REVIEWClinical PET in Oncology
Section snippets
Patient Preparation and FDG Administration
FDG imaging is performed in the fasting state to minimize competitive inhibition of FDG uptake by serum glucose.7 A 4-hour fast is recommended, and a serum glucose level is not obtained routinely prior to FDG administration. If there is a question concerning the length of fasting or if there is the possibility of an elevated serum glucose level because of a history of glucose intolerance, a serum glucose level is obtained prior to FDG administration. If the serum glucose level is <200 mg/dl,
Imaging Protocols
A 30-minute uptake is used for patients with known or suspected brain tumors. A 6-minute acquisition is used in 3-D mode or a 20-minute acquisition is used in 2-D mode. The 3-D mode is preferred because of the higher number of counts obtained in a shorter acquisition time, which minimizes patient motion. The 3-D mode is 4–5 times more sensitive than the 2-D mode, and approximately twice the number of counts can be obtained in the shorter imaging time. The 3-D acquisition does result in more
Image Analysis
FDG-PET scans for oncologic indications are interpreted qualitatively like other imaging studies, and an area of abnormality is detected by comparison with background activity. With FDG imaging, a semiquantitative parameter may be useful in characterizing lesions as benign or malignant that are equivocal by qualitative analysis. This semiquantitative parameter is an index of glucose metabolism that is called the standardized uptake ratio (SUR), which is also referred to as standardized uptake
Image Interpretation
Interpretation of FDG-PET scans for oncologic indications is more difficult than the interpretation of other nuclear medicine studies. The patients have had other imaging studies, which must be reviewed and correlated with the findings of the PET scans. The PET scans should be reviewed on a work station monitor to adjust the image display parameters. The images need to be reviewed in transverse, coronal, and sagittal planes. Resorted projection images may be displayed as a rotating body and may
Brain Tumors
Several applications of PET in brain tumors have been demonstrated.19 PET can be used in determining the degree of malignancy of a tumor and in determining the prognosis of brain tumor patients. PET is useful in determining the appropriate biopsy site in patients with multiple lesions, large homogeneous lesions, and large heterogenous lesions. PET is accurate in differentiating recurrent tumor from necrosis in patients who have undergone radiation therapy and chemotherapy.
Most studies of PET
Lung Cancer
Lung cancer commonly presents as a focal lung abnormality. Imaging with chest radiography, CT, or MRI cannot definitively differentiate benign from malignant focal lung abnormalities in most patients, and a tissue diagnosis is often made by bronchoscopy, percutaneous biopsy, or open lung biopsy. This difficulty in differentiating benign from malignant lesions is true of new lung abnormalities as well as abnormalities that occur after surgical resection or radiation therapy.
The recently
Evaluation of the Solitary Pulmonary Nodule
Solitary pulmonary nodules are identified on routine chest radiographs that are performed as part of preoperative evaluations or physical examinations. In people 35 years of age or older, only one third of the nodules will be carcinoma. In patients with risk factors such as tobacco use, the risk of malignancy is higher. People younger than 35 years of age are much less likely to have a malignant pulmonary nodule. Further radiographic evaluation is often performed with serial plain radiographs
Staging of Bronchogenic Carcinoma
Staging of bronchogenic carcinoma requires accurate evaluation of mediastinal nodes, and staging of the mediastinum is most reliably performed by mediatinoscopy.37, 38, 39 Staging by anatomical studies has been attempted but is only complementary to other techniques.39 For example, adenopathy as defined by CT imaging is both insensitive and non-specific for malignancy. CT and MRI staging of non-small cell lung cancer are reported to have a sensitivity of 52 and 48% and a specificity of 69 and
Evaluation of Response to Therapy
Radiation therapy has been the treatment of choice for patients with unresectable non-small cell lung cancer. FDG-PET can identify changes in tumor glucose uptake after radiation therapy. Some investigators have concluded however, that a decrease in FDG uptake did not necessarily indicate a good prognosis.44 FDG uptake decreases over time in some patients, but other patients do not have a similar response to radiation therapy. Patient outcome data is needed to verify the potential impact that
Colon Cancer
Tumors of the colon and rectum are not well imaged by conventional modalities. CT is often not able to distinguish tumor from other soft tissue masses in the setting of tumor recurrence.
PET has been helpful in differentiating recurrent tumor (Figure 9) from scar.46 In 33 patients with colorectal tumor recurrence, all tumors except one showed high FDG uptake while FDG accumulation was low in area of scar. In a review article published in 1996, Conti et al.47 summarized the results of 6 studies
Breast Cancer
FDG-PET scans have demonstrated increased metabolic activity in breast tumors and breast tumor metastatic lesions (Figure 10). In one study, PET had no false positives in imaging breast tumors and accurately identified tumor in 2 cases where mammograms did not show malignancy because of dense breast parenchyma.50 In another study, 10 of 10 cases were correctly identified by FDG-PET in which the axillary dissection was negative and 9 of 10 cases in which the axillary dissection was positive.51
Melanoma
Several studies have demonstrated the ability of FDG-PET to image melanoma and the distribution of metastatic disease (Figure 11). Using nonattenuation corrected whole-body scans, Steinert and colleagues54 reported excellent results in their study of 33 patients. The sensitivity of PET was 92% with a specificity of 77% without clinical information, which improved to 100% with clinical information such as location of biopsy sites. We have similar results in 35 patients studied at our
Lymphoma
The initial studies on the use of FDG-PET imaging in Hodgkin’s and non-Hodgkin’s lymphoma suggest that both diseases can be accurately detected.55 These studies report that FDG uptake is higher in the higher grade than in the lower grade lymphomas. A recent study by Newman et al.56 has demonstrated that FDG-PET detected all of the more than 50 lesions identified by CT as well as several lesions in lymph nodes of normal size.
Head and Neck Cancer
Nearly 60% of head and neck tumors present with large locally advanced disease, nodal disease, or metastatic disease. Current treatment has only produced less than 30% 5-year survival in advanced disease. Information about response to therapy is needed to improve 5-year survival. The anatomy of the head and neck is complex and lymph tissues of the neck enhance on usual contrast studies causing difficulty in differentiating benign and malignant disease. The same difficulties in differentiating
Other Cancer
Investigators have used PET successfully to image many other cancers. Examples include FDG evaluation of musculoskeletal neoplasms,61 ovarian cancer,62 pancreatic cancer,63 and thyroid cancer.64
Conclusion
PET imaging in oncology is being increasingly utilized because of the additional information provided that is important in patient management. FDG-PET imaging is routinely used in evaluating several malignancies: lung cancer, brain tumors, colorectal cancer, head and neck tumors, lymphoma, and melanoma. Data are now available demonstrating the cost-effectiveness of FDG-PET in several malignancies. Studies of larger numbers of patients are needed to define the role of FDG-PET in other
Acknowledgements
I thank Sharon Hamblen, N.M.T., Abdulaziz Al-Sugair, M.D., and Timothy Turkington, Ph.D., for their help in the preparation and review of this manuscript and Nell Gilbert for her invaluable assistance in organizing the manuscript.
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