He was as "cool as the other side of the pillow" and he will be greatly missed: ESPN's charismatic anchor Stuart Scott passed away on Sunday at age 49 after a seven-year battle with appendiceal cancer, or cancer of the appendix.

Despite his public fight, very little is known about his disease. And Scott himself preferred not to get into details of his own case. He told The New York Times earlier this year, "I never ask what stage I’m in. I haven’t wanted to know. It won’t change anything to me. All I know is that it would cause more worry and a higher degree of freakout. Stage 1, 2 or 8, it doesn’t matter. I’m trying to fight it the best I can.”

But he also had a very rare form of cancer that even experts don't have pinned down. Fewer than 1,000 cases of appendix cancer occur in the United States each year. Cancers of the appendix account for less than 1% of all diagnosed cancers, according to a review published in International Scholarly Research Notices, so choosing the right course of treatment is difficult because doctors simply don't have a lot of previous experience to draw on.

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On top of that, it can be hard to diagnose in the first place because there are often no symptoms until the cancer has spread. And even then, the signs—such as bloating, abdominal pain, nausea, fatigue, and unexplained weight loss—can mimic other gastrointestinal problems. Like most other patients, Scott got his diagnosis after doctors discovered a tumor during surgery to remove his appendix in 2007.

After surgery and chemotherapy, Scott remained cancer-free for four years before a recurrence in 2011 and another recurrence in 2013.

It's unclear exactly what type of appendix cancer Scott had, but there are two main forms of the disease: carcinoid and non-carcinoid tumors.

Carcinoid tumors, which start in hormone-producing cells, make up more than half of all cases of appendix cancer, are more common in women, and usually occur in people in their 40s. Carcinoid tumors can also occur in the stomach, intestines, and lungs. This type of appendiceal cancer has a five-year survival rate of 85%. For people whose cancer has spread outside of the abdomen, however, the five-year survival rate drops to 34%.

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Non-carcinoid tumors form from the mucus-producing skin cells lining the inside of the appendix. These are more likely to spread throughout the abdomen and are associated with more complications. The prognosis for this kind depends on the specific type of tumor and timing of diagnosis, among other factors.

There are no known avoidable risk factors, aside from smoking cigarettes. What is clear is that Scott focused on living. Upon accepting the Jimmy V Perseverance Award at the 2014 ESPYS, he said:

"When you die, that does not mean that you lose to cancer. You beat cancer by how you live, why you live, and the manner in which you live. So live. Live. Fight like hell. And when you get too tired to fight, lay down and rest, and let somebody else fight for you.”

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