The National Association for Proton Therapy, a U.S. proton therapy trade group, said Monday it expanded to include Mayo Clinic proton therapy centers under development in Phoenix, Ariz. and Rochester, Minn. and the McLaren Proton Therapy Center, being built in Flint, Mich.

With the addition of the three centers, the number of centers in development in the United States (at least, that NAPT recognizes) totals seven. Nationwide, nine centers currently operate.

Proton therapy -- which uses massive particle accelerators to fire protons to destroy solid tumors -- is not widespread because it's an expensive undertaking, with most centers costing hundreds of millions of dollars to build.

[Read why there might never be more than 20 or 30 centers in the U.S.]

"NAPT is pleased to welcome Mayo Clinic and McLaren Health into our small, but growing community of proton therapy centers," Leonard Arzt, NAPT's executive director, said in a statement.

The Mayo Clinic's proton therapy centers will use intensity-modulation proton therapy, or IMPT, a form of pencil-beam scanning, NAPT said.

In November, when it announced the project, Mayo said the two four-room centers would cost upwards of $180 million each. The first treatment rooms at each center are expected to open in late 2014 or early 2015, with the remaining six rooms opening over the following year.

McLaren's three-room, 35,000 sq. ft. center, which also uses IMPT, is expected to start treating patients as early as next year. The center is being built on the Flint campus of the Great Lakes Cancer Institute, near the 458-bed McLaren Regional Medical Center.

According to published reports, McLaren's center, using ProTom International's new compact Radiance 330 synchrotron, costs less than one-third the price of most U.S. proton centers. McLaren said that last year, thanks to the ProTom technology, the center, originally planned to be built for $168 million, will cost around $65 million.

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