Her death was announced by her namesake organization, the Dr. Susan Love Foundation for Breast Cancer Research, which said she had recurrent leukemia. Dr. Love was diagnosed in 2012 with acute myelogenous leukemia, a blood and bone marrow cancer.

For years, Dr. Love said, interviewers came to her expecting to hear a tragic story about how she had been inspired to specialize in breast cancer, perhaps because she had been at the bedside of a beloved relative who died of the disease. Certainly not, she said: In fact, “it was pure sexism” that led her to focus on breast surgery in the 1980s, after she became one of only a few women to graduate from her medical school, and one of even fewer women to go into surgery.

“I was a chief resident in surgery at Harvard, and when I finished, nobody offered me a job,” she told a UCLA interviewer in 2018. “And so I hung up a shingle, and the people that were referred to me were all women with breast problems. And I found that I could make a much bigger difference in that arena than I could fixing hernias and doing general surgery.

“There’s always something unique that, as a woman, you can bring to the table that actually makes it better,” she continued. “And the trick, I think, is not to just give in and put up with the [garbage], but to make your own path.”

Dr. Love was among the country’s most prominent advocates for breast cancer research and treatment. She directed breast clinics in Boston and Los Angeles, taught at Harvard Medical School and UCLA’s David Geffen School of Medicine, and was an early champion of cancer treatments that were less invasive than traditional methods, which she described as “slash, burn and poison.”

To the dismay of some colleagues, she argued that radical mastectomies were frequently unnecessary, and were used simply because surgeons were accustomed to the procedure, in which the entire breast is removed. In their place she advocated for partial mastectomies and lumpectomies as a way to preserve breast tissue.

She also broke with colleagues in the way she would soothe and comfort patients, including by holding a person’s hand before they were put under for surgery. “The boys scrub, then come in when the patient’s asleep,” she told the New York Times in 1988. “I got razzed for it, but they’re used to it now.”

In 1990, she gained a wider platform with the publication of “Dr. Susan Love’s Breast Book.” Written with Karen Lindsey, it offered information on breast cancer treatment and prevention and became known as the bible of people with breast cancer. The book has been translated into at least a half-dozen languages, and an updated seventh edition is scheduled to be released this fall.

“Her book demystified breast cancer for the public,” said Fran Visco, a lawyer and breast cancer survivor who serves as the president of the National Breast Cancer Coalition. Previously, she recalled in a phone interview, “you couldn’t say ‘breast,’ let alone ‘breast cancer.’” When one of her relatives was treated for a lump in her breast, Visco added, she was too embarrassed to tell her children, and instead spoke of having “a lump on her arm.”

Determined to find a way to eradicate breast cancer, not just improve diagnosis and treatment, Dr. Love began to imagine a national grass-roots organization modeled on the work that AIDS patients did in pushing for recognition and treatment a decade earlier. She met with Susan Hester, a breast cancer advocate who shared her vision, and they soon partnered with a small group of women to co-found the National Breast Cancer Coalition in 1991.

Within a year, the coalition had 150 member groups. It organized marches and letter-writing campaigns, met with President Bill Clinton in the White House and lobbied for increased federal spending on breast cancer research, which grew from about $90 million in 1992 to $420 million in 1994.

Altogether, the coalition says its advocacy has generated more than $4 billion for research into the disease, which remains formidable: About 264,000 women are diagnosed with breast cancer each year in the United States, and about 42,000 women die, according to the Centers for Disease Control and Prevention. Annually, about 500 men in the United States die of breast cancer.

“It was Susan’s vision that really launched breast cancer advocacy,” Visco said. “She had the vision that women were ready and capable of coming together and being a collective voice, to set an agenda in breast cancer and to lead a movement to end breast cancer. … She led us places where we didn’t envision that we could go.”

Dr. Love’s advocacy efforts also extended to her personal life, as an out lesbian parent in the years before same-sex marriage was legalized. After she had a daughter through artificial insemination, she waged a legal battle to ensure that her partner, fellow surgeon Helen Cooksey, could legally adopt and co-parent their child, Katie. The Massachusetts Supreme Court’s 1993 ruling marked the first time the state recognized a joint adoption by a same-sex couple.

“Helen and I have money and privilege,” Dr. Love said the next year, “so it’s our obligation to pave the way.”

The oldest of five children, Susan Margaret Love was born into an Irish Catholic family in Long Branch, N.J., on Feb. 9, 1948. Her mother was an artist and homemaker, and her father sold forklift trucks for Eaton. His job took the family to Puerto Rico and Mexico City, where Dr. Love graduated from high school.

While studying at the College of Notre Dame of Maryland (now a university), she felt called to enter the Catholic religious order of her instructors. She entered a convent but left after a few months, deciding she had a different calling.

“I wanted to save the world,” she said, “but they wanted to save their own souls.”

Dr. Love continued her pre-med studies at Thomas More College, a women’s affiliate of Fordham University in the Bronx. She graduated in 1970 and received her M.D. four years later from the Brooklyn-based Downstate Medical Center, part of the State University of New York. She completed her surgical training at Boston’s Beth Israel Hospital, a Harvard Medical School affiliate, and in 1988 became the founding director of the Faulkner Breast Centre in Boston.

Four years later, she was recruited to direct what is now the UCLA/Revlon Breast Center in Los Angeles. She retired from regular surgery in 1996, the year after she became the medical director of her namesake foundation, then known as the Santa Barbara Breast Cancer Institute.

The foundation, which is now based in Los Angeles, launched initiatives including the Love Research Army, which pairs cancer scientists with volunteers in an effort to speed research worldwide. Under the leadership of Dr. Love, it also developed a self-reading portable ultrasound device, designed for patients in low- and middle-income countries to determine whether a lump is benign or possibly malignant.

Dr. Love was also a member of the National Cancer Advisory Council and wrote books including “Dr. Susan Love’s Menopause and Hormone Book” (1998), which argued — somewhat controversially — against the widespread practice of using hormone replacement therapy to treat menopausal symptoms, which she linked to an increased risk for cancer.

She and Cooksey, her partner of 40 years, married in 2004 during a brief period in which same-sex marriage was offered in San Francisco. In addition to her wife, survivors include their daughter, Katie Patton-LoveCooksey; two sisters; and a brother.

Dr. Love said that for all her work on breast cancer, her own leukemia diagnosis thrust her into unfamiliar territory. Her youngest sister, Elizabeth M. Love, donated bone marrow for a transplant that enabled her to return to work until her death, and also to continue savoring some of her other interests: dance, travel, good food and drink.

“It just reminds you that none of us are going to get out of here alive, and we don’t know how much time we have,” she told the Times of her diagnosis. “I say this to my daughter, whether it’s changing the world or having a good time, that we should do what we want to do. I drink the expensive wine now.”