 CT4 will be performed for future comparison with CT8, CT12 and CT 16 to exclude Progressive Disease PD and to correlate metabolic and radiological responses and to inform continued treatment. Patients not achieving Deauville score 1-3 at PET 4 will also have a PET scan at completion of treatment after 16 cycles or earlier if brentuximab vedotin discontinued for reasons other than PD 