Background EOC can metastasize through three different pathways: intraperitoneal (in the abdominal cavity), lymphogenous or haematogenous \[5,6\]. Concerning the lymphogenous spread, it is clear that lymphatic spread of EOC occurs mainly to the para-aortic lymph nodes. It is theorized that the tumour cells follow the lymph vessels that accompany the ovarian vessels in the infundibulopelvic ligament up to the upper para-aortic region and renal vein. Additionally, pelvic lymph node metastases are also frequently found. These tumour cells probably follow a different route, migrating via the para-uterine vessels in the broad ligament towards the uterine vessels and further on to the iliac vessels and lymph nodes. In some case reports, isolated inguinal node metastases are also described. The exact mechanism of this route of metastasis is still unclear, but the metastatic cells might follow the course of the round ligament towards the inguinal lymph nodes or follow the iliac vessels towards the femoral vessels.

According to the International Federation of Gynaecology and Obstetrics (FIGO), EOC with lymph node metastases is classified as FIGO stage IIIA1. This significant upstaging is reflected in the need for adjuvant treatment. While patients with cancer confined to the ovaries (FIGO stage I) do not require adjuvant chemotherapy, patients with EOC FIGO stage III require additional treatment. Therefore, the detection of lymph node metastases is of utmost importance in perceived early stage disease.

In case of a clinical early stage ovarian cancer, standard-of-care is a staging laparotomy including bilateral pelvic and para-aortal lymphadenectomy. There is 14% (range 6.1-29.6%) chance of finding lymph node metastases . The incidence is higher in the grade 3 tumours (20.0%) and the serous histological subtype (23.3%). Whereas in grade 1 and mucinous tumours this is respectively 4.0% and 2.6%. By removing a greater number of lymph nodes, the detection rate for metastases rises . Although complete pelvic and para-aortic lymphadenectomy is standard-of-care, radical lymphadenectomy has been associated with serious morbidity and this is a major reason for the difference in the extent of lymph node dissection between centres.

By resecting only the SLN, significant reduction of morbidity and mortality can be obtained in comparison with the standard-of-care. SLN surgery is based on the concept that, if the SLN has no malignant cells, the likelihood that other lymph nodes are affected is reduces to almost non-existent. This means that, at least theoretically, a radical lymphadenectomy could be omitted and thus the associated morbidity and potential mortality. The SLN technique has been proven to be effective in other cancers such as breast cancer and malignant melanoma. In the gynaecological field it has been shown to be effective in vulvar cancer. Several SLN studies have been conducted in patients with ovarian cancer with promising results, as summarized in Table 1.

Rationale As stated, the goal of utilizing SLN in early-stage EOC surgery is to reduce patient morbidity while obtaining optimal oncological outcomes. By resecting only the SLN, the patients would be spared from radical lymphadenectomy, reducing short and long-term complications.

As summarized in Table 1, most studies utilize 99mTc and blue dye to visualize the SLN. Although detection rates are good with these techniques, they have major disadvantages. 99mTC is an isotope, as such it exposed the patient to an irradiation dose. Although total dosage is limited, eliminating isotopes in surgery would reduce the potential harm for the patient and the medical staff during surgery. Furthermore, the process to produce and administer 99mTC is a costly and resource-intensive procedure, requiring the cooperation of nuclear medicine, radio-pharmacy and logistics. As such, reducing the need for 99mTc would benefit the patient, medical staff and the healthcare system in general by reducing costs. Blue dye such as Patent Blue is often used for the detection of SLN. However, it also has major disadvantages. Firstly, it potentially occludes the entire operation field because of dissemination of the dye further than the SLN. Without optimal visualisation of the anatomy, the surgeon can expect more difficult dissection. Secondary, blue dye can cause a severe allergic reaction, leading to anaphylactic shock in up to 2% of patients.

Magtrace® is a brownish ferromagnetic substance capable of identifying the SLN. This novel technique has already been extensively researched in breast cancer and proven to be non-inferior to 99mTc with blue dye. These promising results led to licensing in the European Union. In this trial, the investigators wish to examine the potential of this technique in EOC, eliminating the disadvantages of 99mTc and blue dye. However, this is still a pilot study with unknown efficacy in EOC. Therefore, in order to minimize potential harm to the patient, 99mTc will be co administrated. This has been deliberately chosen due to the extensive literature concerning this sentinel mapping technique (see Table 1). If the efficacy of Magtrace would prove successful, the ultimate goal is to omit 99mTc in further studies and use Magtrace® only to identify the SLN.

In this pilot study, researchers will include two study arms. One in primary staging laparotomy, the other one in secondary staging laparotomy. The rationale of these two arms is based on the clinical practice, were surgeons not always suspect malignancy before resection of the adnexa. In case of a primary staging laparotomy the diagnosis of malignancy is based on a frozen section of the resected adnexa, followed by the sentinel node technique during the same procedure. On the other hand, in some cases early stage EOC is only diagnosed after the primary surgery, when the surgeon resects the ovary with the suspicion of benign disease. If the adnexa are already removed before the diagnosis of malignancy, a secondary staging laparotomy is required and, in this case, a single step approach for SLN is not feasible. The design of two study arms in this pilot study allows to compare the efficacy of Magtrace in native tissue (primary staging laparotomy) with tissue afflicted by previous surgery (secondary staging laparotomy).