This prospective study included 100 patients with histopathologically proven colorectal cancer who were referred to perform post-therapeutic follow-up by PET/CT in the period between January 2014 and January 2018. The study took place in an Egyptian military hospital and was approved by its ethical committee. The inclusion criteria were as follows: (1) histopathologic confirmation of primary CRC including TMN classification; (2) curative resection of the tumor either or not followed by chemoradiation therapy; (3) regular follow-up program every 3 or 6 months, including physical examinations, serum CEA level estimation, and conventional imaging including ultrasound of the liver and CT chest and pelviabdomen; (4) patients with suspected local recurrence or metastasis by routine follow-up program. (5) No age predilection and both sexes were included. Exclusion criteria included patients with strong history of atopic disorders, and those with serum creatinine level above 2 mg/dl, recent surgery less than 6 weeks, radiotherapy within less than 3 months, or chemotherapy within less than 3 weeks.
PET/CT was performed on an integrated scanner (Philips 128 slice CT) that combines both CT and PET capabilities in two sequential gantries, to avoid the need to move the patient between the two components of the study. Patients fasted for at least 6 h before the examination, except for water. Blood glucose levels were measured and should be less than 200 mg/dl. An intravenous dose of (0.18–0.21 mCi/kg, 5–14 mCi) FDG was injected and after 45–60 min; the patients were asked to void just before entering the examination room. Multi-detector CT examination from the base of the skull to the upper thighs was planned with the following parameters: 120 mA, 140 kVp, table speed = 13.5 mm per rotation, and thickness of 4 mm. After CT acquisition, PET scan of the same axial range started with the patient in the same position on the table for 2–3 min per bed position. PET data were acquired by using a matrix of 128 × 128 pixels. CT-based attenuation correction of the emission images was used. Contrast-enhanced CT was performed by the same scanner 20–50 s after giving bolus injection of non-ionic iodinated contrast at dose about 2–3 ml/kg of body weight. Scanning was acquired from the base of the skull till the mid-thigh and may involve the whole body in case of extensive skeletal deposits, using the 2.5-mm thickness section.
The reconstructed attenuation corrected PET images, CT images, and fused images of matching pairs of PET and CT images were reviewed by two experienced nuclear medicine physicians and a radiologist in axial, coronal, and sagittal planes, as well as in maximum intensity projections and in three-dimensional cine mode. Qualitative assessments for the presence of hyper-metabolic lesions were evaluated on corrected PET images. Semi-quantitative evaluation was performed using the standardized uptake value (SUVmax). Lesions which exceeded SUVmax of 2.5 were considered positive for disease involvement whereas findings with SUVmax below 2.5 were considered insignificant. Contrast-enhanced CT images were evaluated for the presence of hepatic focal lesions, lymph node size (more than 10 mm in its short axis), lymph node morphology, pulmonary nodules, peritoneal masses, operative bed masses, and skeletal lesions. Comparison with other clinical and diagnostic methods including laboratory tests (tumor markers) and other pathological findings was performed.
Our reference standard included histopathological or cytological confirmation or at least 6 months of clinical and radiological follow-up to confirm the final diagnosis of recurrence or metastasis.
IBM SPSS statistics (V. 23.0, IBM Corp., USA, 2015) was used for data analysis. Diagnostic validity test was used: it includes agreement and disagreement between PET/CT and CECT. The Chi-square test was used to compare the differences between the two studied imaging modalities. The probability of error equal 0.05 was considered significant; while values at 0.01 and 0.001 are highly significant. PET/CT findings and CECT findings were classified as true positive (TP), false positive (FP), true negative (TN), and false negative (FN), as compared with those of the reference standard. Standard statistical formulas were used to calculate the sensitivity, specificity, and accuracy of 18F-FDG PET/CT and CECT in both the case-based and lesion-based analyses.