A 62-year-old woman presents to the office with a 1-month history of rectal pain, bleeding, and perianal mass. On digital rectal examination and anoscopy, a 3-cm firm mass is noted at the left lateral anal canal. Inguinal examination is unremarkable. Biopsy of the mass shows moderately differentiated squamous cell carcinoma. Staging computed tomography (CT) of the chest and abdomen and magnetic resonance imaging (MRI) of the pelvis demonstrate a T2N1M0 tumor. What is the best next step in her management?