How few cancer cells can be detected by positron emission tomography? A frequent question addressed by an in vitro study

Purpose: Positron emission tomography ( PET) has gained widespread use in cancer diagnosis and treatment, but how many malignant cells are required for a tumour to be detected by PET? Methods: Three human cancer cell lines [ glioblastoma and two subtypes of small cell lung cancer (SCLC)] in concentrations from 10(4) to 10(7) were seeded on six-well plates or plastic tubes and treated with [F-18] fluorodeoxyglucose (FDG) in vitro. FDG retention was measured in a PET/CT scanner and in a calibrated well counter. The clinical situation was simulated using a cylinder phantom with a background concentration of FDG. Results: The theoretical detection limit was found to be around 10(5) malignant cells. In a cylinder phantom the detection limit was increased by a factor of 10. The FDG retention by the glioblastoma cell line was significantly higher than the activity of the SCLC cell line. FDG retention measured by PET and a gamma counter was closely correlated to the number of cells and a linear relationship was found. Discussion: The detection limit of PET is in the magnitude of 10(5) to 10(6) malignant cells. The experimental set-up was robust and well suited as a platform for further investigations of factors influencing the detection limit of PET.