Generally, Cushing's is caused by non-cancerous tumors (like...90 or 99% of the time). In the pituitary, which is the kind I have and am most familiar with, pituitary tumors have various classifications too.
There are secreting tumors and non-secreting tumors, meaning they produce hormones or do not.
Tumors that do not produce hormones are generally called "incidentalomas" because they are often found incidentally when having a head MRI, etc. There is some evidence to suggest they are problematic, but for the most part nothing is done about them unless they are big and compressing other tissues, causing problems. They typically are watched periodically for growth and when hormone testing (and lack of symptomology) suggests they are non-secreting, that's about the gist of it.
Cushing's Disease is caused by a tumor producing too much ACTH (adrenocorticotropic hormone). That hormone triggers the adrenal glands to produce too much cortisol in the body (a steroid). You can have multiple tumors, or tumors producing multiple hormones. And tumors that do not produce ACTH but produce other hormones cause other diseases like Giantism in children, Acromegaly in adults, thyroid disease, etc.
Non-cancerous pituitary tumors are called adenomas, and they are also classified by size. Small tumors, under 10mm in size, are called "micro-adenomas."
Secreting tumors generally cause symptoms and illness, and while it can be hard to identify and recognize at first (they can seem random and unconnected), it is those symptoms and the illness that sends you looking for answers. They are often very small, for example, the size of a pencil eraser or even simply the pencil's tip, and are usually micro-adenomas.
Tumors can be larger, though, and when over the 10mm mark, they are called "macro-adenomas." These tumors often cause problems even due to sheer size. The pituitary sits at the base of the brain, right beneath the optic chiasm and between the carotid arteries in a very small space. Macro-adenomas can compress the nerves, arteries, and even the brain causing dysfunction and damage.
There are some more things worth knowing about Cushing's tumor's, so I'll explain a bit further. ACTH-producing tumors (especially if you have cyclic/episodic/periodic disease) are often not a solid mass like one would expect. They can be goopy, stringy, blobby, liquidy masses that are poorly defined (ie, their edges --where they stop and the normal tissue begins-- may not be easy to see). They may have tendrils, like arms stretching elsewhere in the pituitary. They may even be hyperplasia, meaning that instead of being a defined tumor area causing problems, it is more like pre-tumor tissue with many of the individual cells causing the problem. I picture it like mold that is just starting to spread...the seeds can be all over, but you don't always see it till it grows into a more solid green mass. As you can probably imagine, these "tumors" can be hard to find, hard to remove completely, and thus cures don't always come with the first or even second round of treatment. I'll explain treatment options in another post this month.
Another reason this disease is hard to diagnose is that these tumors can be quite difficult to find prior to surgery on MRI's. The MRI of choice is a 3T (3 teslas --the strength of the magnet used to produce the images) machine and done with "dynamic" pituitary protocols. This means that they not only take pictures before and after contrast dye is injected into your blood stream, but also AS the dye is going in. It takes two people to time it right, and they often have a count-down to make sure the timing of the MRI pictures is started as the contrast is injected into your blood stream. The reason for this is that these tiny tumors do absorb the contrast dye, but they do it at a *very* slightly slower pace. The pituitary should enhance at a uniform rate, like a wave washing over the pituitary till it is all a fairly consistent brightness in the picture. The adenomas stay grey longer, while the tissues around them turn white, but if you don't catch it as it is happening, by the time you take a post-contrast MRI, it may have absorbed the contrast and look like normal pituitary tissue by that point. Pretty tricky, huh?
Some 50% of surgery-proven pituitary tumors were not visualized on MRI prior to surgery (ie, the pathology report confirms it was an ACTH-producing tumor, but they couldn't see it before they got in there themselves). There is some testing that can be done to help determine if the illness is pituitary in origin, and can possibly help to determine which side of your pituitary it is on, that will be/is discussed in this month's post about testing.
To untrained eyes (read "most radiologists") these tumors go completely unnoticed even when they are looking for them. My own first MRI was read clean by a radiologist that is actually fairly experienced with Cushing's and works with my endocrinologist frequently so he accepted the report as accurate. A year later when my illness had worsened and I was gathering sufficient lab results to prove not only that I had the disease but that it was also likely pituitary in origin, a neuro-radiologist and my endocrinologist both looked at the old MRI and agreed that there was one, possibly two microadenomas on my pituitary gland. A new MRI was ordered, the same radiologist that had initially read my first "clean" noted one adenoma, and my endocrinologist *kindly* had him compare both MRI's and he admitted to having missed it originally, but that it was indeed there.
Neurosurgeons also are sometimes loath to be tied down with definitive answers, like saying "I for sure see a tumor here" and frequently say things like "possible micro-adenoma" and then give it a size and location. ;) Or they will call it "an area of hypo-enhancement". Rare is the neuro (and generally loved) that will call a tumor a tumor. ;) Mine did. He saw one area and called it as he saw it. The second area he said was likely...and in surgery he explored the whole gland, but no other tumorous area was found. My pathology report did indeed come back with tissue consistent with pituitary adenoma, that tested positive for many hormones, and it was in a pattern found with hyperplasia cells. Pathology reports have their limits, though...remember, if you are cyclic and your surgery is done while in a low, it can skew the results, and if they remove a small amount of healthy pituitary cells with the tumor cells, it can test positive for anything produced by those cells.
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