WASHINGTON – Six years after she was deemed cancer-free, Kara Kennedy accepted the Presidential Medal of Freedom on behalf of her father, just days before Sen. Edward Kennedy, D-Mass., died while battling a brain tumor.

However, her own lung cancer treatment — surgery and grueling chemotherapy and radiation — left her physically weakened, her brother Patrick Kennedy said. She died Friday at age 51 after her daily workout at a Washington-area health club.

“Her heart gave out,” said Patrick Kennedy, a Democratic former congressman from Rhode Island.

“She’s with Dad.”

Kennedy was a member of the Sport & Health fitness center, though spokeswoman Nancy Terry declined to release further details about the incident, citing member privacy.

Kara Kennedy was born in 1960 to Edward and Joan Bennett Kennedy, just as her father was on the campaign trail for his brother John F. Kennedy during the presidential primaries.

The late senator wrote of his oldest child in his 2009 memoir, “True Compass,” that “I had never seen a more beautiful baby, nor been happier in my life.”

Later, she and her brother Edward Kennedy Jr. helped run their father’s 1988 U.S. Senate campaign.

Her lung cancer diagnosis came in 2002, and the prognosis was grim. But the family refused to accept that, the senator wrote. She was able to have an operation, and Edward Kennedy accompanied his daughter to chemotherapy treatments.

“Kara responded to my exhortations to have faith in herself,” he wrote. “Today, nearly seven years later as I write this, Kara is a healthy, vibrant, active mother of two who is flourishing.”

Her children, Grace and Max, are now teenagers.

Her two brothers have dealt with health issues of their own: Edward Kennedy Jr. lost a leg to bone cancer as a child, and Patrick Kennedy had surgery in 1988 to remove a non-cancerous tumor that was pressing against his spine.

“Her magnificent strength in her successful battle with lung cancer was a quiet inspiration to all us and provided her family and fellow patients with hope,” the Edward M. Kennedy Institute for the United States Senate said in a news release.

Five months before her death, Kara Kennedy wrote of her father and the institute named in his honor in an article published in The Boston Globe Magazine.

She described Christmas 1984, when her father insisted on spending the night helping relief workers feed hungry people in the Ethiopian desert. And how each summer, the family loaded the family into a Winnebago for road trips to hike through historic battlefields and buildings.

“What mattered to my father was not the scale of an accomplishment, but that we did our share to make the world better,” she wrote. “That we learned we were part of something larger than ourselves.”

Kara Kennedy, a graduate of Tufts University, also worked as a filmmaker and in television. She helped produce videos for Very Special Arts, an organization founded by her aunt Jean Kennedy Smith.

She also served as a board member for the Edward M. Kennedy Institute; director emerita and national trustee of the John F. Kennedy Library Foundation; and a national advisory board member for the National Organization on Fetal Alcohol Syndrome.