Positron Emission Tomography and Positron Emission Tomography/Computed Tomography Clinical Applications

Diagnosis.

FDG-PET is sensitive for detecting primary colorectal carcinoma (95% to 100%), but it has unacceptably lower specificity than conventional modalities at initial staging. In practice, FDG-PET is therefore rarely specifically used for the diagnosis of colorectal cancer, although it may incidentally detect such a tumor as PET, and especially PET/CT, becomes more extensively used. At the primary site, however, the positive predictive value of PET is lower than the negative predictive value, owing to the false-positive FDG-PET findings of both physiologic bowel activity and inflammatory processes.

Staging.

In staging patients with colorectal cancer, FDG-PET and PET/CT are mainly used to assess regional lymph node involvement and distant metastases. However, FDG-PET has been shown to have a very low sensitivity of between 22% and 29% for regional lymph node metastases, with CT also demonstrating low values (29%). However, FDG-PET specificity was superior (96% vs. 85%). False-negative findings in regional metastatic lymph nodes on FDG-PET are considered to be due to low sensitivity for microscopic involvement of small lymph nodes or occasionally as a result of increased FDG uptake by the primary tumor, which masks the immediately neighboring structures.

Most patients with colorectal cancer at initial staging have disease limited to the colon or to regional pericolic or mesenteric lymph nodes. For patients with early colorectal cancer, surgery is usually performed with the intention to achieve cure. However, distant metastatic disease may necessitate the surgery for complications, which include hemorrhage, obstruction, and perforation. Given this role of surgery in both regional and advanced disease, surgical and pathologic criteria define the colorectal cancer staging classification.

The standard tumor, node, metastasis (TNM) classification is currently advocated by the American Joint Committee on Cancer. Because of inherent technologic limitations, imaging modalities today are generally unable to match the diagnostic staging information provided by surgical and pathologic findings. PET cannot provide an accurate T-stage determination necessary for TNM nomenclature, in which exact depth of invasion is the primary parameter. PET is usually accurate only in cases of gross serosal penetration and invasion of neighboring structures. CT gives more exact structural information but, unfortunately, usually also cannot adequately resolve the bowel wall layers. N staging necessitates evaluation of mesenteric nodes as well as pericolic nodes, which are frequently small and lie beside the primary tumor. Furthermore, pericolic nodes again are often involved microscopically by tumor, which usually can be diagnosed only by histopathologic evaluation. Despite the clear superiority of surgery and histopathology at TNM staging, preoperative imaging detection of nodal or organ metastases is important in guiding the general management toward palliation or curative tumor resection. Notwithstanding its staging limitations, PET/CT offers combined metabolic and structural information that is particularly useful in cases of more advanced local disease.