Colorectal Cancer - 70 year old male

History
70 year old male with a history of sigmoid carcinoma who underwent resection. Negative PET scan in 9/01 and now has a rising CEA level and negative CT scan. Referred for PET/CT scan for evaluation

Original CT scan
Imaging findings not available

Original PET Scan Findings 09/01 There are no areas of increased uptake to suggest recurrent or metastatic disease. However, sensitivity of this study is decreased due to the patient's hyperglycemia. No evidence of recurrent or metastatic disease.

PET/CT scan 2 months later
Focal area of intense FDG uptake corresponding to mildly enlarged left paraaortic lymph node consistent with recurrence. Additional area of intense FDG uptake identified within the right lobe of the liver without an identifiable liver abnormality using the available technique, although consistent with hepatic metastasis.

Treatment
Chemotherapy

Discussion
One of the pitfalls of PET imaging alone, particularly when a recent CT scan is not available, is trying to distinguish pathologic uptake of FDG from physiologic uptake. When linear bowel uptake is present, if there is a small lesion adjacent to it, it can be very difficult to differentiate it from the background bowel, as in this case.

This patient's initial scan was interpreted as normal with only linear physiologic bowel uptake. Because his CEA level continued to rise, a short term follow up exam was performed about 2 months later. On the follow up exam, a focal area of FDG uptake is now visible away from adjacent linear physiologic bowel uptake. In retrospect, this lesion was present on the initial PET study, but because there was no recent CT available for comparison and the lesion's location was adjacent to bowel, it was difficult prospectively to resolve the two structures.

With PET/CT, because you can inspect all areas of FDG uptake with an accurately co-registered CT and examine the fusion images, differentiation of physiologic from pathologic uptake becomes much, much easier.