In nuclear medicine, estimating the number of radioactive decays that occur in a source organ per unit administered activity of a radiopharmaceutical (i.e., the time-integrated activity coefficient [TIAC]) is an essential task within the internal dosimetry workflow. TIAC estimation is commonly derived by least-squares fitting of various exponential models to organ time–activity data (radiopharmaceutical biodistribution). Rarely, however, are methods used to objectively determine the model that best characterizes the data. Additionally, the uncertainty associated with the resultant TIAC is generally not evaluated. As part of the MIRDsoft initiative, MIRDfit has been developed to offer a biodistribution fitting software solution that provides the following essential features and advantages for internal dose assessment: nuclear medicine–appropriate fit functions; objective metrics for guiding best-fit selection; TIAC uncertainty calculation; quality control and data archiving; integration with MIRDcalc software for dose calculation; and a user-friendly Excel-based interface. For demonstration and comparative validation of MIRDfit’s performance, TIACs were derived from serial imaging studies involving ¹⁸F-FDG and ¹⁷⁷Lu-DOTATATE using MIRDfit. These TIACs were then compared with TIAC estimates obtained using other software. In most cases, the TIACs agreed within approximately 10% between MIRDfit and the other software. MIRDfit has been endorsed by the MIRD Committee of the Society of Nuclear Medicine and Molecular Imaging and has been integrated into the MIRDsoft suite of free dosimetry software; it is available for download at no user cost (https://mirdsoft.org/).

在核医学中，估计放射性药物每单位施用活性（即时间积分活性系数 [TIAC]）在源器官中发生的放射性衰变数量是内部剂量学工作流程中的一项基本任务。TIAC 估计通常是通过将各种指数模型与器官时间-活性数据（放射性药物生物分布）进行最小二乘拟合得出的。然而，很少有方法用于客观地确定最能描述数据的模型。此外，通常不评估与结果TIAC相关的不确定性。作为 MIRDsoft 计划的一部分，MIRDfit 的开发旨在提供生物分布拟合软件解决方案，该解决方案为内部剂量评估提供以下基本功能和优势：核医学适当的拟合函数;指导最适合选择的客观指标;TIAC 不确定度计算;质量控制和数据存档;与 MIRDcalc 软件集成以进行剂量计算;以及用户友好的基于 Excel 的界面。为了演示和比较验证 MIRDfit 的性能，TIAC 来自使用 MIRDfit 的 ¹⁸个 F-FDG 和 ¹⁷⁷个 Lu-DOTATATE 的系列成像研究。然后将这些 TIAC 与使用其他软件获得的 TIAC 估计值进行比较。在大多数情况下，TIAC 在 MIRDfit 和其他软件之间达成的协议在大约 10% 以内。MIRDfit 已获得核医学和分子影像学会 MIRD 委员会的认可，并已集成到 MIRDsoft 免费剂量测定软件套件中;它可免费下载 （https://mirdsoft.org/）。