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Departments of Trauma, Hand and Reconstructive Surgery; Nuclear Medicine; and Pathology, University of Ulm, Ulm, Germany
| ABSTRACT |
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Key Words: diagnosis of bone neoplasms bone neoplasms in infants and children sarcoma staging of bone neoplasms radionuclide imaging in diagnosis of neoplasms
| INTRODUCTION |
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Classification of skeletal lesions is based on an array of differentiated imaging modalities, implicating different surgical approaches. Some lesions require no treatment, some require biopsy, some curettage, and some en bloc resection as the primary therapeutic step. Malignant primary bone tumors such as osteosarcoma or Ewing's sarcoma typically occur at younger ages, and curative treatment is possible in most of these patients. Early detection and precise classification of these tumors helps to improve the prognosis. Misdiagnoses caused by inappropriate imaging techniques can lead to overtreatment of tumorlike lesions or undertreatment of aggressive or malignant bone tumors, with disastrous consequences. In osseous sarcomas, accurate histologic typing and grading determines surgical margins and the necessity for adjuvant treatment: whereas chondrosarcomas require only surgical therapy, high-grade chondroblastic osteosarcoma can be cured only by combined surgery and polychemotherapy. The reliability of the histologic diagnosis primarily depends on the experience of the pathologist and the quality and size of the biopsy specimen. Frequently, misdiagnoses are the result of biopsies taken from a part of the lesion that is not representative of the entire neoplasm.
With some entities, the diagnosis can be difficult even for an experienced bone tumor pathologist. For example, distinguishing proliferative chondroma from well-differentiated chondrosarcoma (1,2), or an aneurysmal bone cyst from telangiectatic osteosarcoma (13), sometimes can be impossible without knowledge of the radiologic appearance. Diagnosis of central, periosteal, or parosteal osteosarcomas, which require different treatment strategies, can be established only with the aid of subtle imaging techniques (1,2).
Elevated uptake of FDG as determined by PET has been reported for malignant mesenchymal tumors in a few studies. FDG PET has been proposed as a diagnostic tool for distinguishing benign from malignant soft tissue and osseous lesions (47), for grading sarcomas (6,810), and for detecting local recurrences (11,12). In addition, FDG PET has been used for monitoring chemotherapy responses (13) and detecting pulmonary metastases in osteogenic sarcoma (14). However, in a recent study of 26 patients with skeletal lesions, the biologic behavior of the tumors did not correlate with their glucose metabolism as quantified by FDG PET (15). The purpose of this study was to determine whether FDG PET is reliable for grading tumors and tumorlike lesions of bone.
| MATERIALS AND METHODS |
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The tumors were localized in the femur (n = 56), pelvis (n = 48), humerus (n = 32), lower leg (n = 30), thorax (n = 25), spine (n = 6), forearm (n = 3), or foot (n = 2). The size of the lesions ranged from 2.9 to 78.6 cm2 on transaxial sections.
Incisional, excisional, or needle biopsy was performed within 8 d after PET. All patients with a diagnosis of primary bone neoplasm underwent tumor resection. The majority of high-grade sarcomas received neoadjuvant chemotherapy. Definitive histopathologic evaluation of the surgical specimen revealed 115 malignant and 87 benign lesions, comprising 70 high-grade sarcomas, 21 low-grade sarcomas, 40 benign tumors, 47 tumorlike lesions, 6 osseous lymphomas, 6 plasmacytomas, and 12 metastases of an unknown primary tumor. Sarcomas were classified as high grade or low grade according to Enneking (16), including histologic, radiographic, and clinical criteria. Likewise, benign tumors were classified as latent (stage 1), active (stage 2), or aggressive (stage 3) (16).
FDG PET
FDG PET studies were performed as described by Stollfuss et al. (17) with a commercially available scanner (ECAT 9310812; Siemens/CTI, Knoxville, TN) that permits simultaneous acquisition of 15 contiguous sections of 6.75-mm thickness with an axial field of view of 10.1 cm. The minimum resolution was 7 mm full width at half maximum at the center of the field of view. Attenuation was corrected with a rotating 68Ge68Ga source. Depending on tumor size, transmission scans with a duration of 8 min per bed position were obtained for at least 3 bed positions, without overlap, on the same day as and before emission scans. Thus, repositioning of the patients was necessary using multiple laser-guided landmarks. The patients fasted for at least 8 h before the study; normal plasma glucose levels were documented before FDG administration. In accord with a study by Ichiya et al. (18), semiquantitative assessment of tumoral FDG uptake was based on static emission scans of the tumor site and the corresponding contralateral area, with 2 or 3 contiguous bed positions, starting no earlier than 45 min (range, 4560 min) after intravenous administration of a body massdependent dose of 120300 MBq FDG. The acquisition time was 10 min per bed position. Images were reconstructed with a multiplicative iterative reconstruction algorithm (19).
Data Analysis
The PET scans were evaluated qualitatively and semiquantitatively by analyzing the zones with elevated FDG uptake on transaxial, coronal, and sagittal sections. Regions of interest (ROIs) were individually defined for each lesion, excluding any areas without detectable uptake within the tumor. ROIs were defined in the transaxial section expressing maximum uptake and were larger than 2.6 cm2 in each instance. Therefore, the ROI was always at least 2 times the minimum resolution of the PET scanner (7 mm). The neoplasm was clearly demarcated from surrounding tissue, and the boundaries of the ROI were just within the apparent hypermetabolic zone of the tumor. Quantitative assessment of FDG uptake was based on transaxial sections exclusively. Tumor-to-background ratios (T/Bs) were calculated using an ROI of identical configuration in the analogous site of the contralateral extremity, pelvic bone, or chest. In patients with a vertebral lesion, an adjacent unaffected vertebral body was used as a reference region. To assess interobserver error, T/Bs were obtained by 2 independent physicians without knowledge of the clinical data and were averaged for statistical evaluation.
Statistical Analysis
The Mann-Whitney test was used to compare the T/Bs of different subgroups, and 95% confidence intervals (CIs) were calculated according to Clopper and Pearson (20).
| RESULTS |
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Histologic diagnoses and the distribution of T/Bs are listed in Table 1. Table 2 correlates FDG uptake with the biologic activity of the investigated tumors. Among the sarcomas, osteosarcomas had a tendency toward higher T/Bs than did Ewing's sarcomas (P < 0.01) and chondrosarcomas (P < 0.001). Glucose metabolism was greater for high-grade malignant lesions than for low-grade sarcomas (P < 0.01), but no difference was observed in comparisons with aggressive benign tumors. The T/Bs of stage 1 and 2 benign lesions were significantly lower than those of stage 3 benign lesions (P < 0.001) and of low-grade sarcomas (P < 0.001). Benign cartilage tumors showed a lower FDG uptake than did chondrosarcomas (P < 0.01). Among the benign lesions, giant cell tumors disclosed a significantly elevated glucose metabolism (P < 0.001).
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FDG PET revealed false-negative results for 6 low-grade sarcomas (G 1) and 2 plasmacytomas. No false-negative results occurred among high-grade sarcomas, lymphomas of bone, and skeletal metastases. In particular, false-negative results were found for 6 patients with low-grade chondrosarcoma (n = 12), among them 1 woman with Ollier's disease, in which malignant transformation of a chondroma had occurred. However, both patients with high-grade chondrosarcoma had markedly elevated glucose metabolism. No false-negative findings occurred for patients with osteosarcoma (n = 44; Fig. 1), Ewing's sarcoma (n = 14), malignant fibrous histiocytoma of bone (n = 6), or angiosarcoma of bone (n = 3).
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| DISCUSSION |
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In planning surgical strategy, one has to consider not only the grade of malignancy but also local aggressiveness, which may occur both in malignant and in benign lesions. Therefore, we primarily attempted to evaluate the biologic activity of osseous tumors and tumorlike lesions using FDG PET. The parameters for quantification of FDG uptake are standardized uptake values and T/Bs. For practicability and independence of transmission scanning, we used T/Bs instead of standardized uptake values. Studies on grading soft-tissue tumors (7) and predicting the response of osteosarcomas to neoadjuvant chemotherapy (13) have shown that the exclusive use of T/Bs estimates the biologic activity of skeletal lesions with great accuracy.
Sarcomas and aggressive benign tumors, both of which require resection with wide surgical margins, showed a markedly elevated glucose metabolism compared with latent and active benign lesions. Therefore, preoperative analysis of glucose metabolism can help in choosing the best surgical procedure for benign tumors with inconclusive radiologic findings. In the identification of skeletal lesions for which curative treatment requires wide surgical excision, analysis of glucose metabolism can improve the diagnostic value of FDG PET with a sensitivity of 93.7% (CI, 86%98%), a specificity of 76.3% (CI, 65%85%), an accuracy of 87.1% (CI, 82%91%), and a positive predictive value of 86.8% (CI, 80%91%).
Grading mesenchymal skeletal lesions using FDG PET can be helpful for equivocal tumors, particularly lesions for which a discrepancy exists between histopathologic diagnosis and clinical findings. If, in a lesion described as harmless by the pathologist, FDG uptake is high, a bone tumor specialist should reevaluate the biopsy specimen, orif the biopsy specimen is considered nonrepresentativea second biopsy should be obtained. Alternatively, the tumor may be resected with extended surgical margins, if such a resection is possible without major risks or functional loss. If FDG uptake is low in a lesion histologically characterized as malignant, the specimen should be reevaluated carefully, especially if mutilating surgical procedures or chemotherapy is intended. In our study, all high-grade sarcomas, lymphomas, and bone metastases were detected as malignant lesions by markedly increased glucose metabolism.
The limitations of FDG PET in discriminating between certain malignant and benign primary bone tumors are obvious: FDG PET failed to reveal malignancy in 6 cases of low-grade chondrosarcoma, whereas the specificity for malignant cartilaginous tumors is high. In a patient with Ollier's disease, multiple chondromas of the long bones showed the same moderately increased glucose metabolism (maximal T/B, 2.7) as did a histologically confirmed malignant transformation of a subtrochanteric lesion into a low-grade chondrosarcoma (T/B, 2.6) in the same patient. Several active or aggressive stage 2 or 3 benign lesions, such as giant cell tumors and some aneurysmatic bone cysts, could not be distinguished from malignant tumors using FDG PET. Interestingly, dynamic gadolinium-DTPAenhanced MRI of aggressive benign bone tumors such as giant cell tumors, for which histologic evaluation of biologic behavior is difficult even for experienced pathologists, showed a pattern of increased signal intensity similar to that of malignant tumors (26). Nevertheless, in a case of telangiectatic osteosarcoma, PET revealed a higher FDG uptake (T/B, 6.6) for the malignancy than for aneurysmatic bone cysts (maximal T/B, 5.9). However, why skeletal lesions with moderate biologic activity, such as fibrous dysplasia or osteofibrous dysplasia, appear strongly hypermetabolic remains unclear. Malignant transformation is known to occur in approximately 1% of fibrous dysplasia cases (1,2). We observed a patient with rhabdomyosarcoma of the innominate bone that disclosed a markedly increased FDG uptake (T/B, 15.9). Some years ago, this patient underwent several surgeries because of histologically proven fibrous dysplasia at the same location.
A general problem with FDG PET in tumor diagnosis is the known accumulation of the glucose analog FDG not only in tumor cells but also in macrophages and granulation tissues (28) and in inflammation, such as osteomyelitis (5,29). We observed 3 false-positive results caused by inflammatory processes.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Michael Schulte, MD, Department of Trauma, Hand and Reconstructive Surgery, Steinhövelstr. 9, D 89075 Ulm, Germany.
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