I don't have experience with osteosarcoma and amputation, but I will share Harley's experience.

The day his surgery was scheduled to take place (and this was an amputation for a recurring infiltrative lipoma no mention of cancer) surgery was canceled because the chest xray revealed two masses -- a 6 cm mass on right lung and a 4 cm mass on the left lung, both peripheral.  The surgeon and I were both surprised to say the least.  I asked if it was metastasis from the limb and she said she very much doubted that.  My thought was something is going on somewhere else.  An ultrasound of the abdomen was done about two weeks later that showed 2 small masses (1 cm) on spleen.  Because masses on spleen were smaller than those on lungs, they did not metastasize.  So where did the lung tumors come from?  Primary lung tumors usually involve just one lung.  Next a biopsy of the larger of lung tumors was done after other tests were done to rule out blasto.  Biopsy confirmed cancer but since it was just a FNA pathologist was only able to narrow it down to one of these:  carcinogen, liposarcoma or histiosarcoma (?).  Liposarcomas can mimic benign sarcomas so in retrospect, what was believed to be a benign infiltrative lipoma could have been liposarcoma.

Now back to Zeus' tumor on his spleen.  Thumb to forefinger is maybe size of golf ball or large marble so maybe 8-10 cm.  My first golden had hemangiosarcoma of the spleen and by the time he actually showed any symptoms the tumor was so large that it was bi-lobed on either side of the spleen and they had to remove spleen.  It was so large that I nearly fainted when I saw the incision.  I think hemangio is fast growing so that would explain why no gradual symptoms.  So I guess my question is:  did you have an abdominal ultrasound prior to this one and were any biopsies done?  Maybe that tumor was already there and maybe it is not malignant, it could just be age-related and not be metastasis from his osteo.  I have learned from Harley's oncologist that it is possible to have two different cancers going on at the same time. 

Three months later, Harley's lung tumors have now grown to about 6 cm and 9 cm which is considered a slow growing cancer or "stable disease".  He really is not showing any significant symptoms of lung cancer and actually we are back to discussing a date for surgery on his leg!  I cannot cure the cancer in his chest, but I can give him more quality of life regarding the leg.

I have learned so much from this site, but the most important thing I've taken away is that life or medical issues are not an exact science.  There is a lot of assumption and maybes out there.  Unless you have MRIs every month of the complete body you really do not know what is there or when it showed up.  All we can do is address the symptoms we are aware of. 

Sorry for such a long post but I wanted to try to draw some similarities here.  I think dogs are actually just like people.  Some are diagnosed with a good prognosis and don't make it and others are diagnosed with a poor prognosis and go on forever.  Only God knows and I bet once in awhile he gets skunked too.

I think Zeus should get french fries with ketchup tonight.

I was going to jump in with the same thing that others have said. My vets said "no worries about pred use" when we were at a similar point.

I'm glad that he's holding his own and that you have these precious days to love him.

Yay! Tripawd Warrior Dude Zeusy!! You are telling that stoopid old cancer schmancer what for

So happy to hear you are having wonderful lovin' time with your mom.

Sending huge hugs and extra yummy treats

TWP Lylee and Joanne

xoxox

Glad to hear Zeus is still doing well! Sorry about the delayed response. Been away on vacation but had to check in to see how "our" Zeussy is going.

Kisses and nose-pokes from me and Rita!
Jackie