Transition Net: 2D backbone to segment 3D brain tumor

•A segmentation method using 2D backbone to segment 3D multimodal brain tumor images is proposed, which can be extended to other 3D tasks.•To solve the problem of large differences between the pre-training domain and the target domain, we propose Transition Head and Transition Decoder to transform the intermediate features.•Weighted region loss is proposed. This loss improves on Dice loss and Focal loss and can drive the model to actively learn difficult samples.