After a surgeon removed a cancerous lump from Karen Medlock’s breast in November, he recommended radiation, a routine next step meant to keep cancer from recurring.

But he did not send her for the kind of radiation most women have received for decades.

Instead, the surgeon referred her to a center in Oakland, Calif., specializing in a newer form of treatment where radioactive “seeds” are inserted in the tumor site. It could be completed in only five days instead of the six weeks typically required for conventional treatment, which irradiates the entire breast using external beams.

To Ms. Medlock, it seemed an obvious choice. The newer treatment  given through a system called MammoSite  has been performed on about 45,000 breast cancer patients in this country since the Food and Drug Administration cleared it for use in 2002.

Only when Ms. Medlock, 49, sought a second opinion did she learn a startling truth: MammoSite is still highly experimental.