Teddy Fortsmann’s doctors misdiagnosed his cancer for about a year, and now that he’s recovering from surgery to remove a malignant tumor from his brain they have concluded that he’s suffering from the same type of cancer that struck Massachusetts senator Teddy Kennedy, FOX Business has learned.

These are some of the details Forstmann, the billionaire businessman and private equity investor, has shared with friends and associates in recent weeks as he undergoes treatment for the disease and tries to remain a presence at his sports-marketing firm, IMG.

What strikes friends of Forstmann the most is that he’s defiant; he believes IMG is in the initial stages of its growth from a pure talent agency into a massive, global marketing company, and he wants to see that growth through. Though he now shares the day-to-day management responsibilities with a long-time executive at the company, Mike Dolan, he continues to make all major decisions, people close to him say.

But Forstmann is also realistic. Friends say he’s angry at his New York based doctors who ascribed his poor health over the past year to various illnesses including meningitis. It wasn’t until he recently went to the Mayo Clinic in Minnesota that he was diagnosed with brain cancer.

Teddy Kennedy died a little more than a year after his diagnosis, but doctors say Forstmann is responding well to his initial treatments of radiation and chemotherapy. Friends say the tumors that could not be removed have shrunk.

A spokesman declined to comment on the matter.

“Teddy’s determined not just to roll over on this,” said one friend with first-hand knowledge of the situation “But he knows that because of the misdiagnosis, the doctors caught the cancer late and he thinks he can take IMG much further than it is today.”

Charles Gasparino joined FOX Business Network (FBN) in February 2010 as Senior Correspondent.