Following PET imaging, tumors were excised, embedded in optimal-cutting-temperature mounting medium (OCT, Sakura Finetek), frozen on dry ice and series of 10 μm frozen sections cut. To determine radiotracer distribution, digital autoradiography was performed by placing tissue sections in a film cassette against a phosphor imaging plate (Fujifilm BAS-MS2325; Fuji Photo Film) at −20 °C for an appropriate exposure period. Phosphor imaging plates were read at a pixel resolution of 25 μm with a Typhoon 7000 IP plate reader (GE Healthcare). Contiguous frozen sections were then used for staining and microscopy. Sections were fixed with ice-cold acetone for 10 minutes and dried at room temperature for 10 minutes before re-hydration in tap water followed by PBS. Superblock (ThermoFisher) was used to block slides for 40 minutes at room temperature. Slides were incubated in primary antibodies HER3-XP (D22C5, Cell Signaling, 1:40) and EGFR-XP (D38B1, Cell Signaling, 1:50) overnight at 4 °C. Slides were developed using Pollink-2 Plus HRP rabbit with DAB (GBI Labs, D39–18) or Klear mouse HRP with DAB (GBI Labs, D52–18) and dehydrated with alcohols and xylenes before being covered with permount. Imaging was performed with a Nikon Eclipse Ci microscope (Nikon) using Spot Basic 5.2 software (Diagnostic Instruments, Inc). Images were analyzed in panoramic viewer 1.15.4 for windows (3D HisTech)58. IHC evaluation and scoring for EGFR59 and HER360 followed previously described protocols60.