For what it is worth, Pofi's suspected cancer was a peripheral nerve sheath tumor in the brachial plexus, so an MRI is really best and only good way to map out the surgical plan to try to get all of the tumor in soft tissue.  But in terms of trying to get a sense of whether there had been spread to lungs, we only did chest X-ray.  The MRI was all on chest/shoulder/armpit.  Everyone I personally know who has had an osteo amputation on their dog had depended on the chest x-ray.  I have known someone who had the amp done after spread to lungs was clear and confirmed because pain was too much.  She still had a good four months past the amp.

We went to a specialist.  My vet practice has two very accomplished surgeons, one of whom had had to perform 2 such amputations early in his career without the aid of MRI - so he called it a presumptive diagnosis and said it was difficult and harrowing as he could not be sure he was getting all the tumor.  The other flat out said he believed we would be best served by the specialist.  

We had same tests as Happy Hannah.  Otis broke his leg on a Sunday, so spent one night in an emergency vet hospital before we could get him transferred on Monday to a facility that had both a qualified surgeon and an oncologist.  The emergency vet said they could do the amputation, but had no oncologist.  I wanted the seamless care of having both on site, and the knowledge that the facility handled osteo cases routinely.  We also did chemo and have completed our four rounds of carboplatin.