At the time of suspected recurrent CRC, the mean age was 55 with a near-equal gender distribution. The majority of patients had undergone surgical resection alone with the remainder having chemotherapy and/or radiotherapy prior to or following resection. In two patients receiving chemoradiation, resection of the primary was not undertaken. Surgical exploration was undertaken within one month of PET scan in 37 patients and within two months of PET scan in the other two.

PET scan was considered normal in 13 (33%) and positive in 26 (67%). At exploration, 27 (69%) patients were found to have histopathologic evidence of tumor recurrence. Of 26 positive PET scans, 22 were true positive and four were false positive. Eight patients were found to have true negative PET scans while five were false negative (Figure 1). In three patients with a false negative PET scan, tumor was mucinous adenocarcinoma and in the other two it was moderately differentiated adenocarcinoma. The overall accuracy of PET was 76.9% with a sensitivity of 81.4%, specificity of 66.6%, positive predictive value (PPV) of 84.6%, and negative predictive value (NPV) of 61.5%.

In the 26 patients with positive PET scans, PET detected a total of 36 lesions, 32 of which were resected. Histopathological findings in these patients are shown in Table 2. PPV value of PET was highest for liver metastases (88.8%) compared to 73.3% for luminal (i.e. anastomotic) recurrences. All recurrences and metastases were consistent with adenocarcinomas with one demonstrating mucinous features. The exact lesion size was discernable in six patients and ranged from 10 to 30 mm. In two patients with pathology proven adenoma with dysplasia in rectum, PET was accepted as true positive for these lesions due to their pre-malignant nature. Pathology for one patient (patient number 24 in Table 2) with a false positive PET scan in the liver (SUVmax of 3.1) showed focal foreign body reaction with necrosis, organizing inflammation, and fibrosis related to previous surgery (Figure 2).

The remaining false positive cases were mainly in the bowel and were secondary to acute/chronic inflammation and granulation tissue. In patients with a false positive PET scan, the time interval between last treatment and PET was 42 months (range:11–96 months). In two cases with a positive PET scan (patient number 8 and 15 in Table 2), CEA became elevated two months after PET scan and subsequent pathology demonstrated tumor recurrence. In one of these two patients, PET showed increased activity in the liver with central photopenia (Figure 3) following treatment with Yttrium-90 microspheres. In one patient (patient number 26 in Table 2) with a false positive PET scan in the rectum, PET scan also demonstrated a bone lesion. Follow-up evaluation demonstrated progression in the bone lesion consistent with metastasis. In two patients with pathology-proven intra-abdominal tumor recurrence, PET also demonstrated mediastinal uptake (patient number 9 and 19 in Table 2). These lesions remained stable on follow-up PET and CT images.