Endometrial carcinoma is the most common gynecological malignancy in the western world, and with a current incidence of 18/100.000 women per year in the European Union, it affects around 65,000 new women each year. Due to the increased life expectancy and obesity, the incidence of endometrial cancer has increased in the last years, and is expected to rise in the coming years. Most patients present with early stage disease and have a favorable outcome. However, patients with metastatic disease have few treatment options with a poor prognosis.

Surgery is the primary treatment modality in endometrial cancer and defines the final surgicopathological stage according to the International Federation of Gynecology and Obstetrics (FIGO) staging system. Adjuvant treatment is based on final tumor grade and FIGO stage and consists of radiotherapy and-or chemotherapy. Hormonal therapy is considered as alternative treatment modality for: 1. patients who are not suitable for surgery, 2. patients who wish to maintain fertility, and 3. patients with metastatic disease. A recent review summarizes the available evidence on the effect of hormonal therapy in patients with advanced and recurrent endometrial cancer. The overall response rate based on a single biopsy taken prior to treatment, was 22%. In ER positive tumors, the response rate was 27% compared to 9% in ER negative tumors. In PR positive tumors 36% of patients responded, compared to 12% in PR negative tumors. Thus, the presence of ER and PR receptors on the tumor cell seems to be relevant for prediction of response to hormonal treatment. Yet, data concerning the percentage of ER and PR expression are lacking, and might underestimate the response to hormonal treatment in individual cases.

Analysis of the genomic landscape of endometrial cancer has shown marked genetic heterogeneity between biopsies of primary tumors and their corresponding metastases, suggesting that only a part of tumor cells in the primary tumor is involved in metastases. Also, loss of ER and PR expression is frequently observed in metastases from ER/PR positive primary tumors. These findings underline the relevance of obtaining a new biopsy to determine ER and PR expression in recurrent disease. This can be challenging since recurrences can be hard to reach by biopsies due to its location and multiplicity.

18F-FES PET CT (FES PET) is a novel imaging method based on positron emission tomography using a specific tracer targeting ER allowing the visualization of ER expression in tissue. FES PET allows non-invasive depiction and quantification of ER expression in all tumor lesions in a patient.

FES PET has been evaluated in breast cancer with a reported sensitivity of 69% to 100%, and a specificity of 80-100% for identifying ER positive tumor when compared to immunohistochemical expression of ER. In endometrial cancer, FES PET has been evaluated in only two studies. The first study included 19 patients with endometrial cancer with different FIGO stages that underwent FES PET prior to surgical resection. In this study a significant correlation between ER uptake on FES PET and ER immunohistochemical expression was observed. In the second study 22 patients were analyzed and classified into high risk (FIGO stage ≥Ic or grade ≥2) and low risk (FIGO stage ≤1b and grade 1) endometrioid type endometrial carcinoma. In the high risk group a significantly lower FES uptake was observed when compared to the low risk group, suggesting that FES PET could assist in identifying patients with high risk endometrial cancer. As shown by van Kruchten et al., FES PET could also be of value in the evaluation of response to hormonal therapy. In this study the results of serial FES PET scans are described in 16 patients with metastatic breast cancer treated with fulvestrant, a selective estrogen downregulator. Response to hormonal therapy was associated with reduced uptake on subsequent FES PET scans. So far, FES PET data are limited in endometrial cancer to one case report in which a decrease of estrogen uptake was observed upon medroxyprogesterone therapy in early stage endometrial cancer consistent with histological and clinical follow-up. Based on previous findings the aim of the current study is to explore the feasibility of FES PET scan in patients with endometrial cancer.