Your age, ( young), pre-menopausal , plus a Ki 67 of 40% (that's really high) and Grade 3 (aggressive growth) would lead me towards taking the chemo for sure. The good news is that as your tumor is Grade 3 and grows quickly there will be lots of cell turnover and it will get smacked down hard by the chemo. Get as much information as you can and then make the decision you can live with. Good luck. ( When I was diagnosed last year, I was told I didn't need chemo, as my tumor was slow growing and chemo didn't work well on it. Now 6 months later my tumor has grown massively (10cm) yes 10, cm (100mm), the endocrine therapy is not working, and they are now starting me on chemo next week. Good luck.