He punched the s*** out of the soap dispenser in the bathroom after the MRI. He also caughed up what appeared the be blood (like he used to) in the flash foward scene in the diner
I thought he just looked at the soap dispenser and kind of reflected for a minute. Didn't he punch that thing back in season one when he was first diagnosed? I thought he just kind of noticed that it was still there after all this time.
It could be just me but when Walt saw the fist indent I took it as him taking a moment to reflect why he started cooking in the first place. He did it so he could leave his family something once the cancer got him. I think seeimg that put everything back into perspective for him and it is why he decided to get out of the business.
I dont think we can conclude with certainty that the cancer is back just yet.