National Physical Laboratory, Teddington, United Kingdom. Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom. Author to whom any correspondence should be addressed.
Phys Med Biol. 2020 Jun 3;65(11):115001. doi: 10.1088/1361-6560/ab8148.
Reference dosimetry in the presence of a strong magnetic field is challenging. Ionisation chambers have shown to be strongly affected by magnetic fields. There is a need for robust and stable detectors in MRI-guided radiotherapy (MRIgRT). This study investigates the behaviour of the alanine dosimeter in magnetic fields and assesses its suitability to act as a reference detector in MRIgRT. Alanine pellets were loaded in a waterproof holder, placed in an electromagnet and irradiated by Co and 6 MV and 8 MV linac beams over a range of magnetic flux densities. Monte Carlo simulations were performed to calculate the absorbed dose, to water and to alanine, with and without magnetic fields. Combining measurements with simulations, the effect of magnetic fields on alanine response was quantified and a correction factor for the presence of magnetic fields on alanine was determined. This study finds that the response of alanine to ionising radiation is modified when the irradiation is in the presence of a magnetic field. The effect is energy independent and may increase the alanine/electron paramagnetic resonance (EPR) signal by 0.2% at 0.35 T and 0.7% at 1.5 T. In alanine dosimetry for MRIgRT, this effect, if left uncorrected, would lead to an overestimate of dose. Accordingly, a correction factor, [Formula: see text], is defined. Values are obtained for this correction as a function of magnetic flux density, with a standard uncertainty which depends on the magnetic field and is 0.6% or less. The strong magnetic field has a measurable effect on alanine dosimetry. For alanine which is used to measure absorbed dose to water in a strong magnetic field, but which has been calibrated in the absence of a magnetic field, a small correction to the reported dose is required. With the inclusion of this correction, alanine/EPR is a suitable reference dosimeter for measurements in MRIgRT.
在强磁场存在的情况下进行参考剂量测定具有挑战性。电离室已被证明会受到磁场的强烈影响。在 MRI 引导的放射治疗（MRIgRT）中需要稳健且稳定的探测器。本研究调查了丙氨酸剂量计在磁场中的行为，并评估了其作为 MRIgRT 中参考探测器的适用性。将丙氨酸小球装入防水支架中，放置在电磁铁中，并在一系列磁通密度下用 Co 和 6 MV 和 8 MV 直线加速器照射。进行了蒙特卡罗模拟以计算有水和无磁场存在时的吸收剂量、水和丙氨酸的吸收剂量。通过将测量值与模拟值相结合，量化了磁场对丙氨酸响应的影响，并确定了磁场对丙氨酸存在的修正因子。本研究发现，当辐照存在磁场时，丙氨酸对电离辐射的响应会发生变化。这种影响与能量无关，在 0.35 T 时，丙氨酸/电子顺磁共振（EPR）信号可能增加 0.2%，在 1.5 T 时增加 0.7%。在 MRIgRT 中的丙氨酸剂量测定中，如果不进行修正，这种影响会导致剂量高估。因此，定义了修正因子 [Formula: see text]。该修正因子的值作为磁通密度的函数获得，其标准不确定度取决于磁场，且不超过 0.6%。强磁场对丙氨酸剂量测定有可测量的影响。对于在强磁场中用于测量水吸收剂量的丙氨酸，但在没有磁场的情况下进行校准的丙氨酸，需要对报告的剂量进行小修正。包含此修正后，丙氨酸/EPR 是 MRIgRT 中测量的合适参考剂量计。
