背景:在过去,GRID疗法被用作使用正电压束治疗大体积和深层肿瘤的治疗方式。现在,随着巨伏束被引入放射治疗,一些放射治疗机构使用GRID疗法与巨伏光子用于大肿瘤的姑息治疗。由于GRID可能是削弱医用直线加速器(LINAC)头部产生的光中子的障碍,以及产生光中子的次要来源,因此,在辐射防护方面,评估GRID对患者的光中子剂量的影响很重要。
方法:在本研究中,使用蒙特卡洛代码MCNPX,对LINAC的完整模型进行了仿真和验证。LINAC的中子源强度(Q),通量(φ)的分布,在有/没有GRID的两种情况下,在治疗表上计算中子的环境剂量当量(H*[10])。最后,在有/没有GRID的情况下,计算MIRD体模的某些组织/器官中中子的吸收剂量和剂量当量。
结果:我们的结果表明,GRID仅将LINAC头部中光中子的产量提高了0.3%。MIRD体模中的计算表明,GRID覆盖的器官/组织中的中子剂量平均比常规放射疗法低48%。此外,在未发现的器官中(通过GRID),这一数额减少到25%。
结论:根据本研究的结果,在GRID治疗技术与常规放射治疗相比,身体组织/器官中的中子剂量急剧减少。因此,不会担心GRID对不必要中子剂量增加的影响,从而导致继发性癌症的风险。
BACKGROUND: In the past, GRID therapy was used as a treatment modality for the treatment of bulky and deeply seated tumors with orthovoltage beams. Now and with the introduction of megavoltage beams to radiotherapy, some of the radiotherapy institutes use GRID therapy with megavoltage photons for the palliative treatment of bulky tumors. Since GRID can be a barrier for weakening the photoneutrons produced in the head of medical linear accelerators (LINAC), as well as a secondary source for producing photoneutrons, therefore, in terms of radiation protection, it is important to evaluate the GRID effect on photoneutron dose to the patients.
METHODS: In this
study, using the Monte Carlo code MCNPX, a full model of a LINAC was simulated and verified. The neutron source strength of the LINAC (Q), the distributions of flux (φ), and ambient dose equivalent (H*[10]) of neutrons were calculated on the treatment table in both cases of with/without the GRID. Finally, absorbed dose and dose equivalent of neutrons in some of the tissues/organs of MIRD phantom were computed with/without the GRID.
RESULTS: Our results indicate that the GRID increases the production of the photoneutrons in the LINAC head only by 0.3%. The calculations in the MIRD phantom show that neutron dose in the organs/tissues covered by the GRID is on average by 48% lower than conventional radiotherapy. In addition, in the uncovered organs (by the GRID), this amount is reduced to 25%.
CONCLUSIONS: Based on the findings of this
study, in GRID therapy technique compared to conventional radiotherapy, the neutron dose in the tissues/organs of the body is dramatically reduced. Therefore, there will be no concern about the GRID effect on the increase of unwanted neutron dose, and consequently the risk of secondary cancer.