retroperitoneal fibrosis ( rf ) is a chronic inflammatory disease , characterized by the presence of retroperitoneal inflammatory tissue , typically surrounding abdominal aorta and/or iliac arteries , and often leading to the involvement of adjacent structures , more frequently the ureters and inferior vena cava [ 17 ] . idiopathic rf ( even reported as ormond 's disease ) represents two thirds of all cases of rf . a true idiopathic form is present in any cases of rf in which no potential etiologic condition may be identified . the pathogenesis of the idiopathic rf appears today to be related to igg4 autoimmune mechanisms  hyper - igg4 disease  . otherwise , rf in the presence of aortic atheromatous inflammation ( atheromatous aortitis ) has been included , more than twenty years ago , among the secondary forms , since this condition appears to be elicited by antigen - acting oxidized - ldl and/or ceroid , that are present within the atheromatous plaque . etiology of other secondary rf refers to medications ( drug - induced rf ) , infections , traumas , surgery , radiation therapy , and malignancies [ 17 ] . clinical presentation of rf is usually characterized by constitutional symptoms and back or abdominal pain . because of the presence of increased serum inflammatory markers levels and positive autoantibodies , and the frequent association with autoimmune diseases ( such as riedel 's thyroiditis , sarcoidosis , inflammatory aneurysm , and autoimmune pancreatitis ) , some authors suspected that rf may result from autoimmune mechanisms   or , even , be considered as distinct autoimmune diseases [ 3 , 811 ] . a frequent complication of rf is unilateral or bilateral ureteral obstruction , secondary to fibrosis and stenosis of the ureters [ 1 , 4 , 5 ] . . the treatment of rf can be medical and/or surgical although a consensus has not been currently reached . the medical therapy is usually based on corticosteroids and immunosuppressive agents ( such as tamoxifen ) at different doses and various combinations on the base of disease activity and often leads to a substantial reduction of size of the retroperitoneal mass . the surgical procedure is required when severe obstructive complications are present [ 4 , 5 , 1315 ] . however , determining the presence of active inflammation , and thus the potential for response to medical therapy , is crucial for the management of patients with rf and can be quite difficult , especially on the basis of only clinical parameters . most commonly used imaging methods in this pathologic entity are computed tomography ( ct ) and magnetic resonance imaging ( mri ) , despite their difficulty in discriminating between active and fibrotic lesions and in differentiating between rf and other retroperitoneal diseases such as tumors [ 16 , 17 ] . fluorine-18-fluorodeoxyglucose positron emission tomography and positron emission tomography / computed tomography ( fdg - pet and pet / ct ) are noninvasive functional imaging techniques which have become established tools in oncology . recently , fdg - pet and pet / ct have been proposed as potentially useful tools for study inflammatory diseases , such as large - vessel vasculitis ( lvv ) and atherosclerosis [ 1923 ] . in fact , fdg is a glucose analogue that identifies areas of high - glucose metabolism . since inflammatory cells , such as neutrophils , lymphocytes , and macrophages , have an increased glucose metabolism , high fdg uptake is revealed in vessel walls and retroperitoneal masses if inflammation is present [ 1923 ] . several studies have evaluated the role of fdg - pet and pet / ct in diagnosing , assessing disease activity , and monitoring rf . a comprehensive computer literature search of pubmed / medline was conducted in order to find relevant published articles on the role of fdg - pet and pet / ct in patients with rf . we used a search algorithm based on a combination of the following terms : ( a )  retroperitoneal fibrosis  or no beginning date limit was used ; the search was updated until july 31st 2012 . studies investigating the role of fdg - pet and pet / ct in patients with rf were eligible for inclusion . review articles or editorials , articles not in the field of interest of this paper , case reports , and preclinical studies were excluded . eleven relevant articles about the role of fdg - pet and pet / ct in patients with rf were found and discussed ( table 1 ) . in 2005 , salvarani et al .   evaluated the presence and extent of large - vessel inflammation in 7 patients with rf or other chronic periaortitis using fdg - pet . these authors found fdg - pet findings suggestive for lvv in patients with rf , demonstrating increased vascular radiopharmaceutical uptake involving abdominal aorta and/or common iliac arteries , which in some cases was also extended to the thoracic aorta and/or its branches . in the same year , vaglio et al . used fdg - pet to evaluate the metabolic activity of residual masses at ct scan after immunosuppressive treatment in patients with rf . six out of seven patients evaluated had no abnormal fdg uptake in the residual mass , and the seventh showed only moderate vascular fdg uptake . during the followup , none of the patients showed clinical , serological , or radiological signs of disease relapse . these findings suggested that fdg - pet is a sensitive mean to evaluate metabolic activity of residual masses in rf patients in clinical remission with detectable masses at ct scans . in 2007 , nakajo et al . reviewed six male patients with rf and confirmed that fdg - pet and pet / ct are useful tools for evaluating disease activity and extent of rf although these methods failed to differentiate between malignancy and rf . in fact , the authors performed early and delayed dual - time point imaging to examine the change of fdg uptake in rf , finding an increase of maximum standardized uptake value ( suvmax ) at delayed images , which was similar to the pattern observed in malignancies . abnormal fdg uptake was also noted in the mediastinum and in the pancreas of some patients , and the diagnosis of mediastinal fibrosis and autoimmune pancreatitis were made , respectively . in 2008 , reported fdg - pet - positive findings in three patients presenting with active rf . in one patient , repeat fdg - pet was performed following immunosuppressive therapy , with complete resolution of the retroperitoneal fdg uptake . the authors confirmed that fdg - pet may be a useful adjunct to anatomic imaging and serum inflammatory markers in distinguishing between active and inactive disease and in assessing the severity of inflammation in rf . this method may also be used in followup to assess therapeutic response if ct findings are unclear . in 2010 , jansen et al . evaluated whether fdg - pet was useful in the diagnostic and therapeutic evaluation of patients with rf treated with tamoxifen . at baseline , patients with a positive fdg - pet scan had a higher serum inflammatory markers level ( p = 0.02 ) and a larger mass size at ct scan ( p = 0.01 ) compared with patients with a negative fdg - pet scan . fdg uptake decreased following treatment , in agreement with inflammatory markers reduction ( p < 0.001 ) , but not with ct - documented mass regression . these findings confirmed that fdg - pet may be useful to evaluate the severity and the extent of rf . furthermore , these authors demonstrated that fdg - pet may be a valuable tool in assessing disease activity during or after treatment in patients with normal inflammatory marker levels and stable residual mass on repeated ct scans . nevertheless , short - term fdg - pet followup did not accurately predict success of tamoxifen treatment . in the same year , piccoli et al .   supported the use of fdg - pet / ct not only for the diagnostic workup but also in the followup of patients with rf in order to tailor medical and surgical approaches . in particular , evaluating seven rf patients with fdg - pet / ct during the diagnostic workup and during followup , the authors found that this method may be useful to identify the best time for a safe removal of the renal stents ( when no active disease is revealed by fdg - pet ) . fdg - pet / ct has been also useful to schedule for tapering of immunosuppressive therapy . recently , pipitone et al .   presented four cases of rf with different etiology ( one idiopathic rf and three secondary rf ) and evaluated the role of fdg - pet in the workup of these different conditions . the authors highlighted that fdg - pet should be part of the investigations used to determine disease extent , activity , and response to therapy both in patients with idiopathic and secondary rf . ha et al .   recently investigated the clinical characteristics , laboratory findings , radiologic findings , treatment , and outcome in 27 korean patients with rf . only 6/27 patients have performed fdg - pet scan , and five out of six patients showed fdg - pet - positive findings . the authors suggested that fdg - pet may be a useful tool to detect active rf . assessed the feasibility and usefulness of fdg - pet / ct in 25 rf patients studied in two italian nuclear medicine centres ; 18/25 patients underwent pet / ct as initial evaluation study , 3/25 patients during followup , 3/25 during steroid therapy in absence of initial evaluation , and 1/25 to reevaluate the disease suspecting a recurrence . fdg - pet / ct was positive in 18/25 patients and negative in 7/25 . among the 10 patients who underwent a second study after steroid therapy , 6/10 showed complete metabolic response , 3/10 partial response , and one patient showed no significant fdg uptake reduction . the authors concluded that fdg - pet / ct is a suitable and useful tool in staging patients which are suspected to be affected by rf ; moreover it could be very useful in treatment response evaluation . evaluated the added value of contrast - enhanced ct ( ce - ct ) combined with fdg - pet as a one - stop diagnostic procedure for the assessment and followup of rf in 10 patients . these authors found that , unlike biologic and ct parameters , fdg uptake , was the most relevant parameter to measure severity of inflammation in rf . ce - ct allowed a better delineation of periaortitis and its extension to adjacent structures . the authors concluded that accurate determination of inflammation level in rf is significantly improved when fdg - pet / ct and ce - ct are performed in the same study and used for better delineation of areas of residual inflammation . lastly , moroni et al .   in their recent prospective study on 22 patients with rf reported that fdg - pet / ct accurately discriminated active from inactive disease in about 94% of cases . a significant correlation ( p < 0.01 ) was found among fdg uptake , and serum levels of inflammatory markers and among fdg uptake and analysis of contrast enhancement at ct images in the retroperitoneal mass . the authors concluded that fdg pet / ct may be considered an alternative imaging method for the assessment of different stages of rf . the diagnosis of rf is based on clinical and radiological findings , and there are no established pet criteria for the diagnosis of this pathological entity . many authors used a visual analysis of pet images for the evaluation of metabolic activity of rf . the visual analysis was usually performed using a four - point graded scale , based on the vessel - to - liver fdg uptake ratio ( 0 : no uptake , 1 : uptake less than that of the liver , 2 : uptake equal to that of the liver , and 3 : uptake greater than that of the liver ) ; a visual score above 1 was usually considered a positive criteria for active rf . however , standardization of fdg - pet studies by using semiquantitative analysis in order to achieve interchangeability in multicenter trials was recently recommended . six out of 11 articles included in this review have used a semiquantitative analysis based on the calculation of the suvmax [ 26 , 27 , 29 , 3234 ] . the semiquantitative assessment of fdg uptake in the rf lesions is also useful for treatment response evaluation . the added value of the hybrid technique pet / ct versus pet alone for rf diagnosis needs to be confirmed by further studies . in fact , only four articles evaluated the usefulness of fdg - pet / ct in patients with rf [ 29 , 3234 ] . hybrid pet / ct allows the evaluation of disease activity and retroperitoneal mass size , as well as the localization of the inflammatory process in the same session . the main limitation of ct scan is the lack of discrimination between active and residual fibrotic tissue ; nevertheless , ct may be useful in differentiating rf from atherosclerosis or in detecting morphological vascular alterations such as stenoses or aneurysms . furthermore , a recent study suggested that the combination of fdg - pet and ce - ct performed in a single procedure would simplify the patient management . this strategy could be more cost - effective and more accurate than separate studies acquired at different points in time although randomized prospective studies are required to validate this hypothesis . overall , fdg - pet and pet / ct seem to be useful methods both in evaluating disease extent and severity at the time of diagnosis and in assessing disease activity during or after treatment , mainly in patients with normal inflammatory marker levels and stable residual mass on repeated ct scans . further prospective and multicentric studies with a larger patient population and cost - effectiveness studies are needed to correctly define the role of fdg - pet and pet / ct in patients with rf . from this overview on the role of fdg - pet and pet / ct in patients with rf , we are led to conclude that these functional imaging techniques seem to be useful both in the diagnosis and in evaluating the treatment response . further studies are needed to substantiate the role of fdg - pet and pet / ct in this setting .
background and purpose . 
 several studies have evaluated the role of fluorine-18-fluorodeoxyglucose positron emission tomography and positron emission tomography / computed tomography ( fdg - pet and pet / ct ) in diagnosing and assessing disease activity in patients with retroperitoneal fibrosis ( rf ) . 
 the aim of our paper is to perform a literature review on this topic . methods . scientific articles that evaluated the usefulness of fdg - pet and pet / ct in patients with rf were searched and discussed . results . 
 eleven studies were found , and the main findings of these articles were described . 
 conclusion . 
 fdg - pet and pet / ct are useful functional imaging methods for assessing patients with rf both in the diagnosis and in the treatment response evaluation . 
 moreover , further studies are needed to substantiate the role of fdg - pet and pet / ct in patients with rf .