Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan 100, Utrecht, 3584CX, The Netherlands.
VSL - Dutch Metrology Institute, Delft, The Netherlands.
Med Phys. 2019 Aug;46(8):3739-3745. doi: 10.1002/mp.13623. Epub 2019 Jun 17.
Reference dosimetry in a strong magnetic field is made more complex due to (a) the change in dose deposition and (b) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, relative orientations of field, chamber and radiation, and minor variations in ion chamber stem or electrode construction. The PTW30013 and IBA FC65-G detectors are waterproof Farmer-type ion chambers that are suitable for reference dosimetry. The magnetic field correction factors have previously been determined for these chamber types. The aim of this study was to assess the chamber-to-chamber variation and determine whether generic chamber type-specific magnetic field correction factors can be applied for each of the PTW30013 and FC65-G type ion chambers when they are oriented anti-parallel (ǁ) to, or perpendicular (⊥) to, the magnetic field.
The experiment was conducted with 12 PTW30013 and 13 FC65-G chambers. The magnetic field correction factors were measured using a practical method. In this study each chamber was cross-calibrated against the local standard chamber twice; with and without magnetic field. Measurements with 1.5 T magnetic field were performed with the 7 MV FFF beam of the MRI-linac. Measurements without magnetic field (0 T) were performed with the 6 MV conventional beam of an Elekta Agility linac. A prototype MR-compatible PTW MP1 phantom was used along with a prototype holder that facilitated measurements with the chamber aligned 90° counter-clockwise (⊥) and 180° (ǁ) to the direction of the magnetic field. A monitor chamber was also mounted on the holder and all measurements were normalized so that the effect of variations in the output of each linac was minimized. Measurements with the local standard chamber were repeated during the experiment to quantify the experimental uncertainty. Recombination was measured in the 6 MV beam. Beam quality correction factors were applied. Differences in recombination and beam quality between beams are constant within each chamber type. By comparing the results for the two cross calibrations the magnetic field correction factors can be determined for each chamber, and the variation within the chamber-type determined.
The magnetic field correction factors within both PTW30013 and FC65-G chamber-types were found to be very consistent, with observed standard deviations for the PTW30013 of 0.19% (ǁ) and 0.13% (⊥), and for the FC65-G of 0.15% (ǁ) and 0.17% (⊥). These variations are comparable with the standard uncertainty (k = 1) of 0.24%.
The consistency of the results for the PTW30013 and FC65-G chambers implies that it is not necessary to derive a new factor for every new PTW30013 or FC65-G chamber. Values for each chamber-type (with careful attention to beam energy, magnetic field strength and beam-field-chamber orientations) can be applied from the literature.
由于(a)剂量沉积的变化和(b)探测器灵敏度的变化，强磁场中的参考剂量测定变得更加复杂。潜在地，它也受到薄空气层、介质之间的界面、场、腔和辐射的相对取向以及离子腔杆或电极结构的微小变化的影响。PTW30013 和 IBA FC65-G 探测器是防水型 Farmer 型离子室，适用于参考剂量测定。这些腔室类型的磁场校正因子先前已经确定。本研究的目的是评估腔室间的变化，并确定当 PTW30013 和 FC65-G 型离子室相对于磁场平行（ǁ）或垂直（⊥）时，是否可以为每个 PTW30013 和 FC65-G 型离子室应用通用的腔室类型特定的磁场校正因子。
该实验使用了 12 个 PTW30013 和 13 个 FC65-G 室进行。使用实际方法测量了磁场校正因子。在这项研究中，每个腔室都使用当地标准腔室进行了两次交叉校准；有和没有磁场。使用 MRI 直线加速器的 7 MV FFF 束进行了 1.5 T 磁场的测量。没有磁场（0 T）的测量是使用 Elekta Agility 直线加速器的 6 MV 常规束进行的。使用原型 MR 兼容的 PTW MP1 体模以及一个原型支架进行了测量，该支架使腔室能够以 90°逆时针（⊥）和 180°（ǁ）的方向与磁场对齐。还在支架上安装了一个监测腔室，并对所有测量进行了归一化，以使每个直线加速器输出变化的影响最小化。在实验过程中重复了对当地标准腔室的测量，以量化实验不确定度。在 6 MV 束中测量了复合。应用了束质校正因子。束之间的复合和束质的差异在每个腔室类型内是恒定的。通过比较两次交叉校准的结果，可以确定每个腔室的磁场校正因子，并确定腔室类型内的变化。
发现 PTW30013 和 FC65-G 腔室类型内的磁场校正因子非常一致，PTW30013 的观测标准偏差为 0.19%（ǁ）和 0.13%（⊥），FC65-G 的观测标准偏差为 0.15%（ǁ）和 0.17%（⊥）。这些变化与标准不确定度（k=1）的 0.24%相当。
PTW30013 和 FC65-G 腔室结果的一致性表明，不需要为每个新的 PTW30013 或 FC65-G 腔室推导新的因子。可以从文献中为每个腔室类型（注意束能、磁场强度和束场腔室取向）应用值。
