I'm not a medical person, so all I can tell you is what I would look for. I would look for credentials and experience, of course. However, I'd also look for someone who treats the cancer patient as a person and not a "case." One who listens and places value on personal concerns, and, if appropriate, listens to the family. I'd also see if I can find some "word of mouth" feedback from previous or current patients (not always possible unless you already have connections to them).
Take care,
Carol