I am very happy to report-- ecstatic would be more like it!-- that Jack is doing fine. The vet prescribed a one-month of antibiotics and at the end of that, late last week, his x-rays showed no signs of the spots that had appeared on the first set. That means that what we saw on the films was not cancer, but rather some unidentified infection that the drugs knocked in the head.
Since we don't know what the infection was I will have to keep a close eye on him in the weeks ahead. But for now 'all is well' and I couldn't be more pleased.