Mandy Cheung, senior radiation therapist, talks to patient Joseph Tracz before doing a laser aligning in the proton beam therapy facility at Massachusetts General Hospital in Boston. (Yoon S. Byun/Boston Globe via Getty Images)

D.C. health officials on Friday approved plans for two of the region’s largest hospital systems to offer a controversial cancer treatment , allowing MedStar Georgetown University Hospital and Sibley Memorial Hospital to establish proton beam therapy programs.

MedStar’s $32 million proposal for one treatment room was approved. But officials said there probably will not be enough demand to justify Sibley’s $132 million program for four treatment rooms, and so approval was granted for only two, according to documents released Friday. Sibley is part of the Johns Hopkins system.

If the decision stands, Washington would be one of only a few U.S. cities to have two centers with the expensive, cutting-edge radiation technology that supporters say is more precise in targeting tumors and safer for healthy tissue than conventional X-rays, especially for some pediatric cancers.

The decision has been closely followed by health experts because critics say it reflects a nationwide medical arms race, as hospitals scramble for dominance by investing millions of dollars in technology that has not been proved better than cheaper alternatives for some cancers.

Today, 11 proton facilities are operating in the United States, and at least 17 more are under construction or in development, including one at the University of Maryland’s proton center in Baltimore, which is expected to start treating patients in two years, experts said. A proton center at Hampton University in Virginia’s Tidewater area, about 180 miles to the south, opened in 2010.

Proton centers in the U.S.

In announcing the decisions, which appeared to be a compromise, Amha W. Selassie, the director of the D.C. health planning commission, cited the District’s high cancer mortality rates.

Cancer is the No. 1 cause of premature deaths in Washington, far greater than heart disease, HIV/AIDS, homicides and accidents, Selassie said.

As a condition of approval, both health systems have also agreed to establish a new cancer screening and treatment clinic in an underserved area in Ward 5 or 7 to prevent and treat the most common cancers — breast, lung, prostate and colorectal.

Selassie acknowledged that no long-term studies exist that show the superiority of proton therapy over conventional radiation. The technology often costs 20 to 30 times more than existing treatments.

The millions of dollars spent to establish such centers, which treat only certain types of cancer and relatively few patients, could be better spent on early intervention efforts to prevent cancer, critics argue.

But in the end, Selassie noted, “the science of proton promises significant technical advances.” This is especially true for tumors near critical structures, such as the brain, the spinal chord and the eye, he said. If a tumor is near the spinal chord, for example, the choice is between “treating the tumor and risking paralysis or not effectively treating the tumor,” he wrote.

While he noted the importance of reining in health-care costs, he wrote in his decision, “it must be balanced with efforts to ensure that the medical innovation and access to cutting-edge treatment are not stifled.”

MedStar executives welcomed the news and plan to begin the one-year construction this fall. Sibley executives said they were pleased to be awarded approval but were reviewing and evaluating the details of the decision.

Any person can request a review of Selassie’s decision, and a final decision could go all the way to the D.C. Court of Appeals.

Experts generally agree that the therapy’s greatest potential is in treating children, particularly those with brain and eye tumors, because the therapy can be administered at higher doses than conventional radiation and causes fewer long-term side effects. District doctors have said it is burdensome for parents to travel to proton centers in Philadelphia and Florida for multiple treatments for their children.

In conventional radiation therapy, X-rays deliver radiation as they enter and exit healthy organs. X-rays are photons, like light. They have no mass, but photon radiation has energies several million times higher than light. Unlike photons, proton therapy uses a beam of protons — positively charged subatomic particles that have mass. Protons deliver a very low entrance dose, and when they arrive at the tumor, they deliver the dose and stop. There is no exit dose and healthy tissue is spared.

Executives at MedStar Georgetown had opposed two proton centers, arguing that its one treatment room, to be built at the Lombardi Cancer Center, would be enough for the region. MedStar estimated treating about 300 patients a year, including about 50 children, focusing on nine tumor types, including prostate, spine and brain.

Selassie disagreed, saying MedStar’s proposal left little room for referral from other hospitals.

In Sibley’s case, however, he noted that there was no evidence that the demand for services was as high as the hospital claimed. He denied Sibley’s request to treat pediatric patients because it does not have a license to treat children.

Sibley, which supports two centers in the District, had sought a $132 million, four-vault facility that would open in 2017, treating 765 adults and 152 children a year at three vaults; the fourth was for research. Selassie approved one for treatment and one for research. He noted that the hospital could apply for additional vaults if the need arose.