A prospective study was conducted on 500 patients from January 2016 to June 2018 (300 male, 200 female) above 50 years of age (mean age 67.5 years) with +ve family history of CRC who underwent CTC. PET/CT was performed to all patients with +ve CTC findings.
Patients with a fasting glucose level 200 mg/dL were excluded (exclusion criteria). Histopathology results were used as standard of reference.
The PET/CT technique
PET/CT scan was performed using Siemens Biograph TruePoint 64 (Siemens Healthcare, Erlangen, Germany).
All patients were asked to fasten for at least 5 h to lower the insulin and blood glucose level. Blood glucose levels were in its normal levels prior to 18-FDG injection. PET images were done 60 min after I.V injection of 5 MBq/kg body weight of FDG (up to 550 MBq).
The standard uptake value (SUV) was defined as the tissue concentration (MBq/mL) of the tracer divided by the activity injected per body weight (MBq/g). The maximum SUV in the volume of interest was considered as the SUVmax for the purpose of analysis. Scans were acquired from skull vertex down to the thigh. Prior to PET imaging, a diagnostic CT scan of the brain, neck, chest, abdomen, and pelvis was performed with oral and without I.V contrast for attenuation correction and anatomic localization. The following parameters were used: 80–100 mA, 140 kV, 5 mm collimation, 0.5 s rotation time, and pitch = 0.984. Patients were in supine position with both arms up; normal respiration was maintained during the scanning. Reconstruction with section thickness of 1.2 mm was done. Subsequently, PET data were acquired in at 5–7 bed positions. 18FDG-PET images were reconstructed; 18FDG-PET, CT, and fused 18FDG-PET/CT images were reviewed on the dedicated workstation. Patients were asked to have FDG PET/CT and CTC imaging performed within 3 days of both examinations.
The PET/CT images were interpreted by an experienced radiology consultant, and reports were compared to those of CT colonography. The reader was not blinded to other modality results.
CT colonography technique
CT colonography was performed after full bowel preparation. MDCT (Toshiba 320-detector CT Medical System) was used with a maximum detector collimation of 2.5 mm, pitch factor 2:1, 200 mAs, 120–150 kV, matrix 512 × 512, full field of view 40 cm. Scanning time was done in one breath hold (20 s). All patients were examined cranio-caudally starting from the level of the diaphragmatic copula down to the anus. Readers used 2D and 3D visualization. CTC was carried out by an experienced radiology consultant.
The FDG PET/CT findings were classified as follows:
True positive findings were occurrence of focal hypermetabolic lesions in a compatible site of advanced adenomas on CTC. False positive findings were occurrence of focal hypermetabolic lesions with no evidence of advanced adenomas in a compatible site on CTC.
False negative findings were lesions which did not display focal hypermetabolic lesions on PET/CT but with positive evidence on CTC.
The FDG PET/CT sensitivity, specificity, positive predictive value, negative predictive value, and lastly the overall accuracy of colorectal neoplasms detection were calculated. Mc Nemar and T test were used to compare the results, with statistical significance p < 0.05. The standard maximum uptake value (SUVmax) of each focal hypermetabolic lesion was quantitative and qualitative evaluated.