Breast cancer is by far the most common cancer among women in the United States and around the world. About 12 percent of women in the U.S. will develop invasive breast cancer in their lifetimes.

Doctors are working hard to better understand this life-threatening disease. Their goal is to find new and more effective ways to treat the more than 266,000 American women diagnosed with invasive breast cancer each year in the United States with the least amount of harmful side effects possible, while at the same time preventing the spread or recurrence of the disease.

Behind the statistics are individuals – understandably fearful, but also hopeful.

A few years ago, I gave my 55-year-old patient with breast cancer – I’ll call her Helen – some good news. Her 2-centimer cancer tumor was confined to her breast and could be removed surgically with a lumpectomy, followed by radiation.

Helen was also estrogen receptor-positive and her cancer had not spread to her lymph nodes under her arms, which put her in the category of half of the breast cancer cases diagnosed in women in the U.S. each year.

Fortunately, Helen was offered and was likely to respond to anti-estrogen therapy. The chances of a dreaded recurrence of her cancer were low.

But Helen’s next question was the one that has troubled cancer specialists for decades. Should she have chemotherapy too?

Chemotherapy has serious side effects. If Helen received it, her hair would likely fall out, and she would probably lose her appetite and become nauseas. Countless chemo patients lose weight and suffer a short-term risk of infection and a long-term risk of leukemia.

Would getting chemotherapy be worth dealing with all the inevitable side effects?

The answer years ago was an automatic “yes.” But the answer has now suddenly changed. For approximately 70,000 women in the intermediate risk category for recurrence, the answer is now “no.”

The new answer for now and for the future is based on the application of genetics to personalized medicine. There is a genetic test that examines the expression of 21 genes and gives a risk score from 0 to 100.

This accurate tool was the focus of a just completed multi-center clinical trial called TAILORx (The Trial Assigning Individualized Options for Treatment). The results were published this week in the New England Journal of Medicine.

The trial studied close to 10,000 women for more than a decade and determined that if the patient had a mid-range genetic recurrence score of 11 to 25, in most cases there was no difference whether she added chemotherapy or not.

Dr. Francisco Esteva, director of the breast medical oncology program at the New York University’s Langone’s Perlmutter Cancer Center, told me that this trial was designed around 2003.

At that time, the standard of care for most of these patients was chemotherapy and hormones, and “lots of patients were treated years ago for no benefit, because we didn’t have a good way to identify the lower risk group reliably,” Dr. Esteva said.

Now, as we enter an era of genetics-driven personalized health care, treatment decisions regarding breast cancer can be made on a patient-by-patient basis using her genetics as a guide. And the good news is that novel multi-gene prognostic tests are now available, including Oncotype, Prosigna, Mammaprint and others.

And it’s not just breast cancer or even just cancer. We are entering a brave new world of genetic-guided therapies. Probabilities are now becoming anchored in your personal genetics, your individual mutations. Diagnoses in the future will be made by the changing the DNA footprint in your bloodstream and treatments will be based on your personal code.

This study is groundbreaking. It is the first time an extensive genetic test has been utilized to accurately predict outcome, the first time that such a test can literally help dictate treatment on a patient-by-patient basis. Younger breast cancer patients with local disease who are hormone receptor-positive and at relatively higher risk may still opt for chemo, but it will be a more informed decision.

The bad old days of hoping to poison the tumor more than you poisoned the body are rapidly being replaced by tailored targeted immune and genetically derived personalized treatments that give a patient the same or better chance of survival with fewer side effects.

Over time, this can make a big difference in the lives of millions of us, for all sorts of diseases, with the goal of giving us longer and healthier lives.