The article closes with that note.

  The American Cancer Society says that in order to meet the definition of a cancer cluster, occurrences must be the same type, in the same area, with the same cause, and affecting a number of people that's "greater than expected" when a baseline for occurrences is established.

  “Nearly 4 out of 10 people in the United States will develop cancer during their lifetimes," the society said on its cancer clusters webpage. "So, it’s not uncommon for several people in a relatively small area to develop cancer around the same time."

The unstated numbers that matter here are many, how many people were thoroughly investigated here, was it the entire staff of the hospital (as many as a thousand, perhaps)? When X many people are thoroughly scanned how common is it for five people to have benign cancers that aren't doing anything, aren't growing, are just there?

If (for example) twenty percent of the time 500 people were scanned, five at least had benign brain cancers, would this report be unusual or suspicious in itself?