Ovarian cysts and tumors: Pathology review

A 61-year-old woman, gravida 2 para 2, comes to the clinic due to pelvic discomfort and fatigue over the past 6 months. The patient was previously healthy, other than a history of polycystic ovarian syndrome. Menarche occurred at age 12, and menopause occurred 2 years ago. Both of the patient’s children were delivered via Cesarean sections due to personal preference. Family history is significant for breast cancer in her cousin at age 56 and colon cancer in her father at age 55. Vitals are within normal limits. BMI is 35 kg/m2. On physical examination, the abdomen is bloated, and shifting dullness is present. Pelvic examination reveals a left-sided adnexal mass. The patient undergoes a hysterectomy and bilateral salpingo-oophorectomy, followed by adjuvant chemotherapy. A pathohistological analysis is consistent with serous cystadenocarcinoma of the ovary. Which of the following is the most significant risk factor for this patient’s condition?