Inferior Petrosal Sinus Sampling (IPSS) is another interesting test, sometimes ordered by endocrinologists, sometimes ordered by neurosurgeon's. It is preformed by experienced interventional radiologists, generally at a large hospital.
If you've ever heard of a cardiac catheter, where they feed a tube or wire up from your femoral artery into your heart...that's similar to what this test is. Except they thread two up towards either side of your pituitary. "Inferior Petrosal Sinus" is the location of the sampling area. When everything is in place, IV's, catheters, etc...they inject you with medication that is meant to force an ACTH producing tumor to produce high levels of ACTH. At timed intervals, they then draw blood samples from your left and right sides at various points (including just below the pituitary). All this data is put into a table, which then can show whether your levels were highest from near the pituitary (which would point to a pituitary tumor) or another area (which would point to an ectopic tumor somewhere else in the body). It *can* also help identify which side of the body or pituitary it is coming from, but at least with pituitary sources, it is only about as accurate as a coin toss in pinpointing sides.
The test can be useful, but really, you want a surgeon that will explore your entire gland to look for tumor tissue anyway, not only rely on a test with poor statistical accuracy. When they explore the gland they make little slices through the entire surface of the pituitary, which generally heals back together well, looking for pockets of tumor tissue (which we jokingly call "fileting"). Many times tumors of florid Cushing's patients are nicely encapsulated circles, but often, cyclic or eptopic patients' tumors are gooey (getting destroyed by suction with no tumor tissue for pathology testing), or oddly shaped (think of dumbbells, octopus tentacles, a thin flat tumor spreading the entire bottom of the gland, multiple tumors, etc). If doctor's aren't experienced in these kinds of tumors and are only looking for a perfect tumor and only looking in one spot, they may miss the tumor entirely, spread gooey tissue around (YIKES!), or miss parts of the tumor...leading to a failed surgery or a brief remission followed by a recurrence of Cushing's Disease, which is already hard to cure. A recurrence may be unavoidable, but you certainly want the best chance possible.
I also strongly encourage you not to settle for the nearest neurosurgeon --what happens in surgery can make all the difference in your quality of life for years to come. Study it out, ask informed questions, talk to their past patients, and make sure you are comfortable with their experience (of removing Cushing's-specific tumors) and their methodology before letting them into your head. It is your head. You have to live with the results.