Despite the high number of EGFR and HER3 present as verified through Bmax values, flow cytometry and immunoblots, the tumor accumulation of the probe in AsPC-1 xenografts uptake was lower compared to BxPC-3 models. With PDAC reportedly possessing dense stroma, we initially rationalized that high levels of collagen, an essential component of the extracellular matrix may perturb tumor penetration of the probe. The trichome stain (Supplementary Fig. S4C), however, did not display any significant disparity in collagen levels between the two types of tumors. Looking at cell density from a pathological perspective, the BxPC-3 tumors were denser than the AsPC-1 tumors as shown by cell count (Supplementary Fig. S4D). This consequently lowered receptor sites available, which potentially explains the lower PET volume of interest (VOI) uptake attained in the AsPC-1 tumors.