PET imaging of anti-4-1BB treatment in CT26 tumor-bearing mice. [18 F]FDG microPET imaging showed an increased signal (% ID/g) of [18 F]FDG at the tumor site and draining lymph nodes (DLNs) of mice treated with 4-1BB mAb (19.1 ± 2.1 4-1BB mAb group vs. 10.3 ± 0.11 control group, p = 0.06). Representative images shown; orange arrows indicate tumor (T) circled in white, and bladder (Bl). Control mice (a) and 4-1BB mAb treated mice (b) on day 6 (left), 14 (middle) and 19 (right) post-tumor implant. All mice were treated with 4-1BB mAb or saline on day 9 and 11 post-tumor inoculation. Muscle serves as a background control. (c) Standardized uptake values (SUVs) of all the imaged mice. Mouse B is slightly rotated in order for the tumor FDG uptake signal not to overlap with the bladder uptake signal.