"We are worried about the potential to have these Humana exchange members seek treatment and have no other option," Zenger said.
Humana spokesman Tom Noland said patients can have access to Huntsman for complex procedures, on a case-by-case basis.
Some state insurance regulators see a problem.
"I want insurers to be able to innovate and come up with new product designs," said Mike Kreidler, insurance commissioner for Washington state. "At the same time, there is a requirement for regulators like myself to be vigilant to make sure there aren't unreasonable compromises."
The Obama administration says it has notified insurers that their networks will get closer scrutiny for next year in the 36 states served by the federal exchange. Cancer care will be a priority, it says.
Associated Press writers Sheila Burke in Nashville, Tenn., Kim Chandler in Montgomery, Ala., Emery Dalesio in Raleigh, N.C., Jeff Karoub in Detroit, Brady McCombs in Salt Lake City, Christine Scalora in Lincoln, Neb., and Christopher Weber in Los Angeles contributed to this report.