A 62-year-old nulligravida woman had 10 months of progressive abdominal distension, diffuse abdominal pain, and 25-kg weight gain. She had no nausea or vomiting, hematochezia, or stool changes; pelvic and abdominal examination showed a large abdominal mass extending from the epigastrium to the pelvis but no other abnormalities; results of laboratory testing, including cancer antigen levels, were unremarkable. What is the diagnosis and what would you do next?