Is my 16-year-old cat's growing, oozing breast tumor cancer?

Thank you for submitting your question regarding Fiona. Unfortunately, there is no way to determine if the mass is cancerous or not just by looking at it. Her veterinarian could perform a simple procedure known as a fine needle aspirated. The doctor would use a small needle to collect a sample of cells to evaluate under the microscope. This can lead to a diagnosis in some cases, but can be inaccurate. For a more accurate diagnosis, the mass would need to surgically removed and the whole mass submitted to the laboratory for a pathologist to review. I understand your hesitancy to do so given her age. That being said, cats have a higher rate of cancerous mammary tumors compared to dogs. Furthermore, the mass appears ulcerated in your picture. This can can associated with malignancy as well. Unfortunately, there is a good chance that it is cancerous. I hope this information helps.