18F-FDG PET/CT in Therapy Response Assessment: Oligometastatic Colorectal Cancer Ibrahim Nasr1, Bader Abdel Maksoud2, Mahmoud Rezk3, Ahmed Badawy4, Walid Almorsy5, Ismail Ali6. 1,2Clinical Oncology and Nuclear Medicine Department, Faculty of Medicine, Zagazig University. 3Radiology Department, National Cancer Institute, Cairo University. 4Oncology and Nuclear Medicine Department, Faculty of Medicine, Cairo University. 5Clinical Oncology Department, Faculty of Medicine, Tanta University. 6RadiologyDepartment, Faculty of Medicine, Zagazig University. Background: Colorectal cancer (CRC) is one of the most widespread cancers worldwide, leading to roughly half a million deaths yearly. The European Society for Medical Oncology (ESMO) defined oligometastatic CRC as a disease with few metastases affecting a small number of sites (5 or occasionally more metastases involving up to 3 sites).In addition to colonoscopy, magnetic resonance imaging (MRI), and digital rectal examination in patients with rectal cancer, response monitoring of CRC is commonly carried out by CT imaging. The use of PET for response monitoring has not been adapted into colorectal cancer guidelines until 2021. However, 18F-FluorodeoxyglucosePositron Emission Tomography Computed Tomography (18F-FDGPET/CT) offers a higher efficiency for assessing treatment outcomes than traditional imaging. This study aims to explore the utility of 18F-FDG PET/CT imaging in the assessment of therapy response in patients with oligometastatic colorectal cancer (OMCRC). Results: The study comprised 79 OMCRC patients(35 and 44 patients with synchronous and metachronous metastasis respectively). In synchronous disease patients 18F-FDG PET/CT scan showed significant reduction of mean size and standardized uptake value (SUV)of the primary site lesions and the mean SUV of lymph nodes (LNs) and lung metastases (P= 0.00, 0.00,0.00, and 0.002, respectively)while, metachronous disease patients had significant reduction in the mean size and SUV of LNs (1.8±0.7 & 4.7±1.3 versus 1.1±1.0 & 2.9±3.0, P=0.001 & 0.00 respectively) and the mean SUV of peritoneal metastases (8.7±4.7 versus 6.8±2.4 P=0.00). Partial metabolic response (PMR) and stable metabolic disease (SMD) were found in more than half of the patients (58.2%). Complete metabolic response (CMR) and Progressive metabolic disease (PMD), on the other hand, were achieved in 41.8% of patients [17 (21.5%) and 16 (20.3%) patients, respectively]. Patients with metachronous disease showed a substantially greater complete response rate than patients with synchronous disease [14.0 (31.8%) versus 3.0 (8.5%) patients, P= 0.015)]. Conclusions:18F-FDG PET/CT can be added as a valuable imaging method for identifying responders and non-responders among OMCRC patients, as it optimizes the selection of patients with CRC for local therapy and has a significant impact on directing their therapy course. Keywords: Fluorodeoxyglucose F18 positron emission tomography, Follow-Up studies, Colorectal neoplasms.