A 56-year-old woman presents for evaluation of increasing abdominal girth, fullness, and abdominal pain. On examination, her abdomen is distended and she has had progressive weight loss of 50 lbs over the last 1 year. Computed tomography is performed (Figure). Her gynecologic examination and transvaginal ultrasonography findings are normal. Findings of colonoscopy are unremarkable. What is the best step in management?