Comparison of detector performance in small 6 MV and 6 MV FFF beams using a Versa HD accelerator

1. Background & purposeInvestigate the applicability of a series of detectors in small field dosimetry and the possible differences between their responses to FF and FFF beams. This work extends upon the series of detectors used by other authors to also include metal-oxide-semiconductor field-effect transistors (MOSFETs) detectors and radiochromic film. We also included a later correction of output factors (OFs) recommended by the recently published IAEA´s code of practice TRS 483 on dosimetry of small static fields used in external beam radiotherapy.2. Materials & methodsThe OFs, profiles, and PDDs of 6 MV and 6 MV FFF beams were measured with 11 different detectors using field sizes between 0.6 × 0.6 cm2 and 10 × 10 cm2.3. ResultsThe OFs of the FFF beams were lower than those of the FF beams for field sizes larger than 3 × 3 cm2 but higher for field sizes smaller than 3 × 3 cm2. After applying the IAEA´s TRS 483 corrections, the final OFs were compatible with our initial results when considering uncertainties involved. Small-volume detectors are preferable for measuring the penumbra of these small fields where this attribute is higher in the crossline direction than in the inline direction. The R100 of equivalent-quality FFF beams was higher compared to the corresponding flattened beams.4. ConclusionsWe observed no difference for the dose responses between 6 MV and 6 MV FFF beams for any of the detectors. OF results, profiles and PDDs were clearly consistent with the previously published literature regarding the Versa HD linac. Correcting our first OFs, taken as ratio of detector charges, with the IAEA´s TRS 483 corrections to obtain the final OFs, did not make the former significantly different.1. 研究背景与目的：本研究旨在探究多款探测器在小射野剂量学中的适用性，及其对均整射束（Flattened Filter, FF）与无均整器射束（Flattening Filter Free, FFF）的响应差异。本研究在既往研究者所使用的探测器系列基础上进行拓展，新增了金属氧化物半导体场效应晶体管（MOSFET）探测器与放射变色胶片（radiochromic film）；同时纳入了最新发布的国际原子能机构（International Atomic Energy Agency, IAEA）TRS 483号外照射静态小射野剂量学实践规范中推荐的输出因子（Output Factor, OF）修正方案。 2. 材料与方法：本研究采用11款不同探测器，对6 MV均整射束与6 MV无均整器射束的输出因子、剂量剖面及百分深度剂量（Percentage Depth Dose, PDD）进行了测量，所用射野尺寸范围为0.6 × 0.6 cm²至10 × 10 cm²。 3. 研究结果：当射野尺寸大于3 × 3 cm²时，无均整器射束的输出因子低于均整射束；而当射野尺寸小于3 × 3 cm²时则反之。采用IAEA TRS 483规范进行修正后，结合测量不确定度分析，最终得到的输出因子与本研究初始结果具有一致性。小体积探测器更适合测量此类小射野的半影：这类小射野的半影在横野方向上的尺寸大于沿野方向。同等照射质量的无均整器射束的R100值高于对应均整射束。 4. 研究结论：所有被测探测器均未表现出6 MV均整射束与6 MV无均整器射束的剂量响应存在差异。本研究得到的输出因子、剂量剖面及百分深度剂量结果，与针对Versa HD型直线加速器的既往发表文献高度吻合。本研究最初以探测器电荷比计算得到的输出因子，经IAEA TRS 483规范修正为最终输出因子后，二者并未出现显著差异。