Background: Peritoneal metastasis (PM) is a late-stage manifestation of several gastro-intestinal cancers. Conventional administrated chemotherapy has limited effect due to the peritoneal-plasma barrier and most patients will die from their disease within a few months. An alternative treatment strategy, Pressurized IntraPeritoneal Aerosol Chemotherapy (PIPAC), has gained interest in the last decade. Prospective phase II studies have demonstrated significant local treatment effect and promising survival data initially within the palliative setting but more recently also with curative intent. The technical aspects of PIPAC are nearly uniform worldwide but there is no consensus on the optimal evaluation of treatment response. Currently, histological features in repeated biopsies from the peritoneum are used as an indicator of treatment response. Thus, easily accessible, and non-invasive alternatives are needed to optimize and personalize patient selection and to improve the response assessment. Both are also essential regarding future development and implementation of this treatment for both curative and palliative purposes. This could be achieved with the use of imaging techniques. Computed tomography (CT) scan holds well-known limitations in the detection of PM. Alternatively, several studies have investigated the use of the Positron emission tomography and CT (PET/CT) scans in the diagnostic setting of PM. This method shows promising results concerning visualization and assessing PM lesions, but no studies have investigated the ability of PET/CT scans to detect changes in PM during PIPAC treatment. Enabling the use of PET/CT scans as an evaluation tool for PIPAC-directed therapy is an important and necessary step to expand the clinical application. Also, non-invasive evaluation methods and consensus of evaluation is of significant importance for the quality of future randomized controlled trials in this field of research. We want to conduct a prospective study to investigate if PET/CT scans can detect changes within PM lesions in patients treated with PIPAC directed therapy. Since the study is the first of its kind, we plan to start with a pilot study including only a few patients. We will use the results to generate important knowledge for subsequent larger randomized studies on and clinical optimization of PIPAC directed therapy in patients with PM.

Objective: To investigate whether FDG-PET/CT scans can detect changes in PM in patients treated with PIPAC and whether the repeated scans are feasible within this group of patients.

Method: The pilot study will include 16 patients with known PM, four from each of the groups: gastric cancer, pancreatic cancer, colo-rectal cancer and ovarian cancer will be included. The patient will be offered standard PIPAC directed treatment (including standard histological evaluation of biopsies of the peritoneum) according to local guidelines. During the first series (one series is equivalent to three PIPAC treatments each five weeks) patients will receive three FDG-PET/CT's, one before PIPAC 1, one after PIPAC 2 and one after PIPAC 3. The scans will be described according to PERCIST (PET response criteria in solid tumors). In case of FDG-negative PM-disease at the first scan the patient will receive only one follow-up scan after PIPAC 2 to investigate whether the treatment if self can cause FDG-uptake in the PM elements.