How is it possible?

My mother was very recently diagnosed with a large pelvic mass, lesion on the liver and pleural effusion. The CT scan indicated ovarian neoplasm. My mother had a hysterectomy and BSO four years ago for fibroids. How can the tumors be ovarian in origin? No indication of cancer was made at the time of surgery, so I'm more than curious about the pathophysiology. Also, how can the CT scan determine the tumors are ovarian neoplasms? No biopsy has been done. Are ovarian masses easily distinguishable from other masses?  Lastly, I realize how much genetics plays a role in ovarian cancer. What is the typical recommendation for immediate female family members (like me). I am over 40 years old with no intention of having more children.  Do the experts recommend a total hysterectomy? bso? both? none?  

Thank you so much for the time.