Radiation may get a little easier for thousands of breast cancer patients: Doctors now can target cancer-killing beams just at the tumor site instead of the whole breast, cutting the usual six-week treatment down to five days.

A major study is under way to prove whether the easier therapy is as effective as the old-fashioned kind — and if so, who's a good candidate and which of three five-day methods works best.

Even before those results are in, Canadian scientists are working to speed treatment still further. They've developed a one-day method, permanently implanting radiation seeds inside the breast to kill stray cancer cells while women go about their normal routines — just like men's prostate cancer can be treated today.

It's an exciting time for this new approach, called partial-breast radiation, already fast gaining in popularity even before the National Cancer Institute-funded study to prove how well it works began recruiting volunteers in March.

But specialists warn that women must carefully weigh the new options — and that the best course for those who want the shorter therapy is to enroll in that study or several others testing the new approaches.

"The question is, how good are they?" says the NCI's Dr. Norman Coleman. "You're not treating the whole breast. What's the loss (in effectiveness) by leaving some of the breast untreated?"

Early stage studies suggest the five-day approaches can work well for at least some patients, but too few women have been tracked for long enough to be sure that partial-breast radiation works as well as standard therapy in preventing cancer's return.

"Patients need to understand where we're at," cautions Dr. Frank Vicini of William Beaumont Hospital in Royal Oak, Mich., who pioneered the five-day approach and is leading the new NCI study.

About 70 percent of the 200,000 American women who will be diagnosed with breast cancer this year qualify for a lumpectomy — removing just the tumor, not the whole breast. A lumpectomy plus radiation cures early stage breast cancer just as well as breast-removing mastectomies do.

But daily radiation treatment for about six weeks is an inconvenience at best, and at worst impossible for women who live far from medical centers or who can't spare that much time away from jobs or home responsibilities. Also, some women suffer serious skin side effects from the beams.

Consequently, many early stage patients choose a mastectomy. Worse, up to a quarter of lumpectomy patients forego radiation altogether, risking a recurrence.

Partial-breast radiation attempts to focus the powerful treatment on just the areas most likely to harbor stray cancer cells, those near the original tumor — and most methods do so from inside the breast.

In the NCI study, 3,000 patients around the country are being randomly assigned to either standard six-week radiation or one of three five-day methods:

--Interstitial brachytherapy, where thin tubes are inserted through the breast and pellets of radioactive iodine are temporarily placed in the tubes twice a day.

--Balloon brachytherapy, given with a machine called the MammoSite, that similarly inserts radioactive pellets into a balloon filling the tumor area.

--External radiation focused just to the tumor site.

In Toronto, Dr. Jean-Philippe Pignol is trying to make partial-breast radiation even faster — a one-day method just like men with prostate cancer can choose.

Pignol has permanently implanted pellets made of a different radioactive material, palladium, into 44 women's breasts. The pellets emit radiation for about two months until they run out; they then sit harmlessly just under the skin.

To work, Pignol and colleagues at Toronto's Sunnybrook and Women's Health Sciences Centre had to create an ultrasound-guided tube to implant the pellets at just the right spot, and talk the pellets' manufacturer into connecting the 60 pellets with strands, almost like a necklace, so they don't shift.

In a study published this month by the American Society for Therapeutic Radiology and Oncology, Pignol's first 16 patients tolerated the treatment well. He's tracking how the women fare over time.

Finally, the Food and Drug Administration last week approved a new machine, Xoft Inc.'s Axxent Electronic Brachytherapy System, to deliver partial-breast radiation through a miniature X-ray system, potentially easier for doctors to handle.

All of these methods must be rigorously tested to prove whether they're equally effective, cautions Vicini.

But probably, he says, "what you're going to end up finding out is there's no one particular technique that works in all patients. Ultimately, we'll need more than one technique."