Correction appended below

For decades, tumor size has been crucial to the treatment and prognosis of breast cancer.

Bigger usually meant the cancer was more advanced and at a higher stage, required more treatment and brought a worse outlook.

But now physicians have biological tests that help them determine how aggressive the cancer is likely to be and the best course of treatment.

One test in particular is considered so reliable that on Jan. 1 it will officially become part of the formula physicians use to determine the breast cancer stage.

The change could affect about a quarter of patients by bumping some down a stage, said Dr. Nathalie Johnson, medical director of the Legacy Cancer Institute.

The test, called Oncotype DX, also influences treatment, eliminating or changing chemotherapy.

It was a blessing for Kathy Landis-Kays, 53, of Portland, who was diagnosed with stage I breast cancer in April. She caught the tumor early but her biology isn't great. She has a gene mutation that causes breast and ovarian tumors. Her cancer was also on the move.

In the past, she could have lost her hair during treatment. But the test indicated that she'd fare just as well with a regime that would not affect her hair.

"The quality of life I get to live during this treatment is significantly improved," Landis-Kays said. "I'm able to have a less toxic treatment."

Her physician, Dr. David Page, a medical oncologist at Providence Health & Services, has been using genetic tests to diagnose breast cancer patients for years. He said the official inclusion of Oncotype DX into the staging formula marks an evolution.

"It's a watershed moment of sorts because it's telling us that measuring a tumor with a ruler and looking at lymph nodes is an antiquated way of doing things," Page said. "We're finally starting to build in more sophisticated tests into the paradigm of telling a woman what stage of cancer it is."

Staging is important in cancer diagnosis. It defines the tumor so doctors involved in diagnosis and treatment, from radiologists to surgeons, know what they're dealing with. It also helps specialists design studies and compare treatments among patients.

And it's a numeral that's easy for patients to understand. Stage I signals an early cancer. Stage IV means it has spread around the body and is no longer curable.

Patients want to know where they fall on that spectrum.

"People ask me about it every day," Johnson said.

The answer can affect a patient's mood and outlook.

"Hearing that you're stage I feels a lot better than hearing you are stage IV," Johnson said.

NAILING DOWN THE DIAGNOSIS

Size still matters in breast cancer in the sense that it indicates the quantity of cancer cells. The more there are, Page said, the more likely it is that some have figured out how to travel to other parts of the body.

When a cancer spreads beyond the breast, or becomes metastatic, it isn't likely to be curable.

But size doesn't determine treatment.

"It isn't about the size of your tumor or even what's in the lymph nodes," Johnson said. "It's the biology of your cancer that makes all the difference."

Tests tell doctors whether the tumor can be attacked with treatments aimed at a molecule, HER-2, that divides and multiplies cancer cells. They also determine which cancers are fed by estrogen or progesterone, two sex hormones. Hormonal therapies can shrink these tumors.

The Oncotype DX test is used for the estrogen-fed tumors, about 80 percent of breast cancer cases.

AN OPTION FOR TREATMENT

Landis-Kays' tumor was in that category. She discovered it last March during a self-exam.

The cancer hadn't spread to her lymph nodes. But genetic tests revealed she has a mutation of a Breast Cancer Gene – BRCA2 – that greatly increases the risk of breast and ovarian cancer.

Actress Angelina Jolie also has the mutation, prompting her to have a double mastectomy to prevent illness.

Landis-Kays underwent the same surgery in May that got rid of the lump. But physicians also discovered another area in her left breast where cancer was forming.

That – and her genetic mutation – meant she needed chemotherapy.

The Oncotype DX test put her in a medium-risk category, with a 17 to 22 percent chance of recurrence. Her doctor said she had a choice of two types of chemo. One would have caused her to lose her hair. The other wouldn't. Both would reduce her risk to less than 10 percent.

She opted to keep her hair.

"To have testing that showed me statistically and scientifically that I'm in a range where I'm making a reasonable choice – that was an amazing gift," Landis-Kays said.

She's been able to keep working and has suffered minimal side effects.

"I feel good," Landis-Kays said. "I'm enjoying my life."

LESS AGGRESSIVE TREATMENT

Chemotherapy used to be the go-to therapy for breast cancer but these days it's not always the best option.

Johnson had one patient several years ago with a very large tumor – 7 centimeters wide – more than three times larger than a small tumor. Tests indicated that her cancer was slow-growing and susceptible to estrogen.

Johnson knew she could use a hormonal therapy but she was reluctant to just rely on that.

"The biology of her tumor looked really good but she had all this disease," Johnson said. "Our standard was to give chemotherapy."

The woman got one dose. It made her so sick that she ended up in an intensive care unit where she stayed for weeks.

The chemotherapy had no effect on her tumor so Johnson tried an anti-estrogen medication that isn't toxic.

Three months later, when the woman was out of the hospital, Johnson re-examined her. She couldn't find a lump. A magnetic resonance imaging test confirmed the cancer was gone.

That taught Johnson a lesson that sometimes less treatment – or targeted medication – is best.

"Even if you have a lot of disease, if the biology is good, you'll do fine without chemo," she said.

Correction: Kathy Landis-Kays' name was misspelled in an earlier version of the story.

-- Lynne Terry