 In addition an exploratory PET scan will be performed after 2 cycles of brentuximab vedotin PET2; investigators will be blinded to the results of PET2 which will not influence patient management in any way. All PET scans will be centrally reviewed for QA purposes by The PET Imaging Centre at St ThomasGuys, under the supervision of Sally Barrington and Mike ODoherty, using the framework developed for the recently completed RAPID and RATHL trials. CT scans after 4 cycles and at the end of treatment may also be reviewed. 