PET Scan Sensitivity for Colon Cancer

Last Modified: August 19, 2007

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Question

Dear OncoLink "Ask The Experts,"

My husband was diagnosed with Stage 3 signet ring cell adenocarcinoma [of the colon] in December 2005 and had 5 months of chemotherapy. Within the past month, his CEA level has risen from 1.5 to 3.4. His doctor ordered a PET scan, and the results came back clear. It showed absolutely nothing. Are there any cancers that don't show up with a PET scan, or is his CEA too low, or is the PET scan right and there is no cancer?

Answer

Peeyush Bhargava, MD, Chief of Nuclear Medicine at The Michael E. DeBakey VA Medical Center, responds:

Colon cancer is an FDG-avid disease (meaning that it usually shows up on PET imaging). In centers where the combination PET-CT scan is available, it is preferred over PET scan alone. This is because the PET-CT combines both the PET and CT images to give more information, based on the metabolic and the anatomic information, respectively. It is generally accepted that PET is limited in the evaluation of subcentimeter lesions (those smaller than 1 centimeter), as lesions this small can be falsely negative. This means that the disease is not seen on the scan, even though it is present. It is also recognized that as a part of PET-CT, a contrast enhanced CT can provide more information. So in summary, the use of PET-CT may provide a more accurate assessment of a patient's clinical status.