Lesley Glenn didn't question the aggressive treatment plan that a community oncology group recommended when she was diagnosed with breast cancer in 2012 at age 47.

But after the Chino Hills, Calif., resident suffered a life-threatening complication, she transferred her care — with permission from her health insurer — to USC Norris Comprehensive Cancer Center, one of the nation's top cancer hospitals. Doctors there put her on the path to where she is today, with no evidence of her advanced cancer.

Just one problem. Six months after she switched, her insurer discontinued coverage of USC because it was not part of her health plan's network of providers.

"It was devastating," Glenn, now 51, recalled. "We appealed it, but my insurer said, 'We believe you can get just as good care in network.' "

Glenn's insurer, like many across the country, was using "narrow networks" — limiting the covered providers —  to keep premiums down.

"Consumers may benefit financially from the fact that these narrow networks generally have lower premiums, but they may face reduced access to the higher-quality providers in their market," said  Daniel Polsky, executive director of the University of Pennsylvania's Leonard Davis Institute of Health Economics.

Polsky is coauthor of a new study that found the narrower the network, the more likely it is to exclude oncologists affiliated with National Cancer Institute-designated cancer centers across the country. These 69 centers — including USC and Penn — are recognized for cutting-edge clinical care and research.

The Penn study examined health plans offered by the insurance marketplaces created under the Affordable Care Act, also known as Obamacare.

However, narrow networks are not new and not limited to the ACA. Glenn's coverage, for example, was through her husband's employer-sponsored health plan.

There is no legal definition of a narrow network, but the management consulting firm McKinsey & Co. defines it as a plan that includes 30 percent to 70 percent of area hospitals. "Ultra-narrow" networks include fewer than 30 percent of area hospitals.

The new study, published this week in the Journal of Clinical Oncology, did not use a specific cutoff. Rather, the researchers used 2014 data to assess the relationship between each network's breadth and its inclusion of oncologists affiliated with NCI-designated centers.

Because such centers tend to be in big cities — Philadelphia has three — the nation's 69 top centers are located in only 51 ACA insurance marketplaces. The remaining 407 markets have no NCI-designated centers, the study found.

Of the 248 networks offered in those 51 markets, 33 networks, or 13 percent, excluded all oncologists from the top cancer centers. Two-thirds of networks in the 51 markets included just under half of the oncologists who were affiliated with top cancer centers, the study found. (The researchers did not break out data for the Philadelphia market.)

Evaluating the quality of cancer care can be difficult for patients. Glenn's original treatment plan called for many months of chemotherapy, followed by a double mastectomy. But she developed a potentially deadly infection called sepsis that began in the chemotherapy port implanted in a vein. She was still recovering from sepsis when the community oncology doctors urged her to resume chemo.

"That's when I decided to get a second opinion," she said. "The USC doctors backed up everything they said with research. They answered my questions. I just felt very good about the care."

At USC, she had a lumpectomy — no double mastectomy and no more chemo — and was preparing for radiation to treat cancer found in a bone in her shoulder. That's when her insurer pulled the plug.

"Thankfully," she said, "my USC doctors helped me find a radiation oncologist in my network."

If her cancer recurs, she said, "I'd fight again to go back to USC."

The Penn researchers suggest that insurers should help cancer patients with exceptional circumstances — for example, those who need specialized procedures or experimental treatments offered through clinical trials — to get out-of-network care without financial penalties.

The researchers also say insurers should include more information about network doctors so consumers can readily judge their credentials and affiliations.

"Patients who really do their homework can find out," said lead author Laura Yasaitis, a health-policy researcher. "The information is out there. But we think it should be easier. You should be able to search by specialty and see the affiliations of these providers. Now, you just get names and addresses of providers."