Cancer tends to be extremely sneaky. It is often a smoldering disease that sometimes we are fortunate enough to catch early, often by mistake looking for something else, but many times very late in the disease process. The reason for that is that cancer is an abnormally fast growth of what starts as normal tissue, but often morphs into abnormal tissue that invades where it shouldn't, spreading and destroying tissue close to the primary tumor and in many cases spreading to other distant sites where filter organs (the lungs, liver, spleen, lymph nodes) filter out the cancer cells allowing a new (metastatic) tumor to grow.As the tumor grows and destroys tissue around it we can see tissue death, which is perfect food
for bacteria. That is why we often see secondary bacterial infections along with tumors, and we can get fooled thinking that is the only problem.Until cancer destroys tissue such that it is no longer functional (the lungs are so filled with abnormal cells that they cannot exchange oxygen, the liver can now longer perform its job as a clean-up organ because normal liver cells are replaced by cancer cells, or the jaw breaks or becomes lytic because cancer cells replace bone) we may not know it is there.So if your kitty seemed to have a nasal discharge that improved with antibiotics then a sinus infection that was treated and responded well makes sense. If we saw some teeth that looked infected, broken or had a lot of tartar then a good cleaning and possibly extracting any teeth too diseased to treat would make sense.Unfortunately I suspect that when they cleaned Sweetie's teeth they realized that there was more there that originally was thought. The jaw likely felt soft and maybe fragmented when the tooth roots were removed. Radiographs of his jaw likely showed proliferation of soft tissue (tumor) and breakdown of normal jawbone (lysis). If a biopsy was taken and submitted then the tumor could be confirmed.I think that this has happened to every veterinarian, and it is heartbreaking for everyone. To go from expecting to lose some teeth, but curing the infection and making him healthy to having to break the news of cancer and all that holds is not what we want to do, or have happen to our patients.You are correct in that with these types of tumors eventually the jaw gets damaged enough that it is too painful to eat, the jaw breaks, or the tumor spread leads to other life-threatening symptoms. But in the meantime we can focus on controlling pain, (Buprenex works pretty well), reducing inflammation, feeding high quality, high energy, soft foods to make sure he gets needed nutrition without hurting him or further damaging the tumor site, and using antibiotics and oral rinses as needed to control secondary infections.I know that you are heartbroken, and I am very sorry for you and Sweetie.Please let me know if you have any further questions.