In some cases, the most difficult part of treating Cushing's syndrome is making the right diagnosis and figuring out what is causing the excessive hormone production. The can be difficult because tumors of both the adrenal glands and pituitary gland are common, and sometimes they exist in a person with Cushing's syndrome without being the cause. For example, in a 2004 review of the medical literature by Shereen Ezzat and colleagues at the University of Toronto examining more than 11,000 people, including normal volunteers and patients with various problems—but not people who were specifically being evaluated for Cushing's syndrome—about 16% were found to have pituitary tumors. Roughly speaking, this means that about 16% of people with Cushing's syndrome will have pituitary tumors that are unrelated to their Cushing's syndrome. Imagine such a person going through major pituitary surgery only to find out that the surgery didn't effectively treat their Cushing's syndrome because the problem was in the adrenal gland or resulted from other, rarer form of Cushing's syndrome! Unfortunately, this has happened many times in the past. For this reason, most of us in the endocrine community favor the evaluation of Cushing's syndrome at a center with lots of experience with the problem. Frequently, specialized and somewhat invasive radiological tests are necessary before a clear diagnosis can be made. Once this is done, surgery is usually successful in the hands of an experienced surgeon.