Fibrosarcomas are highly malignant (spread quickly) tumors. They can occur for no apparent reason and they also occur in some cases as a form of vaccine reaction at vaccination sites. Obviously, this is a serious side effect when vaccinations are implicated as the cause.
I was at a seminar on oncology last year and if I remember correctly, the speaker said that amputation will work to prevent metastasis (spread) of this tumor in about 50% of the cases. Presumably it has already spread in the cats in which this doesn't work. It is recommended that the amputation be followed up with radiation and/or chemotherapy if possible.
Radiation and Chemotherapy Treatment for Fibrosarcoma
Fibrosarcomas on any part of the body in which they can be surgically removed and then treated with radiation therapy and chemotherapy have about a 30% survival rate after 18 months or so, as reported by oncologists on the Veterinary Information Network. I am assuming that it is possible to get a much wider surgical margin when amputating a leg and that the difference in survival times relates to that. It is very hard to get an inch or two of clean margin around one of these tumors in any other manner -- that inch has to be in ALL directions and if the tumor is on the body, it usually means removing the body wall below the tumor, a difficult task in many cases.
As odd as this may sound, I do not know what the survival time without treatment is, nor did I find any study referring to one. From experience, it seems much shorter than 18 months, though.
Amputation is always a difficult choice to make. Most cats do very well on three legs, though. I do not recall a client who was upset at having made a decision to amputate in order to try to prolong a pet's life in our practice after the fact. It is just surprising how well pets do after amputation.