Tumor Receptor Imaging

Tumor receptors play an important role in carcinogenesis and tu-mor growth and have been some of the earliest targets for tumor-specific therapy, for example, the estrogen receptor in breast cancer. Knowledge of receptor expression is key for therapy di-rected at tumor receptors and traditionally has been obtained by assay of biopsymaterial. Tumor receptor imaging offers comple-mentary information that includes evaluation of the entire tumor burden and characterization of the heterogeneity of tumor recep-tor expression. The nature of the ligand–receptor interaction poses a challenge for imaging—notably, the requirement for a low molecular concentration of the imaging probe to avoid satu-rating the receptor and increasing the background because of nonspecific uptake. For this reason, much of the work to date in tumor receptor imaging has been done with radionuclide probes. In this overview of tumor receptor imaging, aspects of re-ceptor biochemistry and biology that underlie tumor receptor imaging are reviewed, with the estrogen–estrogen receptor sys-tem in breast cancer as an illustrative example. Examples of progress in radionuclide receptor imaging for 3 receptor systems— steroid receptors, somatostatin receptors, and growth factor receptors—are highlighted, and recent investigations of receptor imaging with other molecular imaging modalities are reviewed