WASHINGTON — On March 29, Georgetown University Hospital opened a proton-therapy cancer unit that is expected to treat about 300 patients a year at premium prices using what its proponents promote as the most advanced radiology for attacking certain tumors.

At the facility’s heart is a 15-ton particle accelerator that bombards malignancies with beams of magnet-controlled protons designed to stop at tumors rather than shoot through them like standard X-ray waves, mostly sparing healthy tissue.

With the addition, Georgetown joined a medical arms race in which hospitals and private investors, sometimes as partners, are pumping vast sums of money into technology whose effectiveness, in many cases, has not yet been shown to justify its cost.

Although most of the proton centers in the United States are profitable, the industry is littered with financial failure: Nearly a third of the existing centers lose money, have defaulted on debt or have had to overhaul their finances.