Use of Dual-Modality Positron Emission Tomography/Computed Tomography in Oncology

• Objective: To review the literature relating to imaging using positron emission tomography (PET) with 18-fluorodeoxyglucose (FDG) and computed tomography (CT) in oncology. • Methods: Search of the MEDLINE database through 30 June 2005. • Results: There were 22 peer-reviewed publications in which dual-modality FDG-PET/CT was compared with CT alone, PET alone, or other imaging techniques in a total of 1596 cancer patients. Data from these studies suggest that dual-modality FDGPET/CT is superior to either FDG-PET alone or CT alone in detecting, localizing, and characterizing a variety of cancer types. The evidence is limited in that none of the studies used a randomized controlled design in which PET or CT images obtained with stand-alone devices were compared with images acquired using dual-modality instruments. • Conclusion: Although explicit patient selection guidelines for the use of dual-modality FDG-PET/CT have not been published, evidence suggests that dual-modality PET/CT may be considered medically necessary when PET is considered medically necessary for cancer patients. This technology represents a major advance in diagnostic imaging in oncology that will quickly become the standard of care. Positron emission tomography (PET) is a metabolic imaging technique based on the detection of 511 kiloelectron volt (keV) gamma radiation that is produced by an annihilation reaction between a positron (a positively charged electron of characteristic energy) emitted by an unstable radionuclide and an orbital electron of surrounding matter [1–3]. Positron-emitting radionuclides are elemental species with half-lives that range from about 1.3 minutes to nearly 2 hours. They can be incorporated into organic molecules (radiopharmaceuticals) as tracers and do not interfere with cellular functions. PET radiopharmaceuticals are available to evaluate diverse physiologic or biochemical processes [3–6]. The most important in oncology, 18-fluorodeoxyglucose (FDG), is an analogue of D-glucose that becomes trapped within cells after being metabolized through glycolysis [7]. Because cancer cells exhibit increased glucose transport and glycolysis relative to untransformed normal cells, cancerous foci show up as “hot spots” on a FDG-PET image compared with nonmalignant surrounding tissues and organs [8]. FDG-PET has multiple accepted indications in oncology (Table 1) [3,9,10]. The clinical interpretation of an FDG-PET image, however, may be compromised by the limited spatial resolution of PET relative to other methods, in particular computed tomography (CT). It also may be difficult to differentiate foci of pathologic FDG uptake from sites of increased normal physiologic uptake, such as the brain, heart, liver, certain muscles, and soft tissues [11]. The renal collecting systems and the bladder also pose interpretive problems because FDG is excreted through the urinary tract. High-resolution CT is indispensable in cancer diagnosis, treatment, and management but also has specific limitations [12]. In particular, by the time a tumor is sufficiently sized to be detected in a CT image, the disease may well have progressed beyond curable. CT also is not adequate for differentiating posttherapy artifacts from tumor recurrence nor in discerning nonopacified bowel loops from abdominal or pelvic metastases [11]. To address the limitations of PET and CT and enhance the clinical value of the information acquired, separate images may be combined, or fused, either retrospectively or prospectively [13]. Side-by-side visual comparison evolved with the development of computer algorithms and software that electronically coregister images [14,15]. Software-based retrospective techniques are adequate for organs such as the brain, which is fixed relative to external rigid reference markers on the skull. They are less accurate in nonrigid anatomic areas due to patient movement or differences in patient posiFrom the University HealthSystem Consortium, Clinical Knowledge Service, Oak Brook, IL (Drs. Ratko, Cummings, and Matuszewski), and the University of Missouri Hospitals, Division of Nuclear Medicine, Columbia, MO (Dr. Singh). CLINICAL REVIEW