Is the faster-rcnn-resnet50 pretrained on COCO from scratch?

Hello,
I’m using the fasterrcnn_resnet50_fpn and there are two parameters pretrained (full model pretrained on COCO) and pretrained_backbone (only backbone pretrained on ImageNet).
So I was wondering for the COCO model if this model was also trained using the ImageNet backbone or was it completely from scratch ?