A rare initial presentation of a thymic neuroendocrine tumor
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While evaluating the cause of Cushing's syndrome, biochemical confirmation should be sought first as imaging studies might misdirect the diagnosis toward the wrong problem. One of the rare secondary causes that should be kept in mind while evaluating Cushing's syndrome is the thymic neuroendocrine tumor.

Authors report a 24-year-old Filipina woman who was recently diagnosed with hypertension and hypothyroidism. She presented to our hospital with generalized fatigue and weakness that had been going on for three months prior to presentation. She had increased abdominal girth along with acne and occasional difficulty while getting up from chairs or walking. On examination, she was found to be thinly built with a slightly round face containing numerous acne. A slight hump on her back was noted. She also had proximal bilateral muscle weakness, mainly in the lower limbs.

Her blood pressure was found to be 163/67 mmHg. Laboratory tests revealed an 8 AM cortisol level to be high with high ACTH. A low-dose dexamethasone suppression test failed to suppress cortisol. At this point, pituitary magnetic resonance was performed, and it showed a pituitary microadenoma. Then, the high-dose dexamethasone suppression test showed high cortisol levels, which meant failure to suppress cortisol, suggesting the cause to be an ectopic source, rather than central.

Chest, abdomen, and pelvic CT scans were taken; they showed findings of a small thymic neuroendocrine tumor.
Axial contrast-enhanced CT study at the level of the upper abdomen shows a smooth enlargement of both adrenal glands in keeping with hypertrophy. This further confirmed the suspected diagnosis of Cushing's syndrome secondary to a thymic neuroendocrine tumor. She underwent surgical excision of the tumor by right video-assisted thoracoscopic surgery (VATS) with en bloc excision of the mediastinal mass and total removal of anterior mediastinal fat.

Pathological examination of the resected thymus revealed a well-circumscribed tumor sharply demarcated from the surrounding atrophic thymic tissue. The tumor cells are arranged in a nested pattern. The cells are characterized as having round to oval nuclei, salt-and-pepper chromatin, and eosinophilic cytoplasm. The interstitium between the tumor cells is fine with small capillaries.

By immunohistochemistry, the tumor cells are positive with CKAE1/AE3 (paranuclear dot-like pattern), synaptophysin, and ACTH, and negative with chromogranin, TTF-1, and PAX8. The Ki-67 proliferative index is about 1%. The features were consistent with typical carcinoid tumor (ACTH expressing) confined to the thymus. Following the surgery, her ACTH dropped to 6.1 pg/m and cortisol to 399 nmol/L, along with improvement in her symptoms and blood pressure and titration down on her antihypertensives with no postoperative complications.

Source: https://onlinelibrary.wiley.com/doi/10.1002/ccr3.4435?af=R
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