Abstract

Purpose: Bearing partially or fully metallic passive implants represents an exclusion criterion for patients undergoing a magnetic hyperthermia procedure, but there are no specific studies backing this restrictive decision. This work assesses how the secondary magnetic field generated at the surface of two common types of prostheses affects the safety and efficiency of magnetic hyperthermia treatments of localized tumors. The paper also proposes the combination of a multi-criteria decision analysis and a graphical representation of calculated data as an initial screening during the preclinical risk assessment for each patient.

Materials and methods: Heating of a hip joint and a dental implant during the treatment of prostate, colorectal and head and neck tumors have been assessed considering different external field conditions and exposure times. The Maxwell equations including the secondary field produced by metallic prostheses have been solved numerically in a discretized computable human model. The heat exchange problem has been solved through a modified version of the Pennes' bioheat equation assuming a temperature dependency of blood perfusion and metabolic heat, i.e. thermorregulation. The degree of risk has been assessed using a risk index with parameters coming from custom graphs plotting the specific absorption rate (SAR) vs temperature increase, and coefficients derived from a multi-criteria decision analysis performed following the MACBETH approach.

Results: The comparison of two common biomaterials for passive implants - Ti6Al4V and CoCrMo - shows that both specific absorption rate (SAR) and local temperature increase are found to be higher for the hip prosthesis made by Ti6Al4V despite its lower electrical and thermal conductivity. By tracking the time evolution of temperature upon field application, it has been established that there is a 30 s delay between the time point for which the thermal equilibrium is reached at prostheses and tissues. Likewise, damage may appear in those tissues adjacent to the prostheses at initial stages of treatment, since recommended thermal thresholds are soon surpassed for higher field intensities. However, it has also been found that under some operational conditions the typical safety rule of staying below or attain a maximum temperature increase or SAR value is met.

Conclusion: The current exclusion criterion for implant-bearing patients in magnetic hyperthermia should be revised, since it may be too restrictive for a range of the typical field conditions used. Systematic in silico treatment planning using the proposed methodology after a well-focused diagnostic procedure can aid the clinical staff to find the appropriate limits for a safe treatment window.

摘要

目的：承载部分或全金属被动植入物代表了接受磁热疗程序的患者的排除标准，但没有具体研究支持这一限制性决定。这项工作评估了在两种常见假体表面产生的次级磁场如何影响局部肿瘤磁性热疗治疗的安全性和效率。该论文还建议将多标准决策分析和计算数据的图形表示相结合，作为对每位患者进行临床前风险评估期间的初步筛选。

材料和方法：考虑到不同的外场条件和暴露时间，已经评估了在治疗前列腺、结肠直肠和头颈部肿瘤期间对髋关节和牙科植入物的加热。麦克斯韦方程包括由金属假体产生的二次场，已经在离散化的可计算人体模型中进行了数值求解。热交换问题已通过 Pennes 生物热方程的修改版本解决，假设血液灌注和代谢热的温度依赖性，即温度调节。使用风险指数评估风险程度，其中参数来自绘制比吸收率 (SAR) 与温度增加的自定义图表，以及从遵循 MACBETH 方法执行的多标准决策分析得出的系数。

结果：用于被动植入物的两种常见生物材料 Ti6Al4V 和 CoCrMo 的比较表明，尽管 Ti6Al4V 的导电性和导热性较低，但发现由 Ti6Al4V 制成的髋关节假体的比吸收率 (SAR) 和局部温度升高都更高。通过跟踪现场应用时温度的时间演变，已经确定在假肢和组织达到热平衡的时间点之间存在 30 秒的延迟。同样，在治疗的初始阶段，与假体相邻的那些组织中可能会出现损伤，因为对于更高的场强度，很快就会超过推荐的热阈值。然而，

结论：目前磁热疗中植入物患者的排除标准应该修改，因为它可能对所使用的典型现场条件范围过于严格。在重点明确的诊断程序之后使用所提出的方法进行系统的计算机化治疗计划可以帮助临床工作人员找到安全治疗窗口的适当限制。