In May 2011, Cassandra Caton, an 18-year-old with honey-colored hair and the soft features of a child, suddenly went blind in her right eye. Five months later, an ophthalmologist noticed something disturbing. A large growth in the back of her eye had ripped her retina, destroying her vision.

He sent her to Washington University in St. Louis, a three-hour drive from her sparsely furnished apartment in the working-class town of Sedalia, Mo.

And there, Ms. Caton, mother of a 2-year-old daughter, wife of a chicken factory worker, got almost incomprehensibly bad news. The growth was cancer, a melanoma, and it was so huge it filled her eyeball.

“Am I going to die?” Ms. Caton asked. “Is my baby going to have a mommy in five years?”

It is a question that plagues cancer patients. Doctors try to give survival odds based on a tumor’s appearance and size, but often that is just an educated guess.

But Ms. Caton had a new option, something that became possible only in this new genetic age. She could have a genetic test of her tumor that could reveal her prognosis with uncanny precision. The test identifies one of two gene patterns in eye melanomas. Almost everyone in Class 1 — roughly half of patients — is cured when the tumor is removed. As for those in Class 2, 70 to 80 percent will die within five years. Their cancers will re-emerge as growths in the liver. For them, there is no cure and no way to slow the disease.

No test has ever been so accurate in predicting cancer outcomes, researchers said.

The data from studies of the test are “unbelievably impressive,” said Dr. Michael Birrer, an ovarian cancer specialist at Massachusetts General Hospital. “I would die to have something like that in ovarian cancer.”

While for now the ocular melanoma test is in a class by itself, cancer researchers say it is a taste of what may be coming as they continue to investigate the genes of cancer cells. Similar tests, not always as definitive but nonetheless able to give prognostic information, are under development or starting to be used for other cancers, like cancers of the blood.

Having a prognosis allows people to plan their lives, but most do not want

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