There a number of imaging modalities that can stage colon and rectal cancer. A recent study evaluated the impact of FDG-PET on the management of patients with colorectal carcinoma. They noted a change in the clinical stage and major management decisions in approximately 40% of patients. The disease was upstaged in 20 patients (80%) and downstaged in 5 patients (20%). As a result of FDG-PET findings, physicians avoided major surgery in 41% of patients for whom surgery was the intended treatment. On the other hand, false-positive results may occur with FDG-PET in patients with abscesses from nonspecific inflammatory reactions following radiotherapy or tracer uptake in the bowel, bladder, or ureters. Role of PET/CT is less well defined and no guidelines support it for rectal cancer.
NCCN states that PET scan is not routinely indicated. It is not known whether it adds any useful information to that provided by CT.
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nccn.org, rectal cancer, 2018