THU014 Post-surgical Remission After Resection Of Solitary Mediastinal Lymph Node In A Metastatic Occult Ectopic ACTH Syndrome Patient

Disclosure: I.P. de Magalhães: None. C.P. Oliveira: None. L.P. Lins: None. N.X. Andrade: None. M.S. Lima: None. S.A. Siqueira: None. J.S. Fonini: None. E.C. Assis Filho: None. A.W. Mariani: None. M.C. Machado: None. BACKGROUND: The prevalence of occult cases has been increasing in the most recent series of Ectopic ACTH syndrome (EAS) maybe due to the small size of pulmonary neuroendocrine tumors. We present an atypical and very rare case of EAS with initial presentation of a single metastatic mediastinal lymph node that evolved with remission after your resection with no defined primary tumor. CASE REPORT: A 15-year-old female patient presented a clinical feature of Cushing’s syndrome (CS) with a 1-year evolution. Hormonal evaluation confirmed ACTH-dependent CS: urinary free cortisol of 819 µg/24h (3-43), late night salivary cortisol of 409 ng/dL (<274) and ACTH of 105 pg/mL (<63.3). Noninvasive methods for differential diagnosis of ACTH-dependent CS did not clarify the etiology (conflicting results): pituitary MRI negative, suppression of 59.6% of cortisol after high dose dexamethasone suppression test, positive response of ACTH and cortisol at desmopressin test, suppression of 81% of ACTH at acute octreotide SC test, negative serum tumor markers (gastrin, calcitonin, CEA, CA125, alpha-fetoprotein, CA19.9 and bhCG), negative cervical and pelvic ultrasound, negative computed tomography of thorax and negative abdominal MRI. Bilateral and simultaneous petrosal inferior sinus sampling with desmopressin confirmed EAS due to negative central to peripheral ACTH gradient, even after normalized PRL/ACTH gradients. Finally, PET/CT with Ga(68)-DOTATATE revealed a single infracarinal nodule of 1.5x0.6 cm with high capture of radioligand, SUVmax of 23.2, suggestive of affected mediastinal lymph node. The patient was submitted to thoracic surgery and the infracarinal lymph node was dissected and removed. Anatomopathological and immunohistochemical analyses disclosed a lymph node metastatic of a well-differentiated grade neuroendocrine tumor ACTH positive, Ki-67 2%. After surgery, the patient exhibited CS improvement and impressive drop of hormones at 5(th) postsurgical day (serum cortisol <1.0 µg/dL and ACTH of 2.2 pg/mL) confirming remission of CS. CONCLUSION: To our knowledge, approximately 36 cases of EAS due to neuroendocrine tumor of unknown primary origin were reported. However, previous cases commonly exhibited no remission after only removal of metastasis. Our case presented initial remission even with no primary tumor identified (occult primary tumor non-functioning?). Follow up with clinical, hormonal and imaging methods are necessary due to recurrence risk. Presentation: Thursday, June 15, 2023