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Original Report |
1 Hôpitaux de Paris PET Center, Hôpital Tenon, 4 rue de la Chine,
75020, Paris, France.
2 Department of Radiology, Hôpital Tenon, Paris, France.
3 Paris Nord PET Center, Sarcelles, France.
4 Department of Oncology, Institut Gustave Roussy, Villejuif, France.
5 Department of Pathology, Hôpital Tenon, Paris, France.
Received April 23, 2004;
accepted after revision June 30, 2004.
Address correspondence to N. Aide
(naide{at}club-Internet.fr).
Abstract
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CONCLUSION. Remote mesh prostheses can induce FDG uptake because of persistent foreign body reaction. Consequently, each time an unexpected pelvic focus is noticed on FDG PET, the medical history of patients should be carefully reviewed to avoid false-positive results.
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FDG uptake has been frequently reported around surgical scars during healing. But, to our knowledge, the occurrence of foci of FDG uptake several years after surgery, without cancer relapse or infection, has not yet been reported. We present a series of three oncology patients in whom FDG uptake around a mesh prosthesis was discovered several years after an inguinal hernia repair. This led to false-positive results at the lesion level.
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Imaging Studies
PET examinations were performed on a full-ring dedicated PET machine (ADAC,
C-PET). Patients were fasting for at least 6 hr and were at complete muscular
rest. Two megabecquerels of FDG per kilogram of body weight was injected IV
through a saline infusion. The acquisition of PET images was started 1 hr
after injection. Both emission (5 min per step, 3D mode) and transmission (1
min per step) images were acquired. Data were reconstructed by means of an
iterative algorithm, and attenuation correction was performed on the basis of
the segmentation attenuation correction technique. For each patient, maximum
standardized uptake value (SUVmax) of the suspicious FDG foci was
determined by manually drawing regions of interest (ROIs) on the axial images
around the focal FDG uptake zones. FDG uptake in these ROIs was quantified by
calculating the SUV in each pixel according to the following formula: SUV =
activity concentration / (injected dose / body weight). To minimize partial
volume effects, the SUVmax within an ROI was used.
One patient underwent a subsequent PET examination on a PET/CT machine (Biograph, Siemens Medical Solutions) after injection of 5.5 MBq of FDG per kilogram of body weight. Emission data were acquired in the 3D mode 1 hr after injection, with 210 sec per step. The technical parameters used for the CT portion were as follows: pitch, 1; gantry rotation time, 0.8 sec; 110 kV; and 80 mA. In this patient, MRI was performed on a 1.5-T scanner (Sonata, Siemens).
Helical scanning was performed on single-detectors scanners (Picker, PQ 5000; GE Healthcare, ProSpeed). Scans of 5-mm thickness were obtained from abdomen to pelvis before and after contrast administration. The median delay between cross-sectional imaging and PET was 3 weeks (range, 1-4 weeks).
Pathologic Documentation
Histologic and bacteriologic documentation was obtained in one patient by
means of several fineneedle biopsies and two biopsies with a Trucut biopsy
needle (Bard) under echographic control.
Reviewers and Procedures
Imaging was reviewed independently by a cross-sectional radiologist and a
nuclear medicine physician. All PET images were displayed both with and
without attenuation correction on a workstation (either a Sun Microsystems
Ultra 60 workstation or a Siemens e.soft Workstation). Hard-copy CT scans were
reviewed for all patients. In one patient, MRI was viewed on a work-station
(Leonardo, Siemens).
Reviewers assessed and characterized abnormalities in the area of the inguinal hernia repair.
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In patients 1 and 2, in whom serial PET studies were performed, the FDG foci around prostheses remained unchanged and the foci located in metastatic lesions showed either partial response (with pathologic documentation in patient 2) or complete metabolic response (patient 1) on PET.
Correlative contrast-enhanced CT scans showed an ill-defined hyperattenuating lesion in the abdominal wall in patient 1 and a nodular hyperattenuating lesion near the anterior wall of the bladder in patient 2. In patient 3, the CT scan was thought to show normal findings at the time of PET examination, but retrospective analysis revealed a slightly hyperattenuating area in the anterior abdominal wall.
In patient 1, MRI was performed and revealed a slightly hyperintense, ill-defined lesion in the abdominal wall on T1-weighted images after the administration of gadolinium. Histologic proof was obtained at biopsy under echographic control and revealed a macrophagic infiltrate around the fibers of the prosthesis. No neoplastic cells were found, and bacteriologic studies did not reveal any infectious processes.
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We emphasize a new pitfall of PET: FDG uptake around mesh prostheses due to persistent foreign body reaction several years after inguinal hernia repair.
These FDG foci around prostheses were considered to be suspicious in two patients, leading to the search for pathologic proof. In patient 2, although unusual because of its bilateral and almost symmetric pattern, the large area of markedly increased FDG uptake mimicked an atypical pelvic involvement. However, exploration of the pelvic cavity during surgical removal of the liver lesion did not reveal any suspicious lesion. Patient 1 had an unusual history of axillary nodes skip metastasis and she had been previously treated for an advanced carcinoma of the cervix; therefore, the FDG focus was also suspicious. Pathologic documentation was obtained for this patient and revealed an intense foreign body reaction. Bacteriologic studies did not show any infection.
These findings are consistent with the study of Rosch et al. [5], who analyzed inflammatory response to different types of mesh prostheses in a rat model. Their immunohistochemical analysis revealed that macrophages were the predominant cell type in the biomaterial-dependent chronic inflammatory infiltrate. Furthermore, Yamada et al. [6] studied the biodistribution of FDG in cases of chronic inflammation by means of a nonbacterial inflammation model and microautoradiography. They showed that the highest FDG uptake was located where neutrophils and macrophages were present.
Apart from a foreign body reaction, the two other diagnoses that could have been hypothesized are neoplastic involvement and infection. In patient 2, because the FDG foci around prostheses remained unchanged while the foci located in the liver metastasis showed partial response on pathologic documentation, it can be assumed that the foci were not due to a neoplastic process. In patient 3, the FDG foci were linear, bilateral, and almost symmetric. Thus, because the incidence of peritoneal carcinomatosis in lung cancer is low [7], these PET abnormalities were more likely caused by a benign disease. Moreover, no biologic or clinical argument for an active infectious disease was present, either at the time of the PET examination or during the subsequent follow-up. Therefore, the diagnosis of mesh infection was ruled out in these two patients. The evidence from the reports of Rosch et al. [5] and Yamada et al. [6] and the pathologic documentation obtained in patient 1 lead us to speculate that a similar inflammatory response is responsible for the FDG up-take around the mesh prostheses observed in patients 2 and 3.
Anatomic correlation with cross-sectional imaging is important because it can localize the FDG focus in the muscles of the abdominal wall and not in the tumor mass or node. Correlation can be achieved either by visual fusion (a recent CT study is used to anatomically localize PET findings, as in Figs. 1A, 1B, 1C, 1D, 1E, 2A, 2B, 2C, 2D, 3A, and 3B) or by hardware fusion in the case of PET/CT machines (Figs. 1F and 1G). Nevertheless, doubt may persist, especially in the presence of an unusual metastatic history (patient 1). Thus, precise recording of the patient's history is required in case of an unexpected pelvic focus to avoid false-positive results and potentially inappropriate treatment.
The FDG uptake was more intense in patient 2, who had PP mesh, than in patients 1 and 3, who had PTE mesh (SUVmax = 5.6 vs. 2.8 and 4.1, respectively). Once more, this finding is in accordance with the study of Rosch et al. [5], who found a more pronounced inflammatory reaction and cell turnover around PP mesh than around PTE mesh. Indeed, the most important factors influencing the biocompatibility of such materials are the intensity and the duration of the inflammatory process, which depend on the size of the pores of the prosthesis. Thus, prostheses such as those composed of PP with small pores that cause an intense inflammatory reaction have been progressively replaced by large-pore materials such as PTE. Research is ongoing in an attempt to find a mesh material that provides the best biocompatibility without the loss of mechanical resistance in the repaired zone. Because PET is an imaging technique that can give highly accurate quantitative information on a functional process [8], one might assume that FDG PET could be a powerful tool for the evaluation of the inflammatory process induced by new types of mesh prostheses.
In conclusion, one should keep in mind that inguinal mesh prostheses can mimic the FDG PET pattern of carcinomatosis of the anterior abdominal wall, even in the case of a remote repair of inguinal hernia. Such a source of false-positive results should be carefully avoided by reviewing the patient's history each time an unexpected pelvic focus is discovered on FDG PET. Moreover, FDG PET may be of use for the evaluation of the inflammatory response induced by future mesh prostheses.
Acknowledgments
We thank Andrew Lake and Mary Osborne for their assistance in reviewing our
manuscript.
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