It is a general obstacle. Cancer cells are basically normal cells have mutated in such a way that they a) still survive, b) subdivide rampantly and c) fly under the radar of white blood cells, which normally pick up and deal with said anomalies before they become a problem.
So the trick is about getting white blood cells to detect amd attack anomalies that it previously wouldn't, while not attacking healthy cells.
Tumor-destroying sound waves receive FDA approval for liver treatment in humans (news.umich.edu)