RE:RE:RE:RE:RE:RE:QuestionGoTogether,
I think the answer you are looking for is that the laser isn't directed at the cancer cells, instead the goal is to uniformly illuminate the inner surface of the bladder. Hence why they fill it up with water to make it as close to a balloon as possible. And hence why Jeff Cassidy's computer simulations model where to place the fibre optic based to get uniform illumination. They want no "hot spots" so they don't damange the bladder. The instensity is chosen so the 10:1 difference in concentration of PDC in cancer cells causes enough reactive O_2 to kill the cancer cell but not enough in the good cells to kill them.
That is my understanding.
Cheers.