Why are cancer cells more sensetive to radiation than normal cells?

Answer 1

What makes the cells sensitive is that they are dividing rapidly. Cells that do not divide at a rapid rate tend to be more resistant to radiation.

That is why the rapidly dividing cells in the intestines, bone marrow, and the testicles suffer so much during radiation treatment.

Meanwhile, other more stable cells like brain and nerve cells tend to sustain less damage.

The tumour cells are rapidly dividing (that's what makes them malignant, uncontrolled reproduction), so they will be sensitive to the radiation.

Answer 2

http://en.wikipedia.org/wiki/Cancer_stem_cell_theory