All I knew about the child were his initials: R.S. He could have been Roberto Salinas, or Richard Salisbury, or any such combination. He had been 3 years old when he died in the spring of 1948. His body was buried in a small grave somewhere around Boston, but it was unlikely that I would ever exhume his identity.

The identity of “R.S.” had become something of a personal obsession. It was 2004, and I was writing a history of cancer. Early in its conception, I had made a decision to write my book by resurrecting the stories of cancer patients. Typically, histories of medicine are populated by scientific and medical heroes — brave doctors or obsessed lab-rats who invent medicines and seek cures — with an occasional story of a patient. But my instinct was to focus on those who had encountered the illness in their own lives.

The challenge, of course, was that many of these stories and their protagonists had vanished. The women who had borne the onslaught of Halstedian radical mastectomies in the 1940s and ’50s belonged to a past generation: The procedure — found to be no more effective than the non-radical mastectomy — is no longer performed. I dimly remembered meeting one such woman — an 80-year-old grandmother — in a Boston medical clinic in the late 1990s. Her surgery, performed when she was 40, had left her permanently disfigured, with a caved-in shoulder and a swollen right arm. But when I tried to track her down, I could no longer recall her name.

Among all the missing names and stories, though, one particularly gripped my imagination. In the late 1940s, Sidney Farber, a Boston pediatric pathologist, had used a drug called aminopterin to treat acute leukemia in children. The drug had produced brief, flickering remissions — and then the children had relapsed fiercely and died soon after. Farber’s paper describing these responses and relapses had appeared in the New England Journal of Medicine in 1948. The paper was a landmark in medical history: It was the first time that a chemical had been used to treat leukemia. There were about a dozen cases described in the article. One of them was a child named R.S.

R.S.’s story struck me as memorable because — as Farber’s paper noted — he happened to have a twin brother named E.S., a perfectly healthy young boy. (The case illustrates a crucial point about the development of cancer. Identical twins possess nearly identical genes in their cells. Yet a chance event — a genetic “accident” — can change the genes in one twin and thereby precipitate the disease. Thus, although cancer is ultimately genetic in its origin, the gene alterations that cause cancer might occur in one twin, but not the other.)

Author Siddhartha Mukherjee (Deborah Feingold)

If I was lucky, I thought, the twin would still be alive; and, if luckier still, he would be living somewhere around Boston. If I could locate the brother, I could find the identity of R.S.

But where to begin? The few old-timer Boston pediatricians and oncologists whom I knew possessed no memory of the boys. “Post your query on genealogy sites on the web,” one physician suggested. And so I posted — but with no response. “Ask the nurses or the social workers,” another oncologist advised. So I asked — but no one recalled a toddler with a twin from five decades past. A medical memoir written about Farber’s cancer clinic in the 1950s promised a wealth of patient’s names. But the fine print below the article said that the names had all been changed or deleted. The author, whose 3-year-old son had died of Wilm’s tumor in 1956, was also long dead.

In February 2006, after a fruitless two-year search, I traveled on vacation to my parents’ house in New Delhi. Before I left, I learned that a medical biographer with a particular interest in the history of early chemotherapy lived a few blocks from their house in Delhi. On a winter evening in 2006, I walked over to meet him. His place was called “Ella’s Cottage” — not a cottage, exactly, but a brick-and-concrete residence built in Delhi’s Brutalist style, circa 1970.

The biographer was in his late 70s, I think — a voluble scholar from South India. His living room was piled high with manuscripts, photographs, newspapers, books and clippings; his filing system for these, it seemed, was an encyclopedic memory that he could access at will. His wife moved noiselessly about the kitchen and then emerged with glasses of water. The man told me about his visit to Boston in the 1960s. He recalled meeting Farber — a reserved, formal, tightly coiled man. Outside, the insects buzzed noisily in the Delhi evening.

We spoke for two hours, and I got up to leave. And then, scanning the shelves, he pulled out a dusty yellow file and a few books (including one authored by him). Inside, there were letters and notes, and some newspaper clippings from the 1940s and ’50s. Would I like to have a look?

I rifled through. Buried within was a black-and-white newspaper photograph of a 3-year-old child. He was standing with his mother and his twin on a frigid Boston morning in 1947, looking somewhat bewildered in a woolen cap. He appeared rosy and cherubic, indistinguishable from his twin. From Farber’s paper, I knew that he was at the midpoint of his startling remission. In three months, his cancer would relapse, initiating a downhill spiral into death. The caption below identified him as Robert Sandler.

I returned to Boston triumphantly with the photograph. At the Boston Public Library, a 1948 directory revealed his home address. And so 15 minutes later, having journeyed 6,000 miles across the globe and back, I drove to Robert Sandler’s house on Blue Hill Road in Dorchester. This derelict building, partly occupied by a car mechanic, had once housed one of the first recipients of chemotherapy. I looked out, as if through R.S’s eyes, at the leafy arcades of Franklin Park — this might have been the view from his bedroom window — until my eyes clouded over. When I came home that night, my daughter, 3-years-old, was asleep in her bed.

“The Emperor of all Maladies” was published in November 2010. Within a few weeks, I received a phone call. I was in my laboratory, writing a grant, and an unfamiliar phone number — the area code indicated Maine — appeared on my office phone. It was Elliot Sandler, Robert’s twin. His niece had told him about a new book on the history of cancer. Curious, he had walked into a bookstore to find it. And then he had opened it to find his brother’s name emblazoned on the opening page. I had dedicated the book to R.S.: “To Robert Sandler, and to those who came before and after him.”

“I cannot tell you what happened to me when I read that name in a book,” he said. “My knees gave way under me. It was as if my brother had come alive from the dead.”

A few weeks later, I arranged to meet Elliot near Times Square in New York. He was 65 years old. He has his brother’s domed, round head, his sloping eyes and the odd, bewitching half-smile. When I saw him walk through a crowd, coming toward me, I needed no help recognizing him. This was a face that I knew by heart.