DDAVP‑stimulated FDG PET to find hidden pituitary microadenomas

* INCLUSION CRITERIA:

In order to be eligible to participate in this study, an individual must meet all of the following criteria:

1. Subjects aged 8 or older with biochemical evidence of Cushing s disease and a clinical MRI pituitary neuroradiology result of negative or possible adenoma (e.g. "no tumor" or "possible tumor")
2. MRI of the pituitary gland with and without contrast obtained within 9 months of screening
3. For newly diagnosed Cushing s disease cases, IPSS is required.
4. Ability to undergo PET-imaging without general anesthesia
5. Ability to provide informed consent for study participation (parents or guardians in the case of minors)
6. Clinical diagnosis of Cushing s disease based on documented medical records
7. Surgical candidate for and subject agrees to resection of ACTH producing pituitary adenoma within 24 weeks of PET-imaging
8. Normal liver function as evidenced by liver enzyme tests completed within 14 days before injection of radiopharmaceutical: SGOT, SGPT \<= 5 x upper limit of normal; bilirubin \<= 2 x upper limit of normal
9. Tolerance of a previous infusion of DDAVP, including as part of workup and diagnosis of Cushing's disease

EXCLUSION CRITERIA:

An individual who meets any of the following criteria will be excluded from participation in this study:

1. Current pregnancy or lactation
2. Glomerular filtration rate \< 50 mL/min/1.73 m\^2, hepatorenal syndrome, history of urinary retention or post-liver or kidney transplantation.
3. Hyponatremia (serum sodium below 135 mmol/L)
4. Current diagnosis of angina, significant coronary artery disease, congestive heart failure, or SIADH due to risk of fluid overload and/or hyponatremia.
5. Uncontrolled hypertension (blood pressure \>150/95 mmHg) due to risk of further increase if fluid overload occurs.
6. Uncontrolled, severe hypotension (sustained blood pressure \<90/60), or symptomatic hypotension.
7. Current use of any of the following medications:

   * Vasopressors: phenylephrine, dopamine and vasopressin
   * Drugs that can worsen hyponatremia: non-steroidal anti-inflammatory drugs (NSAIDs) within 4 days of testing, loop diuretics (bumetanide, ethacrynic acid, furosemide, torsemide), chlorpromazine, chlorpropamide, cisplatin, opiate agonists within 48 hours of testing, and/or vincristine.
   * Drugs that interfere with DDAVP duration of action or potency: carbamazepine, lamotrigine, and/or tolvaptan
8. Habitual or psychogenic polydipsia, due to increased risk of hyponatremia
9. History of Type IIB von Willebrand s disease due to risk for thrombosis.
10. Elevated blood glucose level above 200 mg/dL on the day of the scan prior to FDG administration.
11. Known intolerance to DDAVP

INCLUSION OF VULNERABLE PARTICIPANTS:

* Children: Children age 8 and older are included in this protocol. More than half of the subjects with CD requiring transsphenoidal surgery at the NIH are children. The knowledge gained by the use of DDAVP PET imaging in children with MR-invisible tumors will provide generalizable knowledge in the treatment of CD in this population. Children under the age of 8 usually require anesthesia for a PET scan, which involves greater risk. Therefore, children under the age of 8 will be excluded from participation.
* Pregnant or lactating women: Pregnant and lactating women will be excluded from participation. The PET radiopharmaceutical used in this study can be harmful to a developing fetus. Therefore women who are able to become pregnant will have a pregnancy test performed within 24 hours before PET imaging. Individuals will not be able to participate in PET scanning if the pregnancy test results positive.