Sad news ... Lance Mackey -- a 4-time Iditarod champion -- has been diagnosed with cancer again after battling the disease in 2001.
The legendary dog musher was first diagnosed with throat cancer back in 2001 -- and after surgery and radiation therapy, he was cancer-free.
But, Mackey announced on Tuesday via Facebook that the illness is back ... saying, "My winter plans just changed yet again."
"I was diagnosed with cancer again and being a single father has made sh* a bit different from my winter plans.