British Journal of Radiology (2006) 79, 37-43
© 2006 British Institute of Radiology
doi: 10.1259/bjr/29320216
Mesenteric panniculitis in oncologic patients: PET-CT findings
R Zissin, MD1,2,3,
U Metser, MD1,3,
D Hain, MD4 and
E Even-Sapir, MD, PhD1,3
1 Department of Nuclear Medicine, Tel-Aviv Sourasky Medical Center, and the 2 Department of Diagnostic Imaging, Sapir Medical Center, Kfar Saba, both affiliated to the 3 Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, and 4 Nuclear Medicine Institute, Shaare Zedek Medical Center, Jerusalem, Israel
Correspondence: Einat Even-Sapir, Department of Nuclear Medicine, Tel-Aviv Sourasky Medical Center, 6 Weizman Street, Tel-Aviv, 64239 Israel
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Abstract
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The aim of this study is to assess the role of PET/CT in differentiating between mesenteric panniculitis (MP) and co-existing MP and mesenteric tumoural involvement. A total of 33 PET/CT examinations, of 19 oncologic patients (16 men and three women with ages ranging from 48 years to 83 years) with findings of MP on the CT part of the study were retrospectively reviewed. The FDG uptake in mesenteric nodules was recorded. The final diagnosis of malignant mesenteric involvement was based on clinical and imaging follow-up. Based on the FDG uptake in mesenteric nodules, patients were categorized as group A: increased mesenteric uptake (n=8) and group B: no mesenteric uptake (n=11). In seven of the eight patients in group A, a co-existing MP and mesenteric tumour involvement was found: one patient had a recurrent cervical carcinoma and the other six patients had lymphoma. In four of these six patients, the positive PET findings disappeared on follow-up PET/CT with complete remission while the CT findings of the MP remained unchanged. In the other two, the PET findings progressed along with clinical deterioration. In the last patient of group A, with rectal carcinoma without evidence of recurrence, the mesenteric FDG uptake was a false positive uptake. In all 11 patients with CT findings of MP and negative PET, no malignant involvement of the mesentery was diagnosed. To conclude, a negative PET has a high diagnostic accuracy in excluding tumoural mesenteric involvement while increased uptake suggests the co-existing of mesenteric deposits, particularly in patients with lymphoma.
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Introduction
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Mesenteric panniculitis (MP), also entitled liposclerotic mesenteritis, mesenteric lipodystrophy, mesenteric lipomatosis and lipogranuloma of the mesentery, is a benign condition characterized by non-specific inflammation involving the adipose tissue of the mesentery, with acute inflammatory changes and fat necrosis being the predominant histological findings. In its chronic phase when fibrosis is dominant, the disease is known as retractile mesenteritis [1–4]. Sclerosing mesenteritis seems the most appropriate diagnostic term of this entity, characterized by a spectrum of histological findings [4]. The specific aetiology of the disease is unknown, although various causes have been suggested, including infection, trauma or ischaemia of the mesentery. The disease has been related to other pathological processes such as vasculitis, granulomatous disease, pancreatitis and malignancy [2]. Its prevalence in abdominal CT examinations is approximately 0.6%, commonly appearing as an incidental finding, mostly in middle or late adulthood [5]. An association between MP and pre-existing malignancy has been reported [5, 6].
The CT features of MP are well recognized and may suggest the diagnosis, but they are non-specific and can appear in other conditions such as mesenteric oedema, granulomatous diseases, primary or secondary abdominal neoplasms and lymphoma [1]. In cases of MP and known intra-abdominal malignancies, differentiating MP from tumoural involvement of mesenteric lymph nodes (LNs) is of crucial importance.
18F-Fluorodeoxyglucose (FDG)/PET imaging has been introduced in addition to conventional cross-sectional imaging methods in the routine practice of oncologic patients. Recently, hybrid systems composed of PET and CT have been introduced and its use is increasing steadily [7]. PET and CT are performed at the same clinical setting resulting with generation of fused PET/CT images, which provides both functional and anatomical data. The potential role of PET/CT in differentiating benign MP from tumoural mesenteric involvement is the topic of the current study. We have reviewed 19 patients with a history of known malignancy, who had incidental MP on the CT component of the PET/CT study, and report the PET/CT features of MP in this oncologic population.
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Material and methods
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The clinical data and PET/CT findings of 19 consecutive patients with MP incidentally diagnosed on the CT part of the study were retrospectively reviewed. The patient group consisted of 16 men and 3 women with ages ranging from 48 years to 83 years (mean age 62±11 years). Five of the 19 patients underwent a PET/CT study for staging and 14 for suspected recurrence or for monitoring response to treatment. Known malignancies included lymphoma (n=10), colorectal cancer (n=5), melanoma (n=2) and lung and cervical carcinomas, one patient each. A total of 33 PET/CT examinations were performed and reviewed in these 19 patients as 11 of them had one to three follow-up (F/U) studies.
The patients fasted at least 4 h prior to the intravenous (IV) injection of 370–666 MBq (10–18 mCi) FDG. Iodinated oral contrast material was administered prior to FDG injection. Glucose levels had been checked prior to the injection of FDG. A PET/CT study was performed only when blood glucose levels were bellow 8.32 mmo l–1. Scanning from the base of the skull through the mid-thigh was performed using the Discovery LS PET/CT system (GE Medical Systems, Milwaukee, WI). Low-dose CT acquisition was performed first with 140 kV, 80 mA, 0.8 s per CT rotation, a pitch of 6 and a table speed of 22.5 mm s–1, without any specific breath-holding instructions. A PET emission scan was carried out immediately following acquisition of the CT, without changing the patient's position. From 5 to 8 bed positions were performed with an acquisition time of 5 min for each one. CT data were used for attenuation correction. Images were generated and interpreted on work station (Xeleris Elgems, Haifa, Israel) equipped with fusion software that enables the display of PET, CT and fused PET/CT images.
The CT criteria for the diagnosis of MP included a well-defined, inhomogeneous fatty mass with higher attenuation than the normal retroperitoneal fat, occasionally with preserved perivascular fat, that contained small nodules and surrounded by a tumoural pseudocapsule (Figure 1
) [5]. When interpreting the PET/CT, the uptake of FDG in the mesenteric nodules was reported. Standardized uptake value (SUV) was measured for any focal increased uptake within the CT mesenteric abnormalities. The latter semiquantitative parameter was automatically obtained on the patient's final report and was calculated as the ratio of activity in tissue per millilitre to the activity in the injected dose per patient body weight. The final diagnosis of the mesenteric pathology was based on clinical and imaging (PET/CT and/or diagnostic CT) F/U: co-existing MP and malignancy was concluded when improvement or disappearance of the mesenteric abnormalities were seen on F/U imaging together with clinical evidence of a favourable response to therapy, or when mesenteric abnormalities progression was seen on F/U imaging along with clinical and imaging evidence of disease progression. The MP was regarded as benign if the mesenteric findings remained stable in a patient who was clinically disease-free or if the findings remained stable on F/U imaging while other sites of disease resolved in response to therapy and the patient was clinically considered in complete remission.
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Figure 1. CT findings of mesenteric panniculitis (MP). Non-enhanced abdominal CT at the mid-abdomen shows a well-defined, inhomogeneous fatty lesion, with higher attenuation than the normal retroperitoneal fat, confined by a highly-attenuated stripe representing a tumoural pseudocapsule (thick arrows), with an engorged mesenteric vessel and scattered discrete nodules of soft-tissue density, some of which are engulfed by a hypodense fatty halo (thin arrow).
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Results
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Based on the FDG uptake within the MP, the study patients were divided into two groups: group A consisting of eight patients with MP and focal increased FDG uptake within mesenteric nodules and group B consisting of 11 patients with MP without increased FDG uptake.
Group A: FDG uptake within MP
FDG uptake was demonstrated within CT mesenteric abnormalities, indistinguishable from a benign MP, in eight patients. The clinical and imaging findings of these patients are summarized in Table 1. A conclusion of malignant mesenteric involvement was made in seven of these patients, one with a metastatic cervical cancer and six with non-Hodgkin's lymphoma (NHL): In four lymphoma patients both the increased FDG uptake and the nodules themselves resolved following chemotherapy, while other CT findings of MP remained unchanged on F/U PET/CT (Figure 2). In the other two, clinical F/U and repeat PET/CT were consistent with tumour progression (Figure 3). In the case of metastatic cervical carcinoma new mesenteric PET findings appeared within known MP, seen previously on two PET/CT studies, along with clinical evidence of tumour recurrence. In coexisting MP and mesenteric tumoural involvement, the MP changes, seen on the CT part of the examination, which were not associated with increased FDG uptake, remained unchanged on F/U PET/CT studies.
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Table 1. Clinical, PET/CT and F/U of eight patients with CT findings of mesenteric panniculitis (MP) and increased FDG uptake within mesenteric nodules
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Figure 2. A 50-year-old man with follicular lymphoma: mesenteric panniculitis (MP) with meseneteric tumoural involvement before and after a favourable response to chemotherapy. (a) A fused PET/CT image shows increased 18F-FDG uptake in an 8 mm nodule (dashed arrow) in the background of MP (arrows). The latter appears as a mesenteric mass of inhomogeneous fatty tissue containing scattered soft-tissue nodules which are not 18FDG-avid. (b) PET/CT images at diagnosis (top images) and following chemotherapy (lower images) show regression in size of the nodule and disappearance of 18FDG uptake (arrows). No change is seen in the other findings of the MP.
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Figure 3. A 76-year-old man with non-Hodgkin's lymphoma (NHL): mesenteric panniculitis (MP) with meseneteric tumoural involvement with disease progression. PET/CT images at diagnosis (top images) and 4 months later (lower images): (a) At the mid-abdomen typical findings of MP with no 18F-FDG uptake are seen, stable on F/U. (b) More caudally, increased 18F-FDG uptake is detected at diagnosis within a 1.2 cm x 1.5 cm mesenteric nodule (SUV – 13.2) (arrows). On F/U the hypermetabolic node, most likely involved with lymphoma, enlarged to 3.8 cm x 4 cm with increasing 18F-FDG uptake (SUV – 18.8) (arrows).
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In the remaining patient with rectal cancer and focal mesenteric FDG uptake (SUV – 3.5) the positive PET was regarded as a false positive study as there was no clinical or imaging evidence of active tumour, together with stability of the MP findings on F/U diagnostic CT studies during a long disease-free period of 28 months.
Group B: MP without increased FDG uptake
In 11 patients no FDG uptake was seen within typical features of MP. The mesenteric soft-tissue nodes ranged between immeasurable, numerous small nodules, to discrete nodes measuring up to 0.9 cm in the short axis and 1.9 cm in the long axis (Figure 4). In all these patients the mesenteric abnormalities seen on the CT part of the study were stable on imaging F/U of a mean of 10.5 months (range: 5–30 months) and we therefore believe that the mesenteric findings were benign.
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Figure 4. A 78-year-old man with diffuse B cell lymphoma and co-existing mesenteric panniculitis (MP). A fused PET/CT image shows typical findings of MP, including inhomogeneous fatty density mass with well-defined nodules of soft-tissue density, confined by a pseudocapsule (arrows), without 18F-FDG uptake. The findings remain stable on a F/U PET/CT, 5 months later, with no clinical evidence of active lymphoma.
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Discussion
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MP is a non-neoplastic inflammatory process of unknown aetiology, affecting the small bowel mesentery. It was rarely diagnosed before the era of ultrasound and CT, but currently it is not uncommonly encountered, often as an incidental imaging finding. Male predominance, as was found in our group, has been previously reported [1–3] though a slight female predominance has been reported ina single publication [5]. Most cases of MP are asymptomatic and are incidentally detected on abdominal CT performed for unrelated conditions [5]. On CT, MP appears as a mass of increased-attenuation mesenteric fat containing small soft-tissue nodes, with a maximal transverse diameter directed toward the left abdomen consistent with the orientation of the jejunal mesentery. The infiltrated fat typically engulfs the mesenteric vessels and displaces adjacent bowel loops without invading them [1, 2, 5]. Hypodense, cystic-like areas and calcifications due to fat necrosis are infrequently seen within this mass [1]. Increased fatty attenuation and small mesenteric nodules, also termed "misty mesentery" may, however, be seen in any pathological process infiltrating the mesentery, such as inflammation, oedema, haemorrhage or metastases [8]. Two CT findings are considered more specific for the diagnosis of MP as they have not been reported in other mesenteric diseases: the presence of tumoural pseudocapsule (found in up to 60% of MP cases) and the "fat ring" sign of hypodense fatty halo surrounding mesenteric nodules and vessels (seen in up to 75% of cases) [1, 2, 4]. Daskalogianki et al have reported the co-existence of MP and various neoplastic diseases, especially lymphoma and gastrointestinal and urogenital adenocarcinomas, in up to 69% of patients with MP [5]. 10 of the 19 study patients with CT findings of MP had lymphoma as the underlying malignancy. Co-existing MP with malignant mesenteric involvement was found in six of the lymphoma patient (60%), representing 85.7% of the 7 study patients with malignant mesenteric involvement. In oncologic patients, therefore, the small soft-tissue mesenteric nodules typically seen within the infiltrated mesenteric fat of MP may be misdiagnosed as metastatic implants. On the other hand, metastatic deposits within a pre-existing MP can also be present, as was described in a single case report on a patient with uterine papillary serous adenocarcinoma in whom multiple nodular metastases were detected on CT within typical MP findings [9].
The results of our study emphasise the potential role of PET/CT in differentiating benign MP and MP with mesenteric tumoural involvement. Fused PET/CT images provide both metabolic and anatomic information with a high accuracy. On CT, lymph node pathology is based on size criteria alone. Enlarged lymph nodes may be reactive while normal-sized nodes may contain early metastatic deposits, which can be reliably detected by the functional (PET) part of the study.
The majority of our patients, including those with a malignant mesenteric involvement, had only subtle CT findings and the differentiation between benign and malignant causes could not be made with confidence based on the CT alone. Our results suggest a potential role for integrated PET/CT in the assessment of MP detected on CT in oncologic patients. PET/CT study can be used to correctly exclude mesenteric tumoural involvement when no FDG uptake is seen within typical CT features of MP. Alternatively, in a patient with an oncologic history, the demonstration of FDG uptake, even in small-sized nodules within characteristic CT findings of MP, is highly suggestive of neoplastic involvement of the mesentery. In PET/CTs of co-existing MP and mesenteric metastatic deposits, the increased FDG uptake was detected in nodules smaller than the benign nodules of the MP that had no increased uptake. The increased FDG uptake of these malignant mesenteric deposits resolved on a F/U study following a favourable response to treatment while the findings of the benign MP remained unchanged. Increased FDG uptake is, however, not tumour-specific as FDG uptake may be seen in benign inflammatory conditions [10], as was the case in one of our patients in whom a slightly increased FDG uptake was detected within MP findings without evidence of malignancy on a long-term F/U of 28 months. As the most consistent histological finding of MP is the presence of an inflammatory infiltrate, it may explain the uptake in that case [4]. We have found a single case report in the English literature regarding the FDG/PET in a patient with sclerosing mesenteritis; a large, speculated, soft-tissue mesenteric mass showed peripheral increased FDG uptake, probably representing the peripheral high metabolic inflammation and inactive central area of fibrosis [11].
The limitations of our study are the relatively small number of patients and the lack of a pathological proof for all lesions. However, as often happens in tumour imaging, not all detected lesions have histological diagnosis and their nature is sometimes based on clinical and imaging F/U. Validation of our findings in larger patient groups is warranted.
To conclude, if MP is suspected on the CT part of the PET/CT study, special attention should be paid to the 18F-FDG-avidity of the findings. A negative PET has high diagnostic accuracy in excluding tumoural mesenteric involvement while increased uptake may suggest the co-existing of metastatic deposits, particularly in patients with lymphoma.
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Acknowledgments
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The authors wish to thank Mrs Limor Zuriel, MSc, for her assistance in the preparation of the manuscript.
Received for publication March 17, 2005.
Revision received May 3, 2005.
Accepted for publication June 1, 2005.
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Abstract
Introduction
