Penn Medicine and Virtua plan to build a $35 million proton cancer therapy center on Virtua's campus in Voorhees, the two nonprofit health systems said Thursday.

Penn already has a proton therapy center on its University City campus. Since the Roberts Proton Therapy Center opened in 2010, at a cost of  $144 million, Penn oncologists have treated almost 4,500 patients there, Penn said. The South Jersey center is expected to open in 2020.

"It's very full, and it keeps growing," said Ralph Muller, chief executive of the University of Pennsylvania Health System. "Essentially, we're running it from 7 in the morning until 10 at night, and running it on Saturdays."

The Roberts center has five treatment gantries, compared with the one planned for Voorhees, where the proton center will be built next to Virtua's medical oncology department.

Proton therapy, controversial for prostate cancer because it costs more but has not been proved to work better than cheaper alternatives, is designed to do less collateral damage to the human body because the beam stops when it hits the cancer instead of traveling all the way through the body.

Muller said about 20 percent of the Roberts center's volume is from prostate cancer, compared with 60 percent and up at some of the free-standing proton treatment centers around the country. "We treat kids. We treat neurotumors, spinal cord, we do lung cancer. We're not as prostate-focused as the other places," he said.

Penn's proton-beam therapy center was the first in the Mid-Atlantic when it opened in early 2010. Since then, centers have opened in Hampton, Va.; Baltimore; Somerset, N.J.; New Brunswick, N.J.; and elsewhere around the country.

Nationwide, there are 27 in operation and 10 under construction or in development, according to the National Association for Proton Therapy. That includes the one announced by Penn and Virtua.

Penn and Virtua, which announced a strategic alliance in October 2015, touted the fact that it will be the first proton center in South Jersey.

"The outstanding value for our patients is that they will receive advanced cancer care close to home and family, with radiation treatment plans developed by Penn's radiation oncologists," said Virtua's new CEO, Dennis W. Pullin.

The proton center will step up the competition in South Jersey for Cooper's MD Anderson Cancer Center, which opened in 2013.

Muller said Penn already gets about 25 percent of its patients from New Jersey, a number that he expects to increase when the nonprofit completes its acquisition of Princeton HealthCare System. He said he expected that deal to be done by Jan. 1.