But Bonnie Falbo of Charlottesville, Va., a caregiver for her 70-year-old husband, who has multiple myeloma, said: “This treatment is one that could save his life one day. Immunotherapy is the future of cancer care. Approving payment for this therapy right now will save the lives of patients who have run out of other options.”

Advocates for patients and physician groups said the coverage policy proposed by the Trump administration was too limited and could delay lifesaving treatment for cancer patients who were very ill.

“We have a new therapy, a result of years of research, and it has been approved by the F.D.A.,” said Kirsten A. Sloan, a vice president of the American Cancer Society Cancer Action Network. “Let’s make sure people have access to it. Let the doctor decide which patients should be eligible. A patient’s health care provider is in the best position to determine when and whether a patient will benefit from CAR T cell therapy and should not be limited by a narrow coverage policy.”

Ted Okon, the executive director of the Community Oncology Alliance, an advocacy group for cancer doctors and patients, said the coverage criteria proposed by the Trump administration were “much more restrictive” than the uses of CAR-T therapy permitted in the F.D.A.-approved label.

Moreover, Mr. Okon said, coverage should not be limited to patients treated at a hospital, as the government proposed. “CAR-T treatment has been slowly moving to the outpatient setting,” he said. “Many community oncology practices have the experience and capabilities to administer CAR-T treatment, saving money for patients and Medicare.”

Hospitals providing CAR-T therapy said they were trying to figure out how to pay for it.

Charles N. Kahn III, the president and chief executive of the Federation of American Hospitals, which represents investor-owned hospitals, said they faced large financial losses in providing the treatment to Medicare patients.

“This is a brave new world,” Mr. Kahn said. “No one anticipated such an extraordinary expense when Medicare’s hospital payment formulas were adopted.”