Patients with unfavourable-risk relapsed or primary refractory classic Hodgkin 's lymphoma who had undergone autologous stem-cell transplantation were randomly assigned , by fixed-block randomisation with a computer-generated random number sequence , to receive 16 cycles of 18 mg/kg brentuximab vedotin or placebo intravenously every 3 weeks , starting 30-45 days after transplantation .