Some back story: my step dad had very aggressive small cell cancer in his stomach, incredibly rare (10 documented cases) and the doctors didn't quite know what to do. They wanted to cut him, and after the initial scans they noticed that the tumor was very large, so they started him with a course of chemo. He was a very fit guy, he would run at least 2 marathons a year, if not more, had morning jogs and didn't really slow down during the day until dinner. He responded very well at first to the chemo, he was still able to go running and still had his hair. The oncologist then discovered that he wasn't on the correct dosage he suggested, and they almost doubled his dosage. This completely wiped him out. He became very tired as the chemo began to take an effect on his system. As an onlooker this was a shocking turn as he changed almost overnight from being fit and healthy to house-ridden and hairless.
However, the chemo had a great effect on him, it actually completely removed all the cancer they could find on all the scans, but they still cut him open to remove the (now benign) growth left on the walls of his stomach. This had a knock on effect of severely reducing his food intake and also allowed the cancer purchase on a newly exposed wound. To cut a long story short, they shouldn't have operated on him in this case, and after 9 months of struggling against it we sadly lost him in the middle of the night (it was actually the eve of my 21st birthday...)