By Lenny Bernstein

The United States faces a shortage of as many as 90,000 physicians by 2025, including a critical need for specialists to treat an aging population that will increasingly live with chronic disease, the association that represents medical schools and teaching hospitals reported Tuesday.

The nation's shortage of primary care physicians has received considerable attention in recent years, but the report predicts that the greatest shortfall, on a percentage basis, will be in the demand for surgeons — especially those who treat diseases more common to older people, such as cancer.

In addition to the growing and aging population, full implementation of the Affordable Care Act in all 50 states would increase demand for doctors as more people are covered by insurance. But Obamacare's impact will be small — just 2 percent of the projected growth in demand, the organization said. The supply of doctors also will grow but not nearly as quickly as the need, officials said.

"An increasingly older, sicker population, as well as people living longer with chronic diseases, such as cancer, is the reason for the increased demand," Darrell G. Kirch told reporters during a telephone news briefing.

The organization called on Congress to raise the federal cap on slots for medical residents at teaching hospitals by 3,000 annually, at a cost it estimated would be about $1 billion per year. The government provides its $40,000 share of the cost of training each U.S. physician — estimated at about $152,000 annually — via the Medicare program. Currently, those hospitals train 27,000 to 29,000 doctors each year.

In 2013, there were about 767,000 doctors practicing in the United States, according to the report.

Policymakers have debated the doctor shortage for years, with some arguing that certain types of doctors are clustered in cities and affluent areas, leaving rural and poor Americans critically underserved. The government runs programs to encourage doctors, especially primary care physicians, to practice in shortage areas. Some states help doctors pay off their medical school debt, which can run into six figures, if they agree to practice in underserved parts of the country.

In a 2013 paper, Linda Green, a mathematician who studies the health care system, argued that the projected shortage of primary care doctors may not occur. The move toward larger practices, which enable physicians to share support staff and office space, can allow them to take on more patients. And the increasing use of physician assistants and nurse practitioners will have the same effect, she wrote.

The new report actually predicts a smaller shortage than a similar report written five years ago. In 2010, the organization said the nation would face a shortfall of 130,600 physicians by 2025. But revised population projections by the U.S. Census and a small increase in the number of doctors have brought the predicted shortfall down, according to the report.

Under a better-case scenario, the doctor shortage 10 years from now would be 46,100, the report notes. That would reflect growth in the number of advanced practice nurses and physician assistants. If the shortage is 90,400, the country would need 31,100 primary care physicians and 63,700 non-primary care doctors — including oncologists, neurologists, psychiatrists and others — to meet the demand.