A 34-year-old patient presents with vague pelvic discomfort. Digital rectal examination (DRE) reveals a 3-cm anterior soft, compressible lesion that is 4 cm from the anal verge. Pelvic magnetic resonance imaging (MRI) shows a presacral solid mass that measures 2.5 cm x 2 cm with its most proximal part at S3 level. There is a clear plane between the lesion and the rectum anteriorly and the sacrum posteriorly. What is the best next step in treatment of this patient?