[Potential sources of diagnostic pitfall and variants in FDG-PET/CT]

F-18 FDG PET/CT in the assessment of patients with unexplained CEA rise after surgical curative resection for colorectal cancer

PURPOSE

We evaluated the role of quantitative assessment by maximum standardized uptake value (SUVmax) on F-18 fluorodeoxyglucose [F-18]FDG positron emission tomography/computed tomography (PET/CT) in stratifying colorectal cancer (CRC) patients with unexplained carcinoembryonic antigen (CEA) rise after surgical curative resection.

MATERIAL AND METHODS

Forty asymptomatic patients (mean age, 64 ± 12 years) with previous CRC and current serum CEA levels >5 ng/ml underwent [F-18] FDG PET/CT 13 ± 3 months after complete surgical resection. The SUVmax was registered on anastomosis and peri-anastomotic tissue lesions, if present. The patients were followed for 24 ± 9 months thereafter. Re-intervention, evidence of newly discovered distant metastases, and death were recognized as main events and constituted surrogate end points. The receiver-operator-curve (ROC) analysis was performed to estimate the optimal SUVmax cut-off to predict patients at high risk of main events. PET/CT results were then related to disease outcome (overall survival; OS).

RESULTS

The mean SUVmax at the anastomotic site was 6.2 ± 3 (range 2.6-15). At multivariate logistic regression analysis, the anastomotic SUVmax remained as the only significant contributor to the prediction of the events (p = 0.004; OR 1.97). The ROC analysis recognized that the optimal threshold of SUVmax to differentiate patients was 5.7. A worse OS was observed in patients presenting with a SUVmax greater than 5.7 as compared to those having lesser (median survival: 16 vs. 31 months; p = 0.002).

CONCLUSIONS

The quantitative assessment by SUVmax on [F-18]FDG PET/CT may be helpful in patients presenting with unexplained CEA rise after curative resection of CRC, by identifying those at risk of main events.

[Thymic uptake of 18FFDG following radioiodine ablation therapy]

We report the case of a 47 year-old woman with a history of non-Hodgkin's lymphoma. During the course of her disease, we performed various (18F)FDG PET/CT that identified several significant incidental findings. First, we incidentally identified a hypermetabolic nodule in the left thyroid lobe, whose final diagnosis was differentiated thyroid carcinoma. Second, metabolic activity was visualized in the thymus secondary to ablative treatment with radioiodine. This uptake disappeared in subsequent studies. Several papers have reported thymic rebound following chemotherapy but we have found only one case report of (18F)FDG uptake after radioiodine treatment. On the other hand, this case is of interest because it supports the benefit of studying the hypermetabolic thyroid nodules incidentally detected on the PET/CT performed for other reasons.

[PET/CT: focal lung uptake of 18F-fluorodeoxyglucose on PET but no structural alterations on CT]

The combination of positron emission tomography (PET) and computed tomography (CT) in a single device (PET/CT) offers a powerful diagnostic tool that opens up new horizons for imaging diagnosis. In order to correctly interpret PET/CT studies, knowledge of the biodistribution of 18F-fluorodeoxyglucose (FDG), the physiological variants as well as the pitfalls, including artefacts, which may be found, is necessary. We report four cases performed during the follow-up diagnostic context of an oncology study performed with 18F-FDG-PET/CT. In every case, this study showed focal uptake in the lung parenchyma in the PET study with no structural lesions being found on the CT scan. Radiotracer extravasation in three of these patients and a recent change in the injection protocol used suggest that an artefact was responsible for these discrepancies.