Let me make one thing perfectly clear: Walter White is NOT faking that his cancer has returned. Some commenters on Reddit have suggested this as a possibility, which it is really not. Walt is a lying liar and would say anything that he thought would save his hide, but (a) chemo is vile poison and no one would suffer having that stuff pumped into their veins if they didn’t have to; and (b) even assuming there are a few sickos who might try to pull off a caper so absurdly self-destructive, they would most likely fail.
Unless they have either been treated for cancer or closely followed someone who has, most people do not realize just how toxic the chemo drugs are. The only reason chemo (sometimes) works is that it attacks and kills cells in your body, which is why needle sticks for chemo nurses represent a significant job hazard. If a nurse misses the vein even slightly when administering chemo, the burns will cause tissue damage.
To qualify for the dubious privilege of having this vile shite pumped into your veins, you must first prove your worth by showing positive results on objective measures like scans and blood tests. You can’t just plop your sorry arse into a chair and order a round of chemo like you are in a barbershop asking to have your head shaved, even if you are Heisenberg.
Taking all of the above into consideration and reinforcing the point with my own unfortunate personal experience, having chemo drugs introduced into your body is at best a highly unpleasant experience and at worst fatal. Although it is true that a few people with a psychological disorder called Munchausen Syndrome may have deliberately dosed themselves with chemo drugs to cause their white blood cell counts to plunge or some such fuckery, I do not believe Walt has Munchausen Syndrome any more than I think he is faking that his cancer has returned.
The writing on a show like Breaking Bad is so rich and complex that I think we do it a disservice when we distract ourselves from the abundant conflicts, ambiguities, and emotional transformations these characters are facing. Even the wildest speculations of Breaking Bad viewers may be interpreted as a positive reflection of how passionately the show manages to involve us, so I can’t really fault anyone for wondering for just one moment if Walt might really go so far as to fake that his cancer has returned.
If you respect the chemistry of lung cancer diagnosis and treatment procedures enough to understand just why it is so terribly improbable that Walt is faking, you must also accept that he may now be facing his own mortality. Having reacted to his diagnosis of fatal lung cancer by gaining near-superhuman control over his own circumstances and surroundings and even over other human beings, Walter Hartwell White has now come full circle. Let the consequences of his dastardly and self-interested actions ensue.