Assessment of out-of-field absorbed dose and equivalent dose in proton fields
Assessment of out-of-field absorbed dose and equivalent dose in proton fields
复制标题
DOI：
10.1118/1.3271390
复制
发表时间：
2010-01-01
期刊：
影响因子：
3.8
通讯作者：
Rosenfeld, Anatoly
中科院分区：
文献类型：
--
作者：
Clasie, Ben;Wroe, Andrew;Rosenfeld, Anatoly
目的：在质子治疗中，与其他形式的放射治疗一样，散射和次级粒子在靶体积外产生不期望的剂量，这可能增加辐射诱导的继发性癌症的风险，并与治疗室中的电子设备相互作用。作者实现了被动散射野外沉积剂量的蒙特卡罗模型，并将其与测量值进行比较，确定了野外等效剂量，并估计了如果采用主动射束扫描技术治疗相同靶体积时的剂量变化。方法：使用顶针电离室和Lucite体模内的Wellhofer MatriXX检测器进行测量，其中场配置基于前列腺癌和髓母细胞瘤的治疗。作者使用GEANT 4蒙特卡罗模拟，证明同意以及与测量内的主要领域，模拟领域提供的测量。部分贡献的剂量分离在模拟中的粒子类型和origin.Results：在场外吸收剂量的实验和模拟之间的协议是在30%以内，在10-20厘米从字段边缘和90%的数据同意在2个标准偏差。在被动散射中，中子对总剂量的贡献主要在布拉格峰下游区域（65%-80%，由于内部产生的中子）和体模内部，距离场边缘超过10-15 cm。对于前列腺癌和颅髓母细胞瘤射野，在体模入口处和距离射野边缘20 cm处的中子加权因子使用10的等效剂量分别为2.2和2.6 mSv/戈伊。在被动散射中，距射野边缘15-20 cm处的等效剂量随深度而减小，而在主动扫描中随深度而增大。因此，主动扫描具有较小的外地等效剂量的因素的30-45在入口region和这个因素随depth.Conclusions：沉积的剂量立即下游的主要领域，在这些情况下，主要是由内部产生的中子，因此，散射和扫描字段可能有类似的风险，在这个区域的第二个癌症。作者证实，主动扫描中的场外剂量降低，但影响随深度而降低。GEANT 4适用于模拟主野外沉积的剂量。与测量的协议是可比的或更好的协议报告的其他实施的Monte Carlo模型。取决于位置，初级场外的吸收剂量主要由初级质子的贡献决定，初级质子可能在黄铜准直装置中散射，也可能没有散射。这是值得注意的，因为低LET质子的品质因数是众所周知的，因此可以准确地评估该区域的相对剂量风险。(C)2010年美国医学物理学家协会。[DOI 10.1118/1.3271390]
Purpose: In proton therapy, as in other forms of radiation therapy, scattered and secondary particles produce undesired dose outside the target volume that may increase the risk of radiation-induced secondary cancer and interact with electronic devices in the treatment room. The authors implement a Monte Carlo model of this dose deposited outside passively scattered fields and compare it to measurements, determine the out-of-field equivalent dose, and estimate the change in the dose if the same target volumes were treated with an active beam scanning technique.Methods: Measurements are done with a thimble ionization chamber and the Wellhofer MatriXX detector inside a Lucite phantom with field configurations based on the treatment of prostate cancer and medulloblastoma. The authors use a GEANT4 Monte Carlo simulation, demonstrated to agree well with measurements inside the primary field, to simulate fields delivered in the measurements. The partial contributions to the dose are separated in the simulation by particle type and origin.Results: The agreement between experiment and simulation in the out-of-field absorbed dose is within 30% at 10-20 cm from the field edge and 90% of the data agrees within 2 standard deviations. In passive scattering, the neutron contribution to the total dose dominates in the region downstream of the Bragg peak (65%-80% due to internally produced neutrons) and inside the phantom at distances more than 10-15 cm from the field edge. The equivalent doses using 10 for the neutron weighting factor at the entrance to the phantom and at 20 cm from the field edge are 2.2 and 2.6 mSv/Gy for the prostate cancer and cranial medulloblastoma fields, respectively. The equivalent dose at 15-20 cm from the field edge decreases with depth in passive scattering and increases with depth in active scanning. Therefore, active scanning has smaller out-of-field equivalent dose by factors of 30-45 in the entrance region and this factor decreases with depth.Conclusions: The dose deposited immediately downstream of the primary field, in these cases, is dominated by internally produced neutrons; therefore, scattered and scanned fields may have similar risk of second cancer in this region. The authors confirm that there is a reduction in the out-of-field dose in active scanning but the effect decreases with depth. GEANT4 is suitable for simulating the dose deposited outside the primary field. The agreement with measurements is comparable to or better than the agreement reported for other implementations of Monte Carlo models. Depending on the position, the absorbed dose outside the primary field is dominated by contributions from primary protons that may or may not have scattered in the brass collimating devices. This is noteworthy as the quality factor of the low LET protons is well known and the relative dose risk in this region can thus be assessed accurately. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3271390]