Mallory Richard C, Woods Kyle, McCullough Kiernan
Colorado Associates in Medical Physics, Colorado Springs, Colorado, USA.
J Appl Clin Med Phys. 2025 Jul;26(7):e70174. doi: 10.1002/acm2.70174.
Brass-mesh-bolus (BMB) has been proposed as an alternative to water-equivalent bolus due to its ease of setup and conformality to patient contour. In high-energy beams, BMB can become radioactive and pose a potential exposure risk to therapists from frequent exposure while handling. Very little research has been performed on the method of activation and dose from clinically realistic activation of BMB, necessitating the need for further investigation.
To determine the expected activation of the BMB, products via neutron-capture were calculated using thermal-neutron cross-section tables, as has been performed in previous literature. A novel consideration is to include the photo-neutron activation components. Measurements were performed using an NaI-scintillator after BMB irradiation, assessing its activity for both in-field and out-of-field activation. The collected gamma spectrum was compared to expected peaks based on differing activation modes. Energy peaks are isolated for half-life measurements to find relative ratios and potential long-term activation risk. Potential exposure to workers after multiple irradiation events was measured using a Ludlum 9DP and BeO OSLDs.
Expected radionuclide products are predicted depending on activation mode: Cu-64, Cu-66, Zn-65, and Zn-69 via neutron capture and Cu-62, Cu-64, Zn-63, and Zn-65 via photo-neutron activation. Gamma-spectrum analysis shows that photo-neutron activation is the dominant source of exposure with decay via positron emission. The effective half-life of the 511 keV positron annihilation peak is measured as 12.16 min, mostly from Cu-62 and Zn-63. Survey measurements indicate the dose to workers of the BMB to be 0.106 mrem per course of a 25 fx, 400 MU/fx, 15 MV treatment if the therapist handles the BMB every fx for 30 s. Using the OSLDs, under the same conditions but handling the bolus for 1 min per fraction, skin dose was estimated as 9.23 mrem per course.
The dominant source of exposure from BMB is the result of photo-neutron activation rather than neutron-capture, as was historically assumed. Penetration of the resultant positron emission is considerable given the presence of annihilation photons compared to previously assumed beta decay and should be considered for exposure to workers.
由于黄铜网模体（BMB）易于设置且能与患者轮廓贴合，已被提议作为水等效模体的替代物。在高能束流中，BMB会具有放射性，在操作过程中因频繁接触而对治疗师构成潜在的暴露风险。关于BMB临床实际活化的活化方法和剂量的研究极少，因此有必要进一步开展调查。
为确定BMB的预期活化情况，如先前文献中所做的那样，使用热中子截面表计算通过中子俘获产生的产物。一个新的考虑因素是纳入光中子活化成分。在BMB辐照后，使用碘化钠闪烁体进行测量，评估其场内和场外活化的活性。将收集到的伽马能谱与基于不同活化模式的预期峰进行比较。分离出能量峰进行半衰期测量，以确定相对比率和潜在的长期活化风险。使用卢德伦9DP和氧化铍光激发光剂量计测量多次辐照事件后工作人员的潜在暴露情况。
根据活化模式预测了预期的放射性核素产物：通过中子俘获产生铜 - 64、铜 - 66、锌 - 65和锌 - 69，通过光中子活化产生铜 - 62、铜 - 64、锌 - 63和锌 - 65。伽马能谱分析表明，光中子活化是通过正电子发射衰变的主要暴露源。511 keV正电子湮灭峰的有效半衰期测量为12.16分钟，主要来自铜 - 62和锌 - 63。调查测量表明，如果治疗师每次照射时处理BMB 30秒，在25次分割、每次分割400 MU、15 MV的治疗过程中，BMB对工作人员的剂量为0.106毫雷姆。使用光激发光剂量计，在相同条件下但每次分割处理模体1分钟，估计每个疗程的皮肤剂量为9.23毫雷姆。
BMB的主要暴露源是光中子活化的结果，而非如以往所认为的中子俘获。与先前假设的β衰变相比，考虑到湮灭光子的存在，由此产生的正电子发射的穿透性相当可观，在评估工作人员暴露时应予以考虑。
