⁹⁰Y-FAPI therapy was recently introduced as novel treatment concept for patients with solid tumors. Lesion and organ-at-risk dosimetry is part of assessing treatment efficacy and safety and requires reliable quantification of tissue uptake. As ⁹⁰Y quantification is limited by the low internal positron-electron pair conversion rate, the increased effective sensitivity, due to improved time-of-flight resolution, of digital silicon photomultiplier-based PET/CT systems might increase quantification accuracy and, consequently, allow for dosimetry in ⁹⁰Y-FAPI therapy. The aim of this study was to explore the conditions for reliable lesion image quantification in ⁹⁰Y-FAPI radionuclide therapy using a digital PET/CT system. Methods: Two tumor phantoms were filled with ⁹⁰Y solution using different sphere activity concentrations and a constant signal-to-background ratio of 40. The minimum detectable activity concentration was determined and its dependency from acquisition time (15 min vs. 30 min per bed) and smoothing levels (all-pass vs. 5-mm Gaussian filter) was investigated. Quantification accuracy was evaluated at varying activity concentrations to estimate the minimum quantifiable activity concentration based on a contour- and an oversize-based quantification approach. A ±20% deviation range between imaged-derived and true activity concentrations was regarded acceptable. Tumor dosimetry of three ⁹⁰Y-FAPI therapy patients is presented to project the phantom results to clinical scenarios. Results: For a lesion size of 40 mm and a clinical acquisition time of 15 min, both minimum detectable and minimum quantifiable activity concentrations were 0.12 MBq/mL. For lesion sizes ≥30 mm, accurate quantification was feasible for detectable lesions. Only for the smallest 10-mm sphere, minimum detectable and minimum quantifiable activity concentration differed substantially (0.43 vs. 1.97 MBq/mL). No notable differences between the two quantification approaches were observed. For the investigated tumors, absorbed dose estimates with a reliable accuracy were achievable. Conclusion: For lesion sizes and activity concentrations that are expected to be observed in ⁹⁰Y-FAPI patients, quantification with reasonable accuracy is possible. Further dosimetry studies are needed to thoroughly investigate efficacy and safety of ⁹⁰Y-FAPI therapy.

⁹⁰ Y-FAPI 疗法最近作为实体瘤患者的新治疗概念被引入。病变和危险器官剂量测定是评估治疗效果和安全性的一部分，需要对组织吸收进行可靠的量化。由于⁹⁰ Y 定量受到低内部正电子-电子对转换率的限制，由于飞行时间分辨率的提高，基于数字硅光电倍增管的 PET/CT 系统的有效灵敏度可能会提高定量精度，因此，允许在⁹⁰ Y-FAPI 治疗中进行剂量测定。本研究的目的是探索使用数字 PET/CT 系统进行^{90 Y-FAPI 放射性核素治疗时可靠的病灶图像量化的条件。}方法：使用不同的球体活性浓度和 40 的恒定信号背景比，将两个肿瘤模型填充 90 Y 溶液。确定最小可检测活性浓度及其与采集时间的依赖性（每床 15 分钟与 30分钟^））和平滑级别（全通与 5 毫米高斯滤波器）进行了研究。在不同的活性浓度下评估量化准确性，以基于轮廓和基于超大的量化方法估计最小可量化活性浓度。图像得出的活性浓度与真实活性浓度之间的 ±20% 偏差范围被认为是可接受的。^{介绍了 3 名90 名}Y-FAPI 治疗患者的肿瘤剂量测定，将模型结果投射到临床场景中。结果：对于 40 mm 的病灶大小和 15 分钟的临床采集时间，最小可检测和最小可量化活性浓度均为 0.12 MBq/mL。对于病灶尺寸≥30 mm，可对可检测病灶进行准确量化。仅对于最小的 10 毫米球体，最小可检测和最小可定量活性浓度存在显着差异（0.43 与 1.97 MBq/mL）。两种量化方法之间没有观察到显着差异。对于所研究的肿瘤，可以实现具有可靠准确性的吸收剂量估计。结论：对于预计在^{90 名}Y-FAPI 患者中观察到的病灶大小和活性浓度，可以进行合理准确度的量化。需要进一步的剂量测定研究来彻底研究⁹⁰ Y-FAPI 治疗的功效和安全性。