I was diagnosed with phyllodes tumor (benign) & read it's rare & may come back malignant. should i consider mastectomy to prevent future reoccurrence?

Well: You are very young and that is a huge step . It is big both physically and mentally. The recurrence rate is not so high. I would wait until it recurred but you could certainly get an opinion from an oncologist

Maybe: Speak with your breast surgeon. Many phylloides tumors of the breast an be cured with lumpectomy alone. Tumor size, grade (how abnormal it looks under the microscope), surgical margins, and othe features may suggest a higher risk of recurrence for which mastectomy, or radiation after lumpectomy may be suggested. There is an online risk calculator at http://mobile.Sgh.Com.Sg/ptrra/.