Fig. 4

In-vivo monitoring of drug-target-interaction with ¹⁸F–Fluoro-Estradiol (FES)-PET/CT in a patient with metastatic estrogen receptor-positive breast cancer being treated with an estrogen receptor-directed agent. The pre-therapy scan (top image) showed multiple chest wall metastases with avid tracer uptake. After initiation of the therapy (bottom image), tracer uptake resolved, while the size of the metastases did not change. It is important to recognize that these findings prove that the drug has hit its target but do not necessarily indicate treatment response. Retained tracer is seen in a central venous catheter on both studies