Background/Objectives: The aim of this study was to establish the relationship between target size and the required diagnostic PET maximum standard uptake value (SUVmax) thresholds needed for successful Biology-guided Radiotherapy (BgRT) delivery on RefleXion X1 PET-linac. The current clinical eligibility recommendation is an SUVmax ≥ 6 at simulation, but the RefleXion system subsequently evaluates Activity Concentration (AC), which must exceed 5 kBq/mL for successful BgRT planning. Methods: A custom 3D-printed phantom containing six spherical targets (8 to 20 mm diameter) was used with varying target-to-background ratios (5:1 to 20:1) of 18F-FDG to systematically achieve a range of SUVmax values for each target size. Images were acquired on Siemens Biograph mCT for SUVmax quantification and RefleXion X1 for AC measurements. Twenty-four BgRT plans were evaluated, and delivery accuracy was validated using ArcCHECK. Additionally, retrospective data from 18 patients across four institutions were analyzed to validate the phantom-derived findings. Results: The PET-linac successfully planned treatments for 13/24 experiments, all achieving an AC > 5 kBq/mL. SUVmax requirements varied by target size: 16–20 mm targets required an SUVmax > 6, consistent with current recommendations, while smaller targets required higher thresholds (e.g., 13 mm: SUVmax > 10, and 11 mm: SUVmax > 15). 8 and 9 mm targets failed to meet AC requirements even at SUVmax 14. Successful deliveries maintained acceptable accuracy, with gamma passing rates of 92.4% ± 5.0% (3%/2 mm) and 97.6% ± 1.9% (3%/3 mm). Analysis revealed that Volume (cc) × SUVmax > 11 consistently predicted successful BgRT planning across all target sizes. This threshold was validated using multi-institutional PET-CT patient data (mean: 11.36, 95% CI: 9.1–12.9), correctly predicting treatment eligibility in 15 of 18 cases. Conclusions: Target size significantly influences BgRT eligibility. We derived a new criterion, Volume(cc) × SUVmax > 11 (95% CI: 9.1–12).
背景/目的：本研究旨在明确RefleXion X1 PET直线加速器上实施生物引导放射治疗（BgRT）时，靶区大小与所需诊断性PET最大标准摄取值（SUVmax）阈值之间的关系。当前临床适用性推荐标准为模拟定位时SUVmax ≥ 6，但RefleXion系统实际评估的是活度浓度（AC），其值需超过5 kBq/mL才能成功制定BgRT计划。 方法：采用定制3D打印体模，内含六个球形靶区（直径8至20毫米），通过调节¹⁸F-FDG靶本比（5:1至20:1）系统性地实现各靶区尺寸对应的SUVmax值范围。在西门子Biograph mCT上进行SUVmax定量成像，在RefleXion X1上进行AC测量。评估了24个BgRT计划，并使用ArcCHECK验证实施精度。此外，通过分析来自四家机构的18例患者回顾性数据，验证体模实验结论。 结果：PET直线加速器成功制定了13/24次实验的治疗计划，所有成功案例均满足AC > 5 kBq/mL。SUVmax要求随靶区尺寸变化：16–20毫米靶区需SUVmax > 6（与现行推荐标准一致），而较小靶区需要更高阈值（如13毫米靶区需SUVmax > 10，11毫米靶区需SUVmax > 15）。8毫米和9毫米靶区即使在SUVmax达14时仍无法满足AC要求。成功实施的治疗计划均保持可接受的精度，伽马通过率分别为92.4% ± 5.0%（3%/2 mm标准）和97.6% ± 1.9%（3%/3 mm标准）。分析发现，体积（立方厘米）× SUVmax > 11这一指标能稳定预测所有靶区尺寸的BgRT计划成功率。该阈值通过多机构PET-CT患者数据验证（均值：11.36，95%置信区间：9.1–12.9），在18例病例中正确预测了15例的治疗适用性。 结论：靶区尺寸显著影响BgRT适用性。我们推导出新标准：体积（立方厘米）× SUVmax > 11（95%置信区间：9.1–12）。
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