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Original Report |
1 All authors: The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, 601 N Wolfe St., Baltimore, MD 21287-0705.
Received April 2, 2003;
accepted after revision May 22, 2003.
Address correspondence to L. M. Fayad.
Abstract
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CONCLUSION. Sacral fractures can show increased uptake of FDG on PET. Therefore, correlative cross-sectional imaging is necessary to avoid the erroneous diagnosis of sacral metastases on PET and prevent inappropriate treatment of patients.
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In oncology patients presenting with lower back pain, metastatic disease to the sacrum is always considered. However, many of these patients have received radiation treatment, and especially in elderly women with osteoporosis, a potential confounding diagnosis is sacral insufficiency fracture.
The purpose of this article is to correlate the finding of sacral FDG uptake on PET scans with corresponding cross-sectional images of the sacrum in three patients with colorectal cancer who had previously received radiation treatment.
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Imaging Studies
All correlative cross-sectional imaging, CT or MRI, was performed on the
same day as FDG PET. In two patients, PET and CT were performed with a
PET–CT camera (Discovery LS, General Electric Medical Systems,
Milwaukee, WI). In these patients, FDG PET images were obtained 60 min after
the IV injection of an average of 14.5 µCi (536.5 MBq) of FDG; CT-based
attenuation correction was performed. For corresponding CT, no IV contrast
material was administered. Scanning parameters were 80 mA, 140 kV, 5-mm
section thickness, 20-mm table feed, and pitch of 4.
In the third patient, FDG PET was performed with a PET camera (Advance, General Electric Medical Systems). PET images were obtained 60 min after IV injection of 10 µCi (370 MBq) of FDG; germanium-attenuation correction was performed. In this patient, MRI was performed on a 1.5-T MRI scanner (Signa, General Electric Medical Systems) using a phased array coil. Axial, coronal, and sagittal fast spin-echo T2-weighted images (TR range/TE, 3,400–5,000/77; echo-train length, 16; number of excitations, 4; matrix, 256 x 256; slice thickness, 8 mm) and axial T1-weighted spin-echo images (TR/TE, 600/9; number of excitations, 2; matrix, 512 x 160; slice thickness, 8 mm) were obtained. Finally, fat-suppressed fast spoiled gradient-echo images (250/2; flip angle, 80°; 1 excitation; matrix, 512 x 160; slice thickness, 8 mm) were obtained before and after IV administration of 15 mL of gadopentetate dimeglumine.
For each patient, standardized uptake values were calculated using a region of interest that included the highest activity area of the sacrum. The standardized uptake value was defined as the concentration of FDG in the sacrum divided by the concentration of FDG in the whole body.
Reviewers and Procedures
Imaging was prospectively reviewed independently by two radiologists, a
dedicated cross-sectional radiologist, and a dedicated nuclear medicine
physician. All PET images were viewed on a work-station (eNTEGRA, General
Electric Medical Systems). CT images for the two patients with corresponding
CT examinations were reviewed in bone window settings (width, 1,776; center,
176) and soft-tissue window settings (width, 410; center, 7) on a remote
workstation (Magicview 1000, Siemens Medical Solutions, Malvern, PA).
Hard-copy MRIs were reviewed for the third patient.
Reviewers assessed and characterized abnormalities of the sacrum. For PET, the presence of increased FDG uptake was determined and configuration of uptake was described. For both CT and MRI, reviewers determined the presence of a sacral abnormality and characterized it as a fracture or abnormality other than a fracture that may indicate metastatic disease. One reviewer recorded the findings on all imaging studies and correlated the results of the PET scans with those of the CT scans and MRIs.
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However, we emphasize a pitfall of PET: sacral insufficiency fractures. We studied three patients with colorectal cancer who, in addition to their risk for bone metastases, had risk factors for the development of insufficiency fractures. These patients were elderly women who had received radiation therapy and were complaining of lower back pain. In one study, 89% of women who had received radiation therapy to the pelvis had insufficiency fractures [5]. Sacral fractures are often unrecognized in patients, despite associated symptomatology, because of the subtlety of findings on conventional radiographs. Compared with radiography, CT offers contrast resolution that is superior for the visualization of fractures, and MRI is sensitive for diagnosing early edema and linear fracture lines [6]. Thus, cross-sectional imaging with CT or MRI can distinguish FDG uptake that is due to insufficiency fractures from FDG uptake that is due to tumoral infiltration. Moreover, with the advent of PET–CT, establishing the presence of associated morphologic changes of a fracture becomes an easier task.
FDG accumulation has been described in the setting of benign fractures [7–10]. However, it is controversial and little is known regarding FDG accumulation in the various stages of fracture. In one of our patients, despite bilateral sacral fractures shown on MRI, there was predominantly unilateral FDG uptake. This discrepancy is most likely due to differences in the ages of the sacral fractures. Fractures that are older than 8 weeks [8] or that are acute [9, 10] may show no FDG uptake. In addition, we calculated standardized uptake values, with the maximal uptake values ranging from 2.2 to 3.6; the utility of these values remains unclear, although a trend toward lower uptake in benign fractures compared with uptake in malignant lesions has been noted [9].
Colorectal carcinomas in our patient population were confined to local disease. There were no distant metastases to the liver or lungs. Because skeletal metastases are uncommon in this population, FDG uptake in the sacrum was more likely to be benign. Furthermore, the diffuse or linear configuration of FDG uptake seen in all three patients may also suggest benign disease. However, isolated skeletal metastases occur with a 16.9% incidence in patients with colorectal carcinoma [11]. Hence, even isolated sacral FDG uptake should alert the physician to the possibility of a sacral metastasis, but once again, correlation with cross-sectional imaging is required to exclude a sacral fracture.
Other pitfalls of PET have been recognized in the evaluation of skeletal metastases. FDG uptake, which is usually minimal in bone marrow, may be increased markedly in patients with marrow stimulation due to massive severe bleeding or in patients who receive hematopoietic stimulation treatment [12]. In addition, benign skeletal lesions other than fractures have been described as PET-avid. Chronic osteomyelitis is an example [13].
Misinterpretation of sacral FDG uptake can be avoided by paying close attention to correlative cross-sectional imaging features, which may distinguish a skeletal metastasis from a sacral fracture. These features may be unilateral or bilateral and may manifest as vertical sacral ala fractures or horizontal body fractures. The typical H-shaped pattern, diagnostic of bilateral insufficiency fractures seen on bone scintigraphy [14], is rarely present [15]. Incomplete variants of the H pattern are more frequently encountered. Such variant patterns can be sought with PET and can alert the radiologist to a potential fracture that may subsequently be confirmed on cross-sectional imaging. CT will accurately depict a vertical fracture component parallel to the sacroiliac joint or a horizontal fracture line in the body. In addition, sclerosis around the fracture line is present in the nonacute stage of fracture [15]. With MRI, a hypointense band on T1-weighted images and hyperintense band on T2-weighted images with a linear signal abnormality are diagnostic of a fracture [6, 15].
With the advent of PET–CT, the importance of careful interpretation of the correlative CT images is underscored. Knowledge and recognition of potential sacral FDG uptake in sacral insufficiency fractures can prevent inappropriate treatment and avoid unnecessary bone biopsies.
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