16 year old male moggie with osteosarcoma and strange mass in mouth

Hi Im looking for some suggeations as to what this light brown mass could be in my cat's mouth (images attached) 4 months ago my 16 year old male moggie has been diagnosed (though not via means of actual testing, as bone aspirations came back inconclusive) with an osteosarcoma growing on his jaw. The tumor has since grown many times larger and has been checked over at the vets several times. One of his premolars right near the center of the mass fell out yesterday and I've noticed a weird brown(ish) lump where the premolar fell out (and presumably where the premolar behind it should be). I'm thinking it might be a simple case of a large tartar build up on the remaining premolar, but his teeth were in pretty good condition just prior to diagnosis, so the tartar seems a bit too far gone in that short time. He went to the vets last week (before the tooth fell out) and the vet said that he was too old to have any real dental work, and that as his teeth weren't great (but still not terrible), palliative care was the best course of action. Any suggestions would be greatly appreciated!    

Hello!  What a fascinating case and what a clear, interesting photo (may people reading this will not realise that such a photo can be extremely hard to take).  Unfortunately it's not the sort of 'interesting case' that we'd like for your pet, as osteosarcoma can be painful and unpleasant.  Obviously I am not your vet and have not examined your loved one;  it sounds from what you say as though your vet is pretty convinced of their diagnosis (bone cancer of the jaw), although they don't have a positive histology sample to confirm it.  Diagnosis of such lesions can sometimes be aided by radiography for metastases and to demonstrate bone lysis (destruction) at the site; perhaps also from lymph-node aspirate and so on.  However, investigation is limited by such factors as age, cost, general-anaesthetic suitability and perceived benefit of the diagnosis when weighed against the potential welfare-costs of the various procedures.  Vets will advise as best they can on the individual cases in front of them based on the examination findings and experience.  Brown friable tissue found in the vicinity may be tartar, but it sounds as though other possibilities may include  infected gum tissue or friable tumour tissue (cancer cells grow rapidly and often don't resemble the tissue around them) or a nidus of necrotic gum tissue (e.g. cells that have died due to blocking off of blood supply) or dental infection in the weakened tissues thereabouts.  If the pathology of the tissue interests you, it can perhaps be tested specifically.  As ever, your vet is the person who can probably comment most accurately having seen it. You will have identified treatment targets with your vet.  Sometimes such targets simply include extending a decent quality of life for as long as possible, or preventing pain, or reaching a diagnosis.  All the best to you at this troubling time.