WASHINGTON: Teen idol Justin Bieber on Wednesday mourned the death of a six-year-old cancer patient who snagged his heart after she became his make-believe wife.

“One of the greatest spirits I have ever known is gone... RIP Avalanna. I love you,” said Bieber on his Twitter account after the family of Avalanna Routh announced her passing on the microblogging site.

Routh, a native of Merrimac, Massachusetts, was nine months old when she was diagnosed with atypical teratoid rhabdoid tumor, or AT/RT, a rare brain disorder that usually kills its victims within a year.

Doctors at the Dana-Farber Cancer Institute in Boston, using a multi-prong therapy regimen that in some cases has overcome the tumor, succeeded in prolonging the child's life.

In August 2011, the hospital staged a pretend wedding at which Routh, a passionate Bieber fan, became “Mrs Bieber.” Six months later, after a Facebook campaign, Bieber invited her to New York to spend Valentine's Day with him.

She said afterwards how she and her “husband” ate cupcakes and played Candy Land, a children's board game, leaving her parents with the more grown-up task of raising public awareness of her disease.

Only about 30 new cases of AT/RT are diagnosed in the United States every year. Patients have a survival rate of just 10 per cent.