Institut de Cancérologie de l'Ouest, Angers, Saint-Herblain, France.
Centre Eugène Marquis, Rennes, France.
Phys Med. 2021 Jan;81:191-196. doi: 10.1016/j.ejmp.2021.01.001. Epub 2021 Jan 16.
The determination of output factors in small field dosimetry is a crucial point, especially when implementing stereotactic radiotherapy (SRT). Herein, a working group of the French medical physicist society (SFPM) was created to collect small field output factors. The objective was to gather and disseminate information on small field output factors based on different detectors for various clinical SRT equipment and measurement configurations.
Participants were surveyed for information about their SRT equipment, including the type of linear particle accelerator (linac), collimator settings, measurement conditions for the output factors and the detectors used. Participants had to report both the ratio of detector readings and the correction factors applied as described in the IAEA TRS-483 code of practice for nominal field sizes smaller or equal to 3 cm. Mean field output factors and their associated standard deviations were calculated when data from at least 3 linacs were available.
23 centres were enrolled in the project. Standard deviations of the mean field output factors were systematically smaller than 1.5% for field sizes larger or equal to 1 cm and reached 5% for the smallest field size (0.5 cm). Deviations with published data were smaller than 2% except for the 0.5 cm circular fixed aperture collimator of the CyberKnife where it reached 3.5%.
These field output factor values obtained via a large multicentre study can be considered as an external cross verification for any radiotherapy centre starting a SRT program and should help minimize systematic errors when determining small field output factors.
在小射野剂量学中，输出因子的确定是一个关键点，尤其是在实施立体定向放射治疗（SRT）时。为此，法国医学物理学家学会（SFPM）成立了一个工作组来收集小射野输出因子。其目的是收集和传播基于不同探测器的不同 SRT 设备和测量配置的小射野输出因子的信息。
调查参与者有关 SRT 设备的信息，包括线性粒子加速器（linac）的类型、准直器设置、输出因子的测量条件以及使用的探测器。参与者必须报告探测器读数的比值以及 IAEA TRS-483 实践规程中所述的应用于标称射野尺寸小于或等于 3cm 的校正因子。当至少有 3 台 linac 的数据可用时，计算平均射野输出因子及其相关标准偏差。
有 23 个中心参与了该项目。对于大于或等于 1cm 的射野尺寸，平均射野输出因子的标准偏差系统地小于 1.5%，对于最小的射野尺寸（0.5cm）则达到 5%。除了 CyberKnife 的 0.5cm 圆形固定孔径准直器外，与已发表数据的偏差小于 2%，在该情况下达到 3.5%。
通过这项大型多中心研究获得的这些射野输出因子值可被视为任何开始 SRT 计划的放射治疗中心的外部交叉验证，并且有助于在确定小射野输出因子时最小化系统误差。
