the main problem is not that rats got cancer. the main problem is margin of safety for usage in women is unknown.
arna tested high doses for both male and female rats. the minimum dose give to rats was 5x human dose, upto 10-15x human dose. male rats did NOT get cancer at lowest dose (5x human dose) and got cancer at higher doses (10x dose) - this shows that 5x dose is safe for male rats.
however, female rats got cancer at lowest dose itself (5x human dose). so the margin of safety for females is UNKNOWN!! no one can say what would have happened if female rats were only given 2x human dose or 3x human dose, and if they would have developed cancer or not. the fact is NO ONE knows, since female rats were found to be developing cancer at lowest dose given to them.