LOS ANGELES — To doctors visiting his company’s booth at a recent medical meeting, Scott Phillips extolled the medical virtues of the company’s equipment for treating cancer with protons. But he also appealed to their financial interests.

With enough patients “it becomes a very lucrative system,” Mr. Phillips, a sales manager with Optivus Proton Therapy, said at the meeting of the American Society for Therapeutic Radiology and Oncology held here in late October. Referring to one proton center that treats up to 175 patients a day, he added, “You can imagine what the return on investment is on that.”

It is also a good return for the companies that make and sell the equipment. The market leader appears to be Ion Beam Applications of Belgium. Others are Hitachi of Japan and Siemens of Germany. Varian Medical Systems acquired a proton equipment company, Accel, earlier this year.

Other companies help finance, build and operate the facilities. The most well known, ProCure Treatment Centers, is signing up community hospitals and even private medical practices. The hospitals or doctors get a small ownership stake, and therefore a small part of the profits, while directing the medical treatments.

ProCure’s first project is being built in Oklahoma City by two medical practices with a total of six radiation oncologists. Some wealthy local residents, led by Aubrey McClendon, who runs a big natural gas company in the area, invested $35 million in the $100 million project.

The company then arranged the rest of the financing through two Belgian banks that do business with Ion Beam Applications, the equipment supplier. Financing the proton centers has been a challenge even for big academic medical centers, which have taken different approaches.

While M. D. Anderson Cancer Center is part of the University of Texas, its proton center is not. To avoid endangering the university’s credit rating, the proton center is a separate, for-profit entity.

M. D. Anderson supplies staff members for the center, but owns only 15 percent. The rest is owned by various investors recruited by an investment bank in Houston.

The University of Pennsylvania’s $140 million proton operation will be part of a nonprofit medical center. It is being built with the medical center’s own money and donations.

Timothy R. Williams, former co-chairman of the radiation oncology society’s health policy committee, said in a talk at the October conference that the profession was threatening to debase itself if doctors were building centers for the money or competitive advantage.