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Your vet is right. About 1 in 10,000 cats have a tendency to develop fibrosarcomas (cancer) at injection sights. Vaccines are the most common trigger, but we see them after antibiotic injections too. The FELV vaccine is the biggest offender and it is usually given in the left hip/leg. Fibrosarcomas are firm, rapidly growing masses so this could be one. If it is a fibrosarcoma simple removal of the mass is surgically difficult and ineffective because it comes right back. Amputation of the entire leg is usually needed to get all of the tumor. Amputation provides a cure in about 75% of cases. In the other 25% we find that the tumor is already spread either up the leg or to other parts of the body. If money is not a concern then it is best to do an MRI before surgery to determine how deep the tumor is invading.
It sounds like your vet is making the right recommendation to get the biopsy and confirm that it is fibrosarcoma and then pursue treatment accordingly.
I hope that helps, but if you have more questions let me know.
I guess we feel that the biopsy isn't going to change the inevitable. It sounds pretty well cut & dried & NOT GOOD! So instead of putting 14 yr. old kitty who we love dearly through the biopsy, we feel we're just going to accept that we're dealing with a terminal situation & enjoy her company as long as we can. I hope we're not copping out by not going with a biopsy, but feel that it sounds like her fate is already sealed so that's what we're going with. Thank you
The only other thing that your vet could do is a needle aspirate. It can be done awake and is relatively painless. It is not as accurate as a biopsy but may allow you to get a diagnosis without the kind of pain and expense of a biopsy. But you are right, if a leg amputation with a 75% cure rate is not in your plans then enjoying the time you have left may be the right decision.
I usually see them 1-3 years post vaccination...... I have never seen on less than 6 months after a vaccine.