A 68-year-old man undergoes follow-up evaluation. The patient has stage IIIB nodular sclerosing Hodgkin lymphoma that was initially diagnosed 3 years ago. Complete remission was achieved following administration of six cycles of doxorubicin, bleomycin, vinblastine, and dacarbazine (ABVD) chemotherapy. Two months ago, stage IIIB Hodgkin lymphoma was again diagnosed. Salvage chemotherapy was initiated with dexamethasone, ifosfamide, cisplatin, and etoposide (DICE). The patient has now completed two cycles of DICE chemotherapy.

On physical examination, the patient is afebrile, blood pressure is 142/80 mm Hg, pulse rate is 80/min, and respiration rate is 12/min. There is no palpable lymphadenopathy. The remainder of the examination is unremarkable. Results of a complete blood count and serum chemistry panel are normal.

PET/CT imaging shows no residual disease.

Which of the following is the most appropriate treatment at this time?