Ectopic secretion of corticotropin(ACTH) by nonpituitary tumors accounts for 1015% of ACTH dependent Cushings syndrome (CS). Generally it is difficult to localize the ACTH secreting tumor by conventional imaging methods because these tumors are often small in size. Here we present a case of ectopic ACTH syndrome diagnosed with octreotide scan.
Case: A 26-year-old male patient presented with moon face, purplish striae, supraclavicular fat pads and proximal muscle weakness. In laboratory examination; early morning cortisol: 33 μg/dl (6.219.4), late evening serum cortisol: 30 μg/dl (2.311.9), serum potassium: 2.3 mmol/l (3.55.1), and urinary cortisol excretion: 258 μg/day (060). There was no cortisol supression with low dose or high dose dexamethasone. As plasma ACTH concentration was 92 pg/ml (060) the patient was diagnosed with ACTH-dependent CS. Pituitary MRI revealed no mass. Inferior petrosal sinus sampling(IPSS) was negative with no increase in ACTH after CRH administration. On thorax CT there was a 14 mm nodule in the middle lobe of the right lung. There was no pathology on 18-FDG-PET scan. But 111-In-Octreotide scan depicted the neuroendocrine tumor located in the right lung. In the lobectomy specimen the pathology of the mass was compatible with classic type carsinoid.
Conclusion: The last step of approaching to a patient with ACTH dependent CS is to detect the source of ACTH secretion. As most of the ACTH secreting tumors arise from thorax it is important to start evaluation from this anatomic localisation. CT or MRI is the first imaging modality of choice. The sensitivity of CT to detect a small tumor such this is 53% whereas it is 37% in MRI. The sensitivity of octreotide scan in detecting ACTH secreting tumor is approximately 3053%. It is the last but not least choice of non invasive procedure on the way going to surgery as in our case.