Although the FR-expression status may be determined by immunohistochemical staining of tumor biopsies there is a need for a non-invasive method to determine the presence of the FR on primary tumors and metastases in humans. For this purpose a radiopharmaceutical product will be used as a radiotracer. Positron Emission Tomography (PET) is an imaging method which allows assessing the distribution of radiotracers (called PET tracer). With this imaging method it is possible to obtain (semi)quantitative measures of FR-expression on tumors of at least 4-10 mm diameter in patients.