Original Report. Sacral Fractures: A Potential Pitfall of FDG Positron Emission Tomography

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1 Downloaded from by on 12/24/17 from IP address opyright RRS. For personal use only; all rights reserved Laura M. Fayad 1 hristian ohade Richard L. Wahl Elliot K. Fishman Received pril 2, 2003; accepted after revision May 22, ll authors: The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, 601 N Wolfe St., altimore, MD ddress correspondence to L. M. Fayad. JR 2003;181: X/03/ merican Roentgen Ray Society Original Report Sacral Fractures: Potential Pitfall of FDG Positron Emission Tomography OJETIVE. Positron emission tomography (PET) with FDG is useful for tumor imaging, but false-positive results can occur. The purpose of this study is to describe three oncology patients with sacral fractures in whom FDG uptake in the sacrum increased on PET. ONLUSION. 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. F DG positron emission tomography (PET) has emerged as a powerful tool for the evaluation of oncology patients, gaining an important role in staging tumors, assessing response to treatment, and detecting recurrences. FDG PET affords high accuracy for the detection of metastatic disease, including skeletal metastases [1, 2]. 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. Materials and Methods review of three patients with histories of colorectal cancer who had undergone FDG PET and correlative T or MRI was performed. ll patients were elderly women (age range, years) and had undergone surgical resection, chemotherapy, and radiation therapy for local disease. Two had tumoral invasion of pelvic organs, and one exhibited metastatic disease to the pelvic lymph nodes. None had known hepatic or pulmonary metastases. Imaging Studies ll correlative cross-sectional imaging, T or MRI, was performed on the same day as FDG PET. In two patients, PET and T were performed with a PET T 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 µi (536.5 Mq) of FDG; T-based attenuation correction was performed. For corresponding T, no IV contrast material was administered. Scanning parameters were 80 m, 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 (dvance, General Electric Medical Systems). PET images were obtained 60 min after IV injection of 10 µi (370 Mq) of FDG; germaniumattenuation 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. xial, 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, ; slice thickness, 8 mm) and axial T1- weighted spin-echo images (TR/TE, 600/9; number of excitations, 2; matrix, ; slice thickness, 8 mm) were obtained. Finally, fat-suppressed fast spoiled gradient-echo images (250/2; flip angle, 80 ; 1 excitation; matrix, ; 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 con- JR:181, November

2 Fayad et al. Downloaded from by on 12/24/17 from IP address opyright RRS. For personal use only; all rights reserved centration 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. ll PET images were viewed on a workstation (entegr, General Electric Medical Systems). T images for the two patients with corresponding T 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, P). Hard-copy MRIs were reviewed for the third patient. Reviewers assessed and characterized abnormalities of the sacrum. For PET, the presence of increased Fig year-old woman with history of adenocarcinoma of cecum who underwent right hemicolectomy with adjuvant chemotherapy and radiation., Maximum-intensity-projection positron emission tomography (PET) scan of whole body shows moderate diffuse FDG uptake in left sacrum (arrow)., xial PET scan shows moderate diffuse FDG uptake in left sacrum (arrow)., xial T scan shows left sacral fracture (arrow). FDG uptake was determined and configuration of uptake was described. For both T 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 T scans and MRIs. Results ll FDG PET scans depicted increased uptake of FDG in the sacrum. In the first patient, moderate diffuse uptake in the left sacrum was identified; the standard uptake value was 2.2 with a mean uptake value of 1.5. T showed a corresponding left sacral fracture without evidence to suggest a sacral metastasis (Fig. 1). In the second patient, bilateral moderate FDG uptake was present in the sacrum in a linear configuration; the standard uptake values were 3.4 on the left and 3.6 on the right with mean uptake values of 2.5 and 2.4, respectively. orrelative T scans showed bilateral sacral fractures without evidence of bony metastases in the sacrum (Fig. 2). In the third patient, moderate diffuse increased uptake was present in the right sacrum; the standard uptake value was 3.6 with a mean uptake value of 1.6. MRI showed bilateral linear fracture lines in the sacrum with surrounding bone marrow edema (Fig. 3). Discussion Whole-body FDG PET is an important imaging tool for the evaluation of oncology patients. Unlike other imaging modalities, FDG PET detects tumors by their rapid glucose metabolism. ross-sectional anatomic imaging methods, on the other hand, rely on morphologic changes to detect malignant lesions. PET has been advocated by some authors to be essential for the staging of tumors because of its high sensitivity and specificity [3]. Furthermore, because of its ability to image the whole body with high accuracy, PET may be an alternative to the series of multiple, time-consuming, and expensive cross-sectional imaging tests that are often performed for complete evaluation of malignancy in select patients [3, 4]. 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 1240 JR:181, November 2003

3 FDG PET of Sacral Fractures Downloaded from by on 12/24/17 from IP address opyright RRS. For personal use only; all rights reserved 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. ompared with radiography, T 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 T 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 T, 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 Fig year-old woman with history of adenocarcinoma of rectum who underwent abdominal perineal resection with adjuvant chemotherapy and radiation treatment., xial positron emission tomography scan shows moderate linear FDG uptake in sacrum bilaterally (arrows). and, xial T scans show vertical () and horizontal () sacral fractures (arrows). 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]. olorectal carcinomas in our patient population were confined to local disease. There were no distant metastases to the liver or lungs. ecause 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. hronic 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. T 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 JR:181, November

4 Downloaded from by on 12/24/17 from IP address opyright RRS. For personal use only; all rights reserved Fayad et al. D Fig year-old woman with history of adenocarcinoma of rectum who underwent low anterior resection with adjuvant chemotherapy and radiation., Maximum-intensity-projection positron emission tomography (PET) scan of whole body shows moderate diffuse FDG uptake in right sacrum (arrow)., xial PET scan again shows moderate diffuse FDG uptake predominantly in right sacrum (arrow)., xial T2-weighted fast spin-echo image (TR/TE, 5,000/77) shows bilateral linear signal abnormalities representing sacral fractures (arrows). D, oronal fat-suppressed T2-weighted fast spin-echo image (3,400/77) shows bilateral linear signal abnormalities representing sacral fractures (arrows), surrounded by bone marrow edema. E, xial T1-weighted spin-echo image (600/9) shows bilateral bandlike decreased signal in sacrum (arrows). E 1242 JR:181, November 2003

5 FDG PET of Sacral Fractures Downloaded from by on 12/24/17 from IP address opyright RRS. For personal use only; all rights reserved 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 T, the importance of careful interpretation of the correlative T images is underscored. Knowledge and recognition of potential sacral FDG uptake in sacral insufficiency fractures can prevent inappropriate treatment and avoid unnecessary bone biopsies. References 1. Franzius, Sciuk J, Daldrup-Link HE, Jurgens H, Schober O. FDG PET for detection of osseous metastases from malignant primary bone tumors: comparison with bone scintigraphy. Eur J Nucl Med 2000;27: ook GH, Fogelman I. The role of positron emission tomography in the management of bone metastases. ancer 2000;88: Staib L, Schirrmeister H, Reske SN, eger HG. Is F18-fluorodeoxyglucose positron emission tomography in recurrent colorectal cancer a contribution to surgical decision making? m J Surg 2000;180: Ogunbiyi O, Flanagan FL, Dehdashti F, et al. Detection of recurrent and metastatic colorectal cancer: comparison of positron emission tomography and computed tomography. nn Surg Oncol 1997;4: lomlie V, Rofstad EK, Talle K, Sundfor K, Winderen M, Lien HH. Incidence of radiation-induced insufficiency fractures of the female pelvis: evaluation with MR imaging. JR 1996;167: Grangier, Garcia J, Howarth NR, May M, Rossier P. Role of MRI in the diagnosis of insufficiency fractures of the sacrum and acetabular roof. Skeletal Radiol 1997;26: Meyer M, Gast T, Raja S, Hubner K. Increased F- 18 FDG accumulation in an acute fracture. lin Nucl Med 1994;19: Shon IH, Fogelman I. F-18 FDG positron emission tomography and benign fractures. lin Nucl Med 2003;28: Kato K, oki J, Endo K. Utility of FDG PET in differential diagnosis of benign and malignant fractures in acute to subacute phase. nn Nucl Med 2003;17: Schmitz, Risse JH, Textor J, et al. FDG PET findings of vertebral compression fractures in osteoporosis: preliminary results. Osteoporos Int 2002;13: Kanthan R, Loewy J, Kanthan S. Skeletal metastases in colorectal carcinomas: a Saskatchewan profile. Dis olon Rectum 1999;42: Gordon, Flanagan FL, Dehdashti F. Wholebody positron emission tomography: normal variations, pitfalls, and technical considerations. JR 1997;169: Robiller F, Stumpe KD, Kossmann T, et al. hronic osteomyelitis of the femur: value of PET imaging. Eur Radiol 2000;10: Schneider R, Yacovone J, Ghelman. Unsuspected sacral fractures: detection by radionuclide bone scanning. JR 1985;144: Peh W, Khong PL, Ho WY, Yeung HW, Luk KD. Sacral insufficiency fractures: spectrum of radiological features. lin Imaging 1995;19: JR:181, November