A 50-year-old woman presents with lower back pain of 3 weeks in duration. Radiologic studies reveal several discrete lytic lesions in the lumbar back and pelvis. Laboratory studies show elevated serum levels of alkaline phosphatase. Serum calcium, serum protein, and peripheral blood smears are normal. Aspiration biopsy of a pelvic lesion shows keratin-positive cells. Which of the following is the most likely diagnosis?