your healthy immune system at work!
Another picture of a killer cell (yellow) besides a large tumor cell (red).
The smaller killer cell has punctured and destroyed the large tumor cell.
A simple picture of a the cell locating and destroying E. Coli bacteria
The small cell on the left is a killer cell ( a cell of your immune system) finding and attacking a larger cancer cell (right). 
The tumor cells structural matrix is all that remains of the encounter with the killer cell.