Small functional pituitary tumors can cause severely disabling symptoms and early death. The gold standard diagnostic approach includes laboratory tests and MRI, with or without inferior petrosal sinus sampling (IPSS). In up to 40% of patients, however, the source of excess hormone production remains unidentified or uncertain. This excludes patients from surgical, Gamma Knife, and CyberKnife therapy and adversely affects overall cure rates. We here assess the diagnostic yield of O-(2-[¹⁸F]fluoroethyl)-l-tyrosine ([¹⁸F]FET) PET/MRI for detection of small functional pituitary tumors in these patients. Methods: This retrospective analysis included patients with Cushing disease (CD) but prior negative or inconclusive MRI results who underwent [¹⁸F]FET PET/MRI between February 1, 2021, and December 1, 2022. PET/MR images and MR images alone were evaluated by experienced nuclear radiologists, neuroradiologists, or radiologists. Postoperative tissue analysis (when performed) was used as a reference standard to assess diagnostic metrics (i.e., sensitivity and positive predictive value). Results were also compared with previously obtained MR images, preceding IPSS, and clinical or biochemical follow-up. Results: Twenty-two patients (68% female; mean age ± SD, 48 ± 15 y; range, 24–68 y) were scanned. All patients showed a clear metabolic focus on [¹⁸F]FET PET, whereas reading of the MRI alone yielded a suspected lesion in only 50%. Fifteen patients underwent surgery directed at the [¹⁸F]FET-positive focus. Tissue analysis confirmed a pituitary adenoma/pituitary neuroendocrine tumor of the corticotroph cell type (TPIT lineage) in 10 of 15 and a pituicytoma in 1 of 15, rendering a sensitivity of 100% and a positive predictive value of 73%. Lateralization was more accurate with [¹⁸F]FET PET/MRI than with IPSS in 33%. Twelve of 16 (75%) patients who received surgical, Gamma Knife, or CyberKnife therapy after [¹⁸F]FET PET/MRI reached short-term remission. Conclusion: [¹⁸F]FET PET/MRI shows a high diagnostic yield for localizing small functional pituitary tumors. This multimodal imaging technique provides a welcome improvement for diagnosis, planning of surgery, and clinical outcome in patients with Cushing disease, particularly those with repeated negative or inconclusive MRI results with or without IPSS.

小型功能性垂体瘤可导致严重的残疾症状和过早死亡。金标准诊断方法包括实验室检查和 MRI，有或没有下岩窦取样 (IPSS)。然而，在高达 40% 的患者中，过量激素产生的来源仍未确定或不确定。这使得患者无法接受手术、伽玛刀和射波刀治疗，并对总体治愈率产生不利影响。我们在此评估O -(2-[ ¹⁸ F]氟乙基)-的诊断率我-酪氨酸（[ ¹⁸ F]FET）PET/MRI 用于检测这些患者的小型功能性垂体瘤。方法：这项回顾性分析包括 2021 年 2 月 1 日至 2022 年 12 月 1 日期间接受 [ ¹⁸ F]FET PET/MRI的库欣病 (CD) 但既往 MRI 结果为阴性或不确定的患者。PET /MR 图像和单独 MR 图像由经验丰富的核放射科医生、神经放射科医生或放射科医生进行评估。术后组织分析（当进行时）用作评估诊断指标（即敏感性和阳性预测值）的参考标准。结果还与之前获得的 MR 图像、之前的 IPSS 以及临床或生化随访进行了比较。结果：对22 名患者（68% 为女性；平均年龄 ± SD，48 ± 15 岁；范围，24-68 岁）进行了扫描。所有患者在 [ ¹⁸ F]FET PET上均显示出明显的代谢焦点，而仅 MRI 读数仅在 50% 的患者中产生可疑病变。 15 名患者接受了针对 [ ¹⁸ F]FET 阳性病灶的手术。组织分析证实 15 例中有 10 例为促肾上腺皮质激素细胞型（TPIT 谱系）垂体腺瘤/垂体神经内分泌肿瘤，15 例中有 1 例为垂体细胞瘤，敏感性为 100%，阳性预测值为 73%。 [ ¹⁸ F]FET PET/MRI的侧化准确率比 IPSS 的准确率高 33%。在 [ ¹⁸ F]FET PET/MRI后接受手术、伽玛刀或射波刀治疗的 16 名患者中，有 12 名 (75%)达到短期缓解。结论： [ ¹⁸ F]FET PET/MRI 对于定位小型功能性垂体肿瘤具有较高的诊断率。这种多模态成像技术为库欣病患者的诊断、手术计划和临床结果提供了可喜的改进，特别是那些在有或没有IPSS的情况下MRI结果反复呈阴性或不确定的患者。