I'm pretty sure stiff joints, achy joints/bones are a side effect of Keytruda. Please confirm with your doctor. My husband has stiff, achy joints/bones, but has bad arthritis and bone lesions so it's hard to tell what's what? (He's been on Alimta + Keytruda and taking a chemo break so only on Keytruda right now.) I read water exercises may help with joint stiffness, etc... Worth a try??? ~Best to you. (FWIW: It seems our doc prefers CT's rather than PET's? A former doc preferred PET's over CT"s? Again, discuss with him. ?I don't know why you couldn't have both if it makes you more comfortable?)