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Background: 68Ga DOTATATE PET/CT (68Ga-PET) has been proposed as a superior method in identifying ectopic ACTH syndrome (EAS). However, recent systematic review suggests its sensitivity is not as high as believed (1). We report a challenging case of EAS whose source was uncovered only after repeated 68Ga-PET. Clinical Case: A 15‐year-old male presented with rapid onset of typical features of Cushing’s syndrome (CS) and metabolic impairment. Hormone evaluation confirmed severe ACTH-dependent CS. Pituitary transsphenoidal surgery was performed due to positive responses in desmopressin stimulation and high dose dexamethasone suppression test, in addition to a 4 mm nodule in pituitary MRI. No tumor was found in surgical specimen and no hormonal improvement was observed after surgery. Inferior petrosal sinus sampling demonstrated no central to peripheral ACTH gradient. Neck US, thorax/abdomen/pelvis CT were negative and PET-CT/FDG was inconclusive. OctreoScan® identified anomalous uptake on left mediastinum and led the patient to a thoracic surgery (TS) with nodule resection at left hilum. Pathology confirmed ACTH positive 10 mm neuroendocrine tumor (NET) infiltrating a lymph node. The patient had transient clinical and hormonal improvement, with recurrence 7 months later. Thoracic CT (T-CT) showed a 7 mm nodule on inferior segment of superior left lobe, PET-CT/FDG and OctreoScan® were negative but abnormal uptake was verified by 68Ga-PET in subcarinal area. A sub centimetric lymph node was resected and pathology confirmed ACTH positive NET, although the patient did not achieve remission. Octreotide LAR, cabergoline and ketoconazole did not control hypercortisolism and bilateral adrenalectomy was performed. Then, T-CT showed stable lung nodule and 2nd 68Ga-PET was negative. One year later, T-CT evidenced growth of lung nodule to 15 mm and 3rd 68Ga-PET demonstrated for the first time, abnormal uptake in this area. The patient underwent resection of left superior lung lobe along with ipsilateral hilar lymph nodes, and histopathology study revealed an ACTH-secreting atypical pulmonary carcinoid tumor with Ki67 of 10% and 5 out of 11 lymph nodes affected. ACTH fell from 288 to 64 pg/mL after surgery. Conclusion: Despite the high sensitivity attributed to 68Ga-PET, false negatives have been reported. In the present case, primary tumor was evidenced by 68Ga-PET seven years after the first resection of a metastatic lymph node, in the 3rd 68Ga-PET assessment and after tumor growth. This adds to the evidence that further studies are needed to better assess the accuracy of 68Ga-PET for EAS. Reference: [1] Varlamov et al. Diagnostic utility of Gallium‐68‐somatostatin receptor PET/CT in ectopic ACTH‐secreting tumors: a systematic literature review and single‐center clinical experience. Pituitary 2019; 22:445–455

MON-250 Late Diagnosis of ACTH-secreting Pulmonary Neuroendocrine Tumor by Repeated 68Ga Dotatate Pet/ct: Influence of Tumor Size in Abnormal Uptake?