A 52-year-old postmenopausal woman from Mumbai presents with progressive abdominal distension and mild abdominal pain for 3 months. On examination, she has a firm, irregular pelvic mass and ascites. Serum CA-125 is 850 U/mL (normal <35). Imaging reveals a 10 cm left ovarian mass with peritoneal nodules and omental caking. Staging laparotomy confirms Stage IIIC epithelial ovarian cancer. She undergoes optimal cytoreductive surgery with no residual disease >1 cm. Which of the following is the most important prognostic factor that will influence her overall survival and treatment intensity?