The 70-year-old actor - who battled leukemia in the late 1990s - will have his symptoms treated, but insiders believe the disease may spread to his "bones, and other areas of his body", which could see him choose to undergo chemotherapy.
A family insider told RadarOnline.com: "Ryan's overall prognosis is very grim. Ryan will most likely begin to suffer from excruciating bone pain, and will need to be treated aggressively with the proper medication when it gets to that point. Ryan's cancer sadly isn't curable, but his symptoms will be treated. He could choose to undergo chemotherapy if the cancer continues to grow and spread."
The 'Love Story' actor - who lost his partner Farrah Fawcett to cancer in 2009 - recently revealed he was "stunned" to be diagnosed with the disease, but he admitted feeling "fortunate" that the condition was spotted early because it will improve his chances of a full recovery.
In a statement, he said: "Recently I was diagnosed with stage four prostate cancer. Although I was shocked and stunned by the news, I feel fortunate that it was detected early and according to my extraordinary team of doctors the prognosis is positive for a full recovery.
"I am deeply grateful for the support of my friends and family during this time, and I urge everyone to get regular check-ups, as early detection is the best defense against this horrible disease that has afflicted so many."