Nath R, Epp E R, Laughlin J S, Swanson W P, Bond V P
Med Phys. 1984 May-Jun;11(3):231-41. doi: 10.1118/1.595497.
The problem of neutrons produced by many of the high-energy x-ray therapy machines (10 MV and above) is reviewed, and the possible risk their presence poses to radiotherapy patients is estimated. A review of the regulatory background containing a summary of the recommendations of the U.S. Council of State Governments (USCSG), and of the International Electro-Technical Commission (IEC), as well as an indication that recommendations will be forthcoming from the National Council on Radiation Protection (NCRP) and the International Commission of Radiological Protection (ICRP) is presented. The neutrons in question are produced by high-energy photons (x rays) incident on the various materials of the target, flattening filter, collimators, and other essential components of the equipment. The neutron yield (per treatment dose) increases rapidly as the megavoltage is increased from 10 to 20 MV, but remains approximately constant above this. Measurements and calculations of the quantity, quality, and spatial distribution of these neutrons and their concomitant dose are summarized. Values of the neutron dose are presented as entrance dose, midline dose (10-cm depth), and integral dose, both within and outside of the treatment volume. These values are much less than the unavoidable photon doses which are largely responsible for treatment side effects. For typical equipment, the average neutron integral dose from accelerator-produced neutrons is about 4-7 g cGy (per treatment cGy), depending on the treatment plan. This translates into an average dose of neutrons [averaged over the body of a typical 70-kg (154 lb) patient] of 0.06-0.10 cGy for a treatment of 1000 cGy. Using these neutron doses and the best available neutron risk coefficients, it is estimated that 50 X 10(-6) fatal malignancies per year due to the neutrons may follow a typical treatment course of 5000 rads of 25-MV x rays. This is only about 1/60th of the average incidence of malignancies for the general population. Thus, the cancer risk to the radiotherapy patient from accelerator-produced neutrons poses an additional risk to the patient that is negligible in comparison.
本文回顾了许多高能X射线治疗机（10兆伏及以上）产生中子的问题，并估计了这些中子的存在对放疗患者可能造成的风险。文中介绍了监管背景，其中总结了美国州政府理事会（USCSG）和国际电工委员会（IEC）的建议，同时指出美国国家辐射防护委员会（NCRP）和国际放射防护委员会（ICRP）也将发布相关建议。所讨论的中子是由入射到靶、均整滤过器、准直器及设备其他关键部件的各种材料上的高能光子（X射线）产生的。当中子能量从10兆伏增加到20兆伏时，中子产额（每治疗剂量）迅速增加，但在此能量之上产额大致保持恒定。文中总结了这些中子的数量、质量和空间分布及其伴随剂量的测量和计算结果。中子剂量值以治疗野内和野外的入射剂量、中线剂量（10厘米深度）和积分剂量表示。这些值远低于主要导致治疗副作用的不可避免的光子剂量。对于典型设备，加速器产生的中子的平均中子积分剂量约为4 - 7克厘戈瑞（每治疗厘戈瑞），具体取决于治疗计划。这相当于对一名典型的70千克（154磅）患者进行1000厘戈瑞治疗时，中子的平均剂量为0.06 - 0.10厘戈瑞。利用这些中子剂量和现有的最佳中子风险系数，估计在典型的25兆伏X射线5000拉德的治疗过程中，每年因中子导致的致命恶性肿瘤发生率为50×10⁻⁶。这仅约为普通人群恶性肿瘤平均发生率的1/60。因此，加速器产生的中子对放疗患者造成的癌症风险相较于其他风险而言可忽略不计。
