"We received sad news in the last few days. My dad met with a specialist in Boston and he has an incurable cancer," Gagne said in a statement released to Canadian media. "We hope a miracle will happen. Right now there are more important things than hockey and I want to be with my family in Quebec City."
In a statement released by the Bruins, Gagne specified the illness as liver cancer.
After Tuesday's practice, coach Claude Julien said it was unknown how long Gagne will be away from the team.
In his first season in Boston, Gagne, 34, has three goals and one assist for four points in 23 games for the Bruins.