Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

中文翻译：

---

AAPM Task Group 155 报告：小场和非平衡条件下的兆伏级光子束剂量学

由于横向带电粒子平衡 (LCPE) 的丧失、初级光子源的遮挡以及合适的辐射探测器的选择有限，用于预先治疗技术的小场剂量测定带来了挑战。这些挑战极大地影响了剂量测定的准确性。许多引人注目的辐射事件表明，人们对小场剂量测定的适当方法缺乏了解。这些事件令人担忧，因为在各种专门的放射治疗技术中小区域的使用继续快速增长。小场和复合场中的参考和相对剂量学是国际原子能机构 (IAEA) 剂量学实践规范的主题，该规范已作为 TRS-483 和 AAPM 摘要出版物 (IAEA TRS 483；外束放射治疗中使用的小静场剂量学：IAEA/AAPM 参考和相对剂量测定国际操作规范，技术报告系列第 483 号；Palmans 等，Med Phys 45(11):e1123, 2018)。AAPM 任务组 155 (TG-155) 的职责是总结小场剂量学的当前知识，并为小兆电压光子束中的相对剂量测定提供最佳实践建议。提供了 LCPE 问题的概述以及光子束扰动随场尺寸减小的变化。包括有关适当检测器系统和测量方法的建议。现有已发表的关于小光子场剂量学参数的数据（例如，百分比深度剂量、组织体模比率/组织最大比率、离轴比率、和场输出因素）以及对各种探测器的必要扰动校正进行了审查。提供了关于测量中的误差和不确定性分析的讨论。在治疗计划系统中设计射束模型以模拟小场需要特别注意主射束源和准直装置在能量注量和剂量计算中的影响。回顾了适用于小范围剂量建模的通量和剂量计算引擎的一般要求。商业治疗计划系统中的实现差异很大，本报告的目的是为医学物理学家提供洞察力，并为放射治疗计划系统的波束模型开发人员提供指导。