Proton therapy, which generally costs twice as much as traditional radiation, requires a large investment — not just in technology but in real estate. Proton beams must travel horizontally on a level plain, requiring a long open space. For a building of 140,000 square feet, the Proton Center will have a large footprint, without the typical height of a New York City building.

“It’s just a difficult issue in a congested metropolitan area to find an acre of property,” said Norton L. Travis, the spokesman for the Proton Center. “One that will have good access for patients.”

The $300-million, three-story center has been in development for eight years, though construction didn’t start until July 2015. The Proton Center is made up of a consortium of three of New York’s leading cancer treatment providers — Sloan Kettering, Mt. Sinai and Montefiore Health System. Coming together to purchase and share the expensive equipment made the project financially viable.

Forty percent of the construction cost is in the equipment needed for treatment, said Mr. Travis. An 80-ton cyclotron — the particle accelerator that generates the protons — was assembled in Germany, shipped to the United States, and lowered into the building in October. The beam transport system, electromagnets that steer the protons through a vacuum to the four treatment rooms, have also been delivered and are currently being set up.