Importance of 18F-FDG-PET/CT in Detection of Early Colorectal Cancer Relapse and its Effect on Therapy Plan. Talaat, O1. Mahmoud, E2. Hassan, M3. Ali, A4. Nasr, I4 1Oncology and Nuclear Medicine Department, National Cancer Institute, Cairo University. 2Radiodiagnosis Department, National Cancer Institute, Cairo University. 3Surgical Oncology Department, National Cancer Institute, Cairo University. 4Oncology and Nuclear Medicine Department, Faculty of Medicine, Zagazig University, Egypt. Egyptian J. Nucl. Med., Vol. 19, No. 2, December: 2019 ABSTRACT: Introduction: Colorectal cancer (CRC) is the 7th commonest cancer in Egypt, representing 3.5% of male and 3% of female cancers. About 40% of patients with CRC develop early recurrence within the first two years after completion of their treatment. Aim of the work: To evaluate the importance of 18F-FDG-PET/CT in detection of early relapse and to assess its effect on management in patients with CRC and equivocal CECT findings. Patients and Methods: Forty eight patients with treated colorectal cancer, under follow up were subjected to full clinical and laboratory assessment, CECT and PET/CT imaging plus histopathological examination of the biopsied sites of suspected recurrence. Results: Among 48 patients with CRC, PET/CT and histopatholgy were concordant in 41 patients (31 positive & 10.0 negative) and discordant in 7 patients (6 false positive & 1.0 false negative). Overall SN, SP, PPV, NPV and accuracy of PET/CT were 96.9%, 62.5%, 83.8%, 90.9% and 75.6% respectively. PET/CT and CECT findings exhibit significant association in detection of local recurrence, hepatic lesions, loco-regional LNs and distant lesions (p value 0.001, 0.001, 0.004 and 0.003 respectively). PET/CT led to overall changes in the therapy plan for 26/48 patients (54.2%), 12/48 patients (25%) from follow up to be positive for recurrence and recommended for therapy, one patient (2.1%) from false positive to be negative and continue under follow up and 13/48 patients (27.1%) underwent modification in their plan either by addition or withdrawal of other therapy lines. 22/48 patients (45.8%) didn’t show changes in their proposed plan. The changes in follow up, chemotherapy and radiofrequency strategies before and after PET/CT were significant (p value < 0.001, 0.004 & 0.038). Conclusion: PET/CT is efficient than CECT in detection of early CRC relapse. PET/CT has also a significant impact on directing management through improving the accuracy and decreasing the failure rate of the suggested therapy plan. Key Words: Colorectal Cancer; 18F-FDG-PET/CT; CECT.