BACKGROUND: Thymic neuroendocrine tumor as a cause of Cushing syndrome is extremely rare in children. CASE PRESENTATION: We report a case of a 10-year-old girl who presented with typical symptoms and signs of hypercortisolemia, including bone fractures, growth retardation, and kidney stones. The patient was managed with oral ketoconazole, during which she experienced adrenal insufficiency, possibly due to either cyclic adrenocorticotropic hormone (ACTH) secretion or concurrent COVID-19 infection. The patient underwent a diagnostic work-up which indicated the possibility of an ACTH-secreting pituitary neuroendocrine tumor. However, after a transsphenoidal surgery, the diagnosis was not confirmed on histopathological examination. Subsequent bilateral inferior petrosal sinus sampling showed strong indications of the presence of ectopic ACTH syndrome. Detailed rereading of functional imaging studies, including 18F-FDG PET/MRI and 68Ga DOTATOC PET/CT, ultimately identified a small lesion in the thymus. The patient underwent videothoracoscopic thymectomy that confirmed a neuroendocrine tumor with ACTH positivity on histopathological examination. CONCLUSION: This case presents some unique challenges related to the diagnosis, management, and treatment of thymic neuroendocrine tumor in a child. We can conclude that ketoconazole treatment was effective in managing hypercortisolemia in our patient. Further, a combination of functional imaging studies can be a useful tool in locating the source of ectopic ACTH secretion. Lastly, in cases of discrepancy in the results of stimulation tests, bilateral inferior petrosal sinus sampling is highly recommended to differentiate between Cushing disease and ectopic ACTH syndrome.

3rd Department of Medicine 1st Faculty of Medicine Charles University Prague Czech Republic

Department of Children and Adolescents 3rd Faculty of Medicine Charles University University Hospital Kralovske Vinohrady Šrobárova 50 Prague 100 34 Czech Republic

Department of Medicine Military University Hospital Prague Czech Republic

Department of Pediatrics Thomayer University Hospital Prague Czech Republic

Developmental Endocrinology Metabolism Genetics and Endocrine Oncology Affinity Group Eunice Kennedy Shriver NICHD NIH Bethesda MD USA

Institute of Nuclear Medicine 1st Faculty of Medicine Charles University General University Hospital Prague Czech Republic

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