But she didn't let that stop her from singing. She posted songs online, gained a large and loyal audience and helped raise money for cancer research.
Allison's family announced on Tuesday that, after a week in hospital, the 19-year-old had died.
"She left memories and a legacy that will live on forever in our hearts and minds. She never quit fighting until her last breath; she showed more strength than anyone thought was possible," said a post on her blog written by her sister, Riley, and boyfriend John.
Allison was attending Queen's University when she returned home to get treatment for clear cell sarcoma, an inoperable form of cancer.
As soon as she gained attention online for her voice, she began fundraising for cancer charities and Princess Margaret Hospital, where she was being treated. She sang the national anthem at two Maple Leafs games, and lent her singing voice to cancer fundraisers across the city.
The Maple Leafs held a moment of silence in her honour at Tuesday night's game.
Her work with cancer charities will continue, as Carley's Angels is the Allison family's charity to help others with cancer treatments.
"Her life may have been short, but the lives she was able to touch and inspire is unparalleled to some people that have lived vast amounts of years," wrote her sister and boyfriend.