Both CT and (more often) MRI are used to stage carcinoma of the cervix. Lymph node staging is fundamental to the management of cervical cancer in terms of both survival and treatment. Nodal staging with structural imaging methods is limited, however, by the arbitrary 1-cm cutoff for the differentiation of malignancy from benign disease. It is now accepted that small nodes can harbor disease and that large nodes may only be reactive. A more accurate assessment of nodal involvement, perhaps using PET to add functional information, is required.

• Unknown primary tumors. Unknown primary tumors can be localized, staged, and restaged with FDG-PET/CT.⁹ The modality may also be used to monitor response to therapy and for radiotherapy planning.

INFECTION, INFLAMMATION

Infectious and noninfectious inflammatory conditions may demonstrate high FDG uptake. This is because glucose metabolism is elevated by stimulated inflammatory cells, macrophage proliferation, and healing.

FDG accumulation in inflammatory or infectious tissue may reduce specificity in patients with cancer. In patients with suspected infection and no known history of cancer, FDG-PET/CT can be a useful tool for localizing the infectious disease.

Differentiation of the cause of FDG uptake in complex clinical cases—following tumor resection, for example—is possible with sophisticated methods such as kinetic analysis and dual or more time-point PET examinations. Dual and time-point PET scans are performed after two to three hours to see the difference in SUV values after FDG injection. The CT component of PET/CT can also help when differentiating between residual and recurrent cancer and infectious or inflammatory lesions.

FDG-PET/CT avoids many of the disadvantages associated with radiolabeled leukocyte scans, which include the complex cell-labeling procedure, handling and possible cross-contamination of blood samples, high radiation dose, low counts (leukocyte), and time from imaging to diagnosis.

FDG-PET has also been used in the imaging evaluation of patients with fever of unknown origin. One study involving PET reported a positive predictive value of 87% and a negative predictive value of 95% in detecting sites of infection or inflammation.10 PET offers many advantages over gallium scintigraphy in this setting, including higher spatial resolution, better image quality, and decreased time from imaging to diagnosis.

Interest is growing in the use of PET/CT for the evaluation of patients with suspected osteomyelitis and infected limb prosthesis implants. A report of 39 patients with suspected soft-tissue and bone infections found pooled sensitivity and specificities for FDG-PET of 98% and 75%, respectively.¹¹ FDG-PET has a high negative predictive value for osteomyelitis, such that a negative scan effectively excludes that condition. High sensitivity (100%) and specificity (88%) have also been reported for PET in the diagnosis of chronic musculoskeletal infections in both the axial and appendicular skeleton.¹²

FDG-PET/CT could potentially play a role in localizing infection in patients with disseminated tuberculosis.¹³ Biopsies guided by FDG-PET/CT could also help distinguish between lymphoma and opportunistic infections in immunodeficient patients. Subsequent follow-up should be carried out to assess the response to the therapy. Little has been published on this application at the moment, though investigations are ongoing.