The Live Wire hitmaker's relative Tom was diagnosed with the disease earlier this year (12) and Sixx admits he has been struggling to hold it together as his grandpa enters the final stages of his life.
In a post on Facebook.com, he writes, "Watching him change daily breaks my heart as he is slowly breaking down. The doctors gave him 3 to 6 months to live 4 months ago. This prostate cancer is a ticking time bomb and our hearts are breaking in slow motion.
"The reality of it all is kicking in for my whole family...This man was there for me like a father. He never let me down. He was a role model and my biggest fan. The water runs deep between us so the blade of the knife also hits the bone. Love is a two-edged sword."
And the rocker reveals he'll be airing his emotions on Facebook in the hope of helping others who are watching a loved one struggle with life.
Sixx adds, "Why do I tell you this? 'Cause maybe I am not the only one in pain. We all deal with it in different ways. I can't run to a bottle or a bindle of powder (drugs), so I talk about it and, as it happens, usually write music about it."