I personally believe it looks malignant (cancer). A cystic structure would be more regular, spheric, fluid-filled. This looks abnormally shaped and well imbedded.
There are 3 tests your vet can perform after they've cleared the infection out:
- FNA (fina needle aspirate) - can be done conscious, in the consult room; they pop a needle into that lump and have a look under the microscope; unfortunately, this option is very limited as there is very little quantity inside the needle. Slides can be sent to the Laboratory as well to be analised.
- Biopsy: the best option! But it requires a General anesthetic. Again, we can do this just after we've cleared all the infection. I personally like to take chest X-rays if I have them under, to make sure we are not having any nasty surprises in the lungs. This option tells us, black on white, what's the nature of that lump, how to get it out, can we do chemotherapy on it? (if needed)
- Excisional surgery: again, done under General anesthesia, less safe if no biopsy is done first as we might not take enough margins and re-intervention might be needed if it comes back as a highly malignant cancer.