[18F]AlF-FAPI-74 PET/CT for preoperative assessment of the peritoneal cancer index and comparison with MRI-based, surgical and pathological assessment in colorectal cancer patients eligible for CRS-HIPEC: study protocol for a prospective observational proof-of-concept study.

INTRODUCTION: Colorectal cancer patients with peritoneal metastases have a very poor prognosis. A minority of these patients is eligible for curative cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC). The peritoneal cancer index (PCI) is an important criterion to select patients for CRS-HIPEC. Due to challenges in peritoneal metastasis detection by imaging, the PCI is currently routinely assessed by invasive diagnostic laparoscopy in addition to CT and/or MRI. Yet, open-close procedures and early disease recurrence following CRS-HIPEC are common, indicating the need for better patient selection tools. Fibroblast activation protein (FAP)-targeted imaging has recently emerged as a promising strategy for visualising peritoneal disease. The aim of this study is to assess the potential value of FAP inhibitor positron emission tomography/CT (FAPI-PET/CT) as an alternative non-invasive tool for quantitative PCI assessment. METHODS AND ANALYSIS: TROMPET is a prospective observational proof-of-concept study. A total of 25 colorectal cancer patients with suspected or verified peritoneal metastases who are eligible for CRS-HIPEC based on MRI will be included in this study. Patients younger than 18, pregnant and/or breastfeeding, with any contraindication(s) for MRI, PET, CT and/or CRS-HIPEC, and/or with a known additional malignancy within the past five years are excluded. Participants will receive [18F]AlF-FAPI-74 PET/CT prior to surgery. The primary objective is to determine the correlation between PCI scores determined by FAPI-PET/CT and 'true' PCI scores determined by histopathological analysis of all resected lesions. The secondary objectives include the correlations between PCI scores determined by FAPI-PET/CT, by MRI and during surgery, the potential of FAPI-PET/CT to detect extraperitoneal metastases, and molecular and immunohistochemical analysis of resected tissue to provide insight into the nature of FAPI-PET-positive lesions. The primary endpoint is all PCI scores determined by FAPI-PET and histopathology and their correlation on patient level. If this study shows that the PCI score can be accurately determined preoperatively by FAPI-PET/CT, it will form the basis for further developing FAPI-PET/CT as a quantitative, standardised, non-invasive diagnostic tool for selecting patients for CRS-HIPEC. Moreover, the 'radiology-pathology' setup of the study will allow us to characterise FAPI-PET-positive and PET-negative lesions in detail, providing further insight into the strengths and potential pitfalls of FAPI-PET/CT in the detection of peritoneal metastases from colorectal cancer. ETHICS AND DISSEMINATION: This study is approved by the assigned Medical-Research-Ethics-Committee (METC NedMec) on 19-08-2024. All participants will provide written informed consent. Study results will be disseminated through (inter)national meetings and peer-reviewed publications. TRIAL REGISTRATION NUMBER: 2024-512301-16-01.