Is whole-lung CT scanning still necessary in all cases of ACTH-dependent Cushing's syndrome in the era of petrosal sinus sampling?

We reviewed 31 patients in whom both bilateral inferior petrosal sinus sampling without CRH stimulation, and a CT scan of the lungs were done. Twenty-five had normal lung CT scans, of whom 23 had a higher inferior petrosal sinus:peripheral ACTH ratio greater than or equal to 1.5. After careful follow-up, none was subsequently shown to have ectopic ACTH syndrome. Six had abnormal lung CT scans, of whom two had ratios greater than or equal to 1.5. In these two patients, other investigations suggested pituitary disease, and pituitary surgery led to apparent cure. Of the remaining four patients, who had ratios <1.5, two had incidental lung findings, and pituitary abnormalities were demonstrated at pituitary surgery. The third underwent bilateral adrenalectomy, and no evidence of ectopic ACTH syndrome has emerged as yet after 4 years follow-up. The fourth had a small-cell carcinoma of the lung, confirmed histologically. Our series suggests that whole-lung CT scanning is only necessary in cases of ACTH-dependent Cushing's syndrome where bilateral inferior petrosal sinus sampling has not demonstrated a significant increase in petrosal sinus ACTH levels as compared with the peripheral level. Thus, in our experience the test is now only necessary in those patients (approximately 25%) where the ratio is less than or equal to 1.5.