Oncologist Siddhartha Mukherjee was treating one of his patients, a woman with advanced abdominal cancer who had relapsed multiple times, when she asked him what seemed like a simple question.

"She said, 'I'm willing to go on, but before I go on, I need to know what it is I'm battling,' " Mukherjee tells NPR's Terry Gross.

But, as Mukherjee explains, describing his patient's illness wasn't so simple. Defining cancer, he says, is something doctors and scientists have been struggling to do since the disease's first documented appearance thousands of years ago.

"Cancer is not just a dividing cell," he says. "It's a complex disease: It invades, it metastasizes, it evades the immune system. So there are many, many other stages of [defining] cancer which are still in their infancy."

Mukherjee's new book, The Emperor of All Maladies: A Biography of Cancer, grew out of his desire to better understand the disease he treats, through examining the way cancer has been described and treated throughout history. He chronicles the ways therapies evolved, particularly in the 20th century, as more treatment options became available and scientists worked to understand the underlying genetic mutations that caused the disease.

"If there's a seminal discovery in oncology in the last 20 years, it's that idea that cancer genes are often mutated versions of normal genes," he says. "And the arrival of that moment really sent a chill down the spine of cancer biologists. Because here we were hoping that cancer would turn out to be some kind of exogenous event — a virus or something that could then be removed from our environment and our bodies and we could be rid of it — but [it turns out] that cancer genes are sitting inside of each and every one of our chromosomes, waiting to be corrupted or activated."

As the genetic understanding of cancer evolves, Mukherjee says, oncologists will be able to integrate that knowledge to develop more targeted treatment options — particularly as they find commonalities between different types of cancer.

"A breast cancer might turn out to have a close resemblance to a gastric cancer," he says. "And this kind of reorganization of cancer in terms of its internal genetic anatomy has really changed the way we treat and approach cancer in general."

Dr. Siddhartha Mukherjee is an assistant professor of medicine at Columbia University and a staff cancer oncologist at Columbia University Medical Center. His articles have been published in Nature, The New England Journal of Medicine, and The New York Times.

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