It was a phone call no one would want to make or receive. And when Christopher Reeve‘s widow Dana told her loved ones that she had been diagnosed with lung cancer, she could barely contain her anger.

Remembering how she had just lost her mother and husband in the space of four tragic months, she cursed before declaring: “It is so unfair.”

Sources close to the 44-year-old singer-actress say that as she poured her heart out to her father and sister, she revealed that the only symptom of her life-threatening disease had been an irritating cough. Although she has been told the cancer is inoperable, she is bravely undergoing chemotherapy treatments at the famed Sloan Kettering Cancer Institute in New York City, The National Enquirer has learned.

And her 13-year-old son Will, still struggling with the loss of his iconic father, is said to have become a tower of strength — offering to be there for her whenever she needs him.

Dana announced to the world that she was fighting cancer on August 9, a day after The National Enquirer posted an exclusive website story revealing she had been diagnosed with the disease.

How she first learned of her shocking diagnosis can now be revealed. It was in February that Dana became troubled by what seemed more like an irritation than a medical problem. Said a family insider: “Family members noticed Dana had developed a slight, persistent cough.”

Dana wasn’t concerned at the time. She was still grieving the loss four months earlier of her 52-year-old Superman star husband, who had been paralyzed by a 1995 horse-riding accident.

Then, on February 10, her beloved mother Helen, 71, passed away from complications of ovarian cancer surgery. Dana, however, found strength by immersing herself in the passions of her life.

She worked overtime as chairwoman of the Christopher Reeve Paralysis Foundation, which funds research into treatments for paralysis caused by spinal cord injury. And she was also reviving a singing career she had put on hold while she cared for her husband.

A cabaret gig on June 10 and 11 at Feinstein’s, a club in New York’s Regency Hotel, was upcoming and she was rehearsing long and hard.

But that cough was still there. “It was more a nuisance, really just a little cough,” said the family insider. “But Dana had it all through the rehearsals.”

Still, it didn’t seem to affect Dana’s performance when her big night came. Entertainment writer John Hoglund, who was at one of the shows, told The National Enquirer that Dana had the audience weeping at her intensely personal performance.

She sang songs to her son and late mother and then dedicated “I’ll Be Seeing You” to her dead husband.

“At the end, she looked up and blew a kiss toward the heavens to him,” said Hoglund.

But that cough was still there. It persisted as she made appearances in mid-June promoting a children’s book to raise money for the foundation.

The family insider said: “Dana went to the doctor finally in late June. The cancer first showed up on an X-ray. Then a scan was performed and it was bad. Dana was told the cancer was inoperable.”

Dana gave the grim news to her dad Charles Morosini, 72, a retired cardiologist, and her sister Deborah, 47, a pathologist, in a three-way phone conversation.

“Dana had a few expletives she used to describe her medical diagnosis,” said the insider. “She told her dad, ‘It is so unfair.’ He agreed but told her she was a fighter and could beat it. By the end of the call Dana was determined to survive.”

Dana, who has never smoked, has already undergone several weeks of treatment.

“She started chemotherapy at Sloan-Kettering Institute in July,” said the insider. “She goes for treatments three weeks in a row and then has a week off before another three weeks of treatments begin.

While Dana is still physically strong, she “is looking like she has already lost weight from her struggle,” said the close source. Although Dana does not lack support from her family. Her father Dr. Morosini told The National Enquirer: “She’s tough and she’ll get through this with her usual grace and resilience.

“She’s a brave lady and has the right attitude to beat cancer. I’m hoping she’ll be the poster girl for lung cancer when she survives this ordeal.”

Dana also has son Will, who was three when his father was paralyzed. “Will is a tough kid,” said the insider.

“When Dana told him she had lung cancer, Will took it fine. He told his mom he was there for her anytime she needed him.”