Peritoneal metastases (PM), commonly originating from gastrointestinal and ovarian cancers, are associated with a poor prognosis and limited treatment options due to the diffuse nature of the disease and the inability of systemic chemotherapy to adequately penetrate the peritoneal surface. Intraperitoneal (IP) chemotherapy has emerged as a potential treatment modality for patients with extensive PM, allowing for higher local drug concentrations while minimizing systemic toxicity. However, current response evaluation methods, such as CT scans, have low sensitivity for detecting PM, and second-look laparoscopy, although more accurate, is invasive and unsuitable for repeated assessments. The ⁶⁸Ga-FAPI PET/CT scan offers a promising non-invasive alternative for assessing peritoneal response to IP chemotherapy. This imaging technique targets the fibroblast activation protein, which is overexpressed in cancer-associated fibroblasts within the tumor microenvironment. Early studies have shown that ⁶⁸Ga-FAPI PET/CT is a highly sensitive too in detecting peritoneal metastases. Given these advantages, the investigators hypothesize that ⁶⁸Ga-FAPI PET/CT could improve the evaluation of treatment response in patients undergoing IP chemotherapy for PM, offering a non-invasive and accurate alternative to current methods. The primary objective of this study is to investigate the accuracy of ⁶⁸Ga-FAPI PET/CT for evaluating peritoneal response in patients with peritoneal metastases undergoing repeated IP chemotherapy.

Inclusion Criteria:

• A planned start of repeated IP chemotherapy (with or without the combination of systemic chemotherapy) for macroscopic peritoneal metastases;
• WHO-performance score of 0 to 1;
• Aged 18 years or older;
• Written informed consent according to the ICH-GCP and national/local regulations.

Exclusion Criteria:

• Impaired renal function, defined as eGFR <25 ml/min/1,73 m2. An exception can be made in consultation with the treating physician;
• Pregnancy/breastfeeding. For the latter, temporary discontinuation may be considered;
• Known allergic reaction to therapeutic radiopharmaceuticals;
• Inability to lie still on the back for the duration of PET-CT;
• Any (other) condition, disease, metabolic dysfunction, physical examination finding, or clinical laboratory finding that may affect the interpretation of the results, or which might contribute substantially to the patient's experience of study burden (such as non-suppressible claustrophobia).