Insanely Disturbing sums it up best
But heres my analysis of it
You have a man, who may represent man as a whole or an element of man. Who is lonely and depressed. The tree represents the numerous entertainment apparatuses (t.v, videogames, etc.) that try to make us feel happy but in the end cannot. The man is in a place where he is not himself and is in a deranged state (much like you Mr.Firth) and the dog represents a somewhat "normal" and not insane form of the man himself, not an actual dog. The tumor represents a false friend that simply wants to use you, and is more harmful that beneficial to you. Finally his "tumor" brings him to a breaking point in where either his life changes or he dies (suicide?). The "dog" (his voice of reason) finally resonates with him, and he leaves his depressed and contorted state, but not without physical and emotional scars (and deformities left) and also a part of him dies (he switched places with the dog, and that part of him dies). He finally tells the tumor (false friend) a sarcastic "thank you" because without the tumor he would not have gotten better but the tumor really didn't care either which way.
that's just what I think I'm sure David Firth probably disagrees with me alot, but that's what i got out of it.