Dual-time-point 18F-FDG PET/CT in Patients with Colorectal Cancer: Clinical Value of Early Delayed Scanning

Objective: In dual-time-point PET/CT, early delayed scanning (D-1) just after the completion of whole body scanning (E) is easy to perform without additional radiation exposure and repositioning. Our aim was to assess the clinical value of D-1 compared with conventional delayed scanning (D-2).

Methods: Our institutional review board approved this retrospective study. Fifty-four patients with known or suspected colorectal cancer underwent (18)F-FDG PET/CT at our institution. The E scan at 1-h post-injection was followed by D-1 at 85 ± 7 min post-injection and D-2 at 124 ± 7 min post-injection. The clinical value of D-1 was evaluated by comparing diagnostic performance with D-2 for differentiating physiologic from pathological uptake and for staging colorectal cancer. Colonoscopic findings, histopathological results and clinical follow-up including radiological findings were used as reference standards.

Results: Thirty-two, eight and 73 focal or short segmental FDG foci by physiologic processes in the colon/rectum, the small intestine and the ureter, respectively, noted in the E scan were evaluated in this study. Using D-1 and D-2, 14/32 (44%) and 17/32 (53%) in the colon/rectum, 5/8 (63%) and 8/8 (100%) in the small intestine, and 55/73 (75%) and 69/73 (95%) in the ureter, respectively, were accurately interpreted as physiologic with the change of intensity and/or shape/location. A significant difference between D-1 and D-2 was observed in the ureter, but not in the bowel. The 55 colorectal cancers were finally diagnosed in 52 patients. In the staging of colorectal cancer, there were no significant differences among the three scans in the lesion-based detectability, the patient-based sensitivity, specificity and accuracy for the identification of primary tumors, nodal and hepatic metastases, and dissemination.

Conclusions: Neither D-1 nor D-2 improved staging of colorectal cancer. However, delayed scans yielded information useful for differentiating physiologic uptake from pathological uptake and D-1 may provide comparable efficacy with D-2 in the bowel. Because of the ease of acquisition, the D-1 scan was considered a practical way to reduce false-positives in the abdomen and possibly helpful to avoid unnecessary additional invasive examinations, such as colonoscopy.