A MD-PhD in Oncology
Since you indicated that you are focusing on oncology, it seems likely that a PhD will enhance your oncology practice. This thread on The Student Doctor Network explores the pros and cons of an MD-PhD in Radiation Oncology. The general consensus is that it is worthwhile if you plan to be active in research as an MD, otherwise it may be a waste of time, money, and energy. 

As Shion mentioned in a comment, most MD-PhD's become medical research professors, while it sounds as though you will be in active medical practice. There is a possibility that having a PhD may exclude you from consideration by the hiring boards at non-research-focused institutions. At research-intensive insitutions, the PhD will probably help your case, and certainly can't hurt. It appears that many practicing physicians at top US cancer centers are MD-PhD's. See here, here, and here.