wedge factors vary by less than 1.9% of the measured dose in water for clinically relevant field sizes. Simulated tissue maximum ratios in water match measured values within 3.3% at depths up to 20 cm. Although the absorbed dose for water and adipose tissue are within 2% at a depth of 1.7 cm, the absorbed dose in muscle and bone can be as much as 12 to 40% lower than the absorbed dose in water. The reported studies are significant from a historical perspective and as additional validation of a new tool for patient quality assurance and as an aid in ongoing efforts to clinically implement advanced treatment techniques, such as intensity modulated neutron therapy, at the UW.
华盛顿大学（UW）临床中子治疗系统（CNTS）通过 50.5MeV 质子与 Be 靶的相互作用产生高传能线密度快中子，其深度剂量特性与 6MV X 射线相似。与早期中子治疗临床试验中使用的固定射束角度和原始阻挡相比，CNTS 具有一个全 360°旋转的龙门架、内部楔形物和多叶准直器（MLC）。自 1984 年 10 月以来，超过 3178 名患者使用 UW CNTS 接受了适形中子治疗。在这项工作中，通过测量和蒙特卡罗模拟的比较，记录了 CNTS 的物理和剂量学特性。还使用模型 17 电离室（IC-17）的高分辨率计算机断层扫描来提高模拟绝对校准几何形状的准确性。IC-17 的响应很好地近似于从几十 keV 到约 20MeV 的中子和光子的单位质量释放的动能（KERMA）。在 20MeV 以上，模拟的模型 17 离子室响应比水中的中子 KERMA 高 20%-30%。对于 CNTS 中子，在水中模拟的轴上和轴外输出因子与测量值匹配，对于等效正方形面积从 2.8cm 到 30.7cm 的矩形和不规则形状的场，在 2%以内。楔形因子在水临床相关场尺寸的测量剂量中变化小于 1.9%。在水模拟的组织最大比与测量值在 3.3%以内，深度高达 20cm。尽管在 1.7cm 深度处水和脂肪组织的吸收剂量在 2%以内，但肌肉和骨骼中的吸收剂量可以比水低 12%至 40%。从历史角度来看，这些研究意义重大，是对新工具进行患者质量保证的进一步验证，也是在 UW 临床实施先进治疗技术（如强度调制中子治疗）的持续努力的辅助工具。
