MESENTERIC PANNICULITIS PART 2: PREVALENCE AND NATURAL COURSE: MDCT PROSPECTIVE STUDY

underdiagnosed uncommon benign inflammatory condition of unknown etiology that involves the adipose tissue of the mesentery and for which an extremely varied terminology has been used, causing considerable confusion. Now it can be evaluated as a single disease with two pathological subgroups: MP, representing the very large major subgroup where inflammation and fat necrosis predominate and retractile mesenteritis, rarely found, where fibrosis and retraction predominate. In histo-pathological terms the preferred terminology is sclerosing mesenteritis. mean age 65 +/15, 25 years) and 333 males (20-89 years, mean age 63,66 +/14,65 years) was constituted. The patients were scanned with two different 64-row MDCT machines (Lightspeed Ultra and Lightspeed VCT, General Electric, Wisconsin). The collimation was 64 x 0.65 mm with a pitch comprised between 0.516 and 1.375. The 1.25/0.65 mm or 1.25/1.25 axial series were used for axial cineview analysis and dynamic MPR on a Workstation (Advantage Workstation 4.3 and 4.4, General Electric, Wisconsin). On the basis of their clinical records the patients were classified as “neoplastic” (diagnosis and /or follow-up) (n = 165 or 27%) and “non neoplastic” patients (n = 448 or 73%). The “neoplastic” group was composed of 107 males (32% of males) and 58 females (20,7% of females) and the “non neoplastic” group was composed of 226 males The goal of this study was to reestimate the prevalence of MP in general population through a large prospective study, to compare the results with those of the literature and to evaluate the natural course of the condition.

A positive CT diagnosis of MP required the presence of at least 3 typical signs comprising: a welldefined "mass effect" on neighbouring structures (sign 1) constituted by mesenteric fat tissue of inhomogeneous higher attenuation than adjacent abdominal fat (sign 2) containing small soft tissue nodes -lymphadenopathies -(sign 3) which may be surrounded, like mesenteric vessels, by an hypoattenuated fatty halo (sign 4). The last sign (sign 5) was constituted by the presence of a hyperattenuating pseudocapsule surrounding the mesenteric fat tissue. 242 JBR-BTR, 2011, 94 (5) Fig. 1. -A: volume rendering view, B: sagittal and coronal oblique MPR views and C: selective volume rendering view of nodes obtained in a 57 year-old patient presenting with a sharp mesenteric panniculitis. The CT was performed to study the renal arteries in a context of hypertension. All typical sings are present and the typical nodes, surrounded by a halo (white arrows) have a ramified and irregular shape. The results are expressed in percentage.
A case was being considered positive when at least 3 signs were simultaneously present. Thus the minimal positive score was 3 (score 1 for at least 3 signs) and the maximal score was 15 (score 3 for all five signs). Globally MP was arbitrary characterised as minimal (score </= 5), moderate (score 5 to 9) or marked (score 10 to 15).
The medical records of the "positive" patients were systematically reviewed and all their previous and subsequent abdominal CT examinations were retrieved from our picture archiving system (PACS) and secondarily reinterpreted by the same radiologist to follow the evolution of MP.
Signs 1 to 3 were present in all cases but the "halo sign" (sign 4) (figures 1-5) and a "pseudocapsule" (sign 5), which are considered in the literature as the most typical and specific CT signs of MP were absent respectively in 6 (12,5%) and 27 (56%) of the 48 "positive" cases.
A CT follow-up was available in 25 (52%) of the 48 positive patients of the group 1. The follow-up period was varying from 9 to 104 months

Discussion
Mesenteric Panniculitis (MP) is a benign fibrosing and inflammatory condition that involves the adipose tissue of the mesentery (1-5) and this uncommon disorder of unknown etiology is likely underdiagnosed (6). In histo-pathological terms, the preferred terminology is Sclerosing Mesenteritis (1,3,(7)(8)(9). It represents the more accurate term because of Classically on CT, MP appears as a mass of increased-attenuation mesenteric fat containing small softtissue nodes, with a maximal transverse diameter directed toward the left abdomen consistent with the orientation of the jejunal mesentery. 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. Two CT findings are considered more specific for the diagnosis of MP as they have not been reported in the presence of some degree of fibrosis as a common denominator (4,9).
Most studies have indicated that the disease is more common in men, more frequent between the 6 th and 7 th decades of life, with a male/female ratio of 2-3:1, and several reports have indicated it to be more common in Caucasian men (1-2, 4, 6, 10-11). In our series the male predominance appeared less evident with a male/ female ration of 1,28:1. Pediatric cases are exceptional, probably because children have less mesenteric fat when compared to adults. 244 JBR-BTR, 2011, 94 (5)
The overall prevalence of MP in abdominal CT examinations has previously been estimated at approximately 0.6%, commonly appearing as an incidental finding, mostly in middle or late adulthood (1,4,8,19). It is our opinion that this prevalence has probably been extremely underestimated when compared with the results of our prospective study which shows a mean prevalence of 7,83% when at least 3 CT signs are found (group 1) and still of 3,42% when all five CT signs are considered (group 2) including the "halo sign" and the presence of a "pseudocapsule" . The reason probably resides in the major technological evolution experienced in CT imaging during the last decade. When compared with the CT parameters of the study of Daskalogiannaki (19) (where 10-mmthick continuous axial views are analysed) there is no doubt that the dynamic multiplanar examination of hundreds of millimetric views allows a much higher sensitive diagnosis of sometimes subtle mesenteric abnormalities.
In the large but single series of MP of Daskalogiannaki was related to malignancy in 69% of patients (19). These malignancies included mostly urogenital or gastrointestinal adeno-or do they only represents the unrelated features which lead patients to imaging studies? As suggested by our results the relatively high prevalence of MP probably explains a spontaneous association with numerous and probably unrelated clinical situations.
The course of MP is favorable in most cases, because the disease usually progresses slowly and subsides spontaneously. In about 20% of patients -but it is not sure that this high percentage is not overestimated -MP is associated with significant morbidity and a chronic debilitating course (6). Nevertheless Daskalogiannaki (19) found a great stability of the CT findings of MP in 20/21 patients in which follow-up abdominal CT examinations were available within an interval of 5 months to 3 years. Only one patient showed a slight increase of the fatty mass. Our results are exactly the same. In the 25 patients in which a follow up was availablewithin a mean interval of 36 +/-21 months -21 (84%) showed an extremely stable appearance of their MP (Fig. 2-3). A discrete increase in score was observed in only 2 patients and a subtle regression was observed in 2 patients.

Conclusions
The prevalence of MP was much higher in our study than previously reported in the literature and the reason probably resides in the major technological evolution experienced in CT imaging during the last decade. This high prevalence probably explains the spontaneous association with the numerous and probably unrelated clinical situations found in the literature. Finally the vast majority of cases are considered as idiopathic, benign an asymptomatic. Except a discrete higher prevalence found in patients presenting with bladder and/or prostatic neoplasms and with lymphoma in group 1 the general prevalence of MP in our study doesn't significantly differs in the "neoplastic" and "non neoplasic" groups of patients. We conclude that the value of MP in term of predictivity of an associated neoplasm is probably not relevant.
Finally PET/CT is proved useful to correctly exclude mesenteric tumoral involvement in patients presenting with typical MP and follow-up studies show a great stability of the CT findings of MP in about 85% of cases.
carcinoma or lymphoma but also breast and lung cancer and melanoma. In our study MP is found heaving not only a much greater prevalence than previously reported but moreover this prevalence doesn't not significantly differs in the two -"neoplastic" and "non neoplastic"groups. MP thus probably represents an idiopathic phenomenon whose value in term of predictivity of associated neoplasm is probably not relevant. A discrete higher prevalence of MP was only found in patients presenting with vesical and prostatic neoplasms and with lymphoma in group 1.
Daskalogiannaki et al. (19) also noted that an association between MP and lymphoma had been suggested in the previous literature. In our series, MP was effectively associated with lymphoma in a higher percentage (15,4% of patients of the group 1) than in the general population.
A wide variety of clinical manifestations of MP have been described which are very inconstant and nonspecific (1, 3) but as many as half of the cases reported MP has been an incidental finding in asymptomatically presenting patients (3,16). It's also our opinion that this proportion of asymptomatic presentations is extremely much higher.
Such a wide variety of potential clinical manifestations would first mean that a large number of illnesses must be considered for differential diagnosis. For us the real unanswered question is: are all these various symptoms really related to MP Table II. -Prevalence of Mesenteric panniculitis in the different groups of patients for the two positive groups: group 1 characterized by at least 3 "positive" CT signs and group 2 in which all 5 typical CT signs are present including the "halo sign" and the presence of a "pseudocapsule" .