The differential diagnosis of retroperitoneal tumor associated with aortic aneurysm is a challenge. A 79-year-old frail male elderly, who had the history of lymphoma, was found to have a retroperitoneal tumor adjacent to the left common iliac artery aneurysm. Although there was no distant metastasis detected, the patient was in poor nutritional status, and he complained of pain in the lower left leg. The diagnostic procedure and potential treatment options were discussed in the cancer board. How to set the goal of his treatment was quite difficult. The patient decided to receive the best supportive care, and he did not undergo a biopsy of the tumor to make a pathological diagnosis.