Possible reasons could be the individual tumor heterogeneity, a dedifferentiation of PCa leading to a decoupling of tumor mass and PSA level, absent or very low PSMA expression of some tumors (e.g. neuroendocrine PCa), tumors adjacent to the urinary bladder, small tumour size below the spatial resolution of the PET scanner or the stage of technology of the PET scanner. This evidence concerns the gene KLK3 and neoplasm.