Can aortic aneurysm growth rate be predicted in clinical practice using 18-fluorodeoxyglucose positron emission tomography (18-FDG PET)?

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1 Can aortic aneurysm growth rate be predicted in clinical practice using 18-fluorodeoxyglucose positron emission tomography (18-FDG PET)? Poster No.: C-1743 Congress: ECR 2011 Type: Authors: Keywords: DOI: Scientific Paper D. Stobo, F. W. Poon, A. Reid; Glasgow/UK Arteries / Aorta, Vascular, Cardiovascular system, PET, CT, Diagnostic procedure, Aneurysms, Inflammation /ecr2011/C-1743 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 11

2 Purpose Standard practice in the management of abdominal aortic aneurysms not meriting immediate intervention is to offer serial imaging follow-up. Treatment is recommended once aneurysm rupture risk becomes unacceptable, currently defined on structural grounds by diameter in excess of 5.4 cm [1]. The approach is effective but resource intensive and may induce unnecessary anxiety in many patients with aneurysms which will never cause morbidity. A diagnostic test performed once at the time of initial aneurysm diagnosis to accurately determine long-term rupture risk would be ideal. Patients with low risk lesions could then be reassured and avoid follow-up while those with higher risk aneurysms could have them treated at an earlier stage when endovascular or surgical repair may be more straightforward. Inflammatory processes are known to be involved in aneurysm pathophysiology [2] and we postulate that quantification of these may have a role in determining prognosis. Glucose-dependent metabolic activity can now be readily measured and localised in clinical practice using 18-fluorodeoxyglucose positron emission tomography with computed tomography (FDG PET-CT). The aim of this study is to seek evidence of any correlation between aneurysm FDG uptake at PET-CT and growth rate. Methods and Materials Reports for all whole body FDG PET-CT studies performed in this institution over a 26- month period containing the term 'aneurysm' were electronically extracted. Non-aortic aneurysms were excluded. The earliest and most recent CT scans available for each patient including the entire aneurysm were examined on a workstation (the non-contrast CT component of the PET-CT study was eligible for inclusion). Maximal AP diameters at both time points were measured by one investigator (DBS) and used to calculate longterm growth rate in millimetres per year. Patients with no CT imaging other than the PET- CT study available or with interval between CT scans of less than 60 days were excluded. Where multiple CT examinations separated by at least 60 days were available, the exercise was repeated to determine short-term growth rate from PET-CT to the next available CT scan. Page 2 of 11

3 PET-CT studies were examined on a dedicated workstation with a volume of interest defined to encompass the entire aneurysm and measure maximal standardised uptake value (SUV max ). Care was taken to avoid measuring SUV max from voxels outwith the aneurysm, for example from FDG within the adjacent ureters. Note was made of cases exhibiting clear focal mural uptake. Spearman's rank correlation co-efficients were calculated to seek evidence of a relationship between growth rate and SUV max. Linear regression models were derived from the data in cases where correlation was demonstrated. Additional data were gathered from patient records to allow subgroup analysis by age, gender, PET-CT indication, aneurysm size at time of PET-CT and follow-up interval. Results 153 reports containing the term 'aneurysm' were identified. 18 patients with non-aortic pathology and 51 with insufficient follow-up imaging were excluded. 84 patients were therefore included in the study. Mean long-term follow-up duration was 1.0 y with mean growth rate 2.2 mm/y. Average short-term follow-up interval was 0.4 y. Mean aneurysm size at the time of PET-CT scan was 42 mm with mean SUV max 2.5. No correlation was demonstrated between aneurysm SUV max and short-term growth rate (#=0.037;P=0.762), long-term growth (Figure 1) or aneurysm size (Figure 2). The following table shows the subgroup analysis results. Factor Subgroup No of Patients # P Age Gender PET-CT indication y y y y Male Female Lung cancer Other cancer Page 3 of 11

4 Aneurysm size Follow-up interval Focal mural uptake Non-cancer < 35 mm mm * mm * > 54 mm < 6 months months months > 24 months Present Absent No correlation was demonstrated within the age, gender, PET-CT indication, follow-up interval or focal mural uptake subgroups. Within the aneurysm size subgroups, significant positive correlation was demonstrated between growth rate and SUV max in patients with aneurysm diameter mm at PET-CT. Negative correlation was demonstrated in patients with aneurysms of diameter mm. Linear regression models gave wide prediction bands crossing zero for both groups exhibiting correlation (Figure 3) therefore the current data cannot be used to adequately predict aneurysm growth from SUV max. A typical pattern of diffuse aneurysmal FDG uptake is shown in Figure 4. Five patients were noted by the reviewing radiologist to have focal mural uptake as in Figure 5. Interestingly, four of these were among the seven most FDG avid aneurysms in the study; however the significance of this observation in such a small subgroup is uncertain. One patient was noted to have aneurysm rupture on the final CT study available, however SUV max at PET-CT had been unremarkable at 1.9 and there was no focal mural uptake in this case. Images for this section: Page 4 of 11

5 Fig. 1: SUVmax plotted against long-term aneurysm growth rate for all patients. No significant correlation was demonstrated (#=0.072;P=0.515). Fig. 2: SUVmax plotted against aneurysm size for all patients. No significant correlation was demonstrated (#=0.179;P=0.104). Page 5 of 11

6 Fig. 3: Linear regression models for subgroups exhibiting significant correlation with regression line, 95% confidence interval (CI) and 95% prediction interval (PI) shown. The upper image shows a tendency to slower growth with rising SUVmax in those with aneurysm diameters mm while the lower image shows a tendency to faster growth with rising SUVmax in those with aneurysm diameters mm. Page 6 of 11

7 Fig. 4: Sagittal fused FDG PET-CT image showing the typical appearance of an abdominal aortic aneurysm (arrow) in a 64-year-old man. Maximal AP diameter in this case was 37 mm with SUVmax 3.7. Page 7 of 11

8 Fig. 5: Transverse fused FDG PET-CT image showing a 64 mm AP diameter abdominal aortic aneurysm in an 80-year-old male patient. This exhibits focal mural uptake (arrow) with SUVmax 4.2. Page 8 of 11

9 Conclusion These data show some correlation between abdominal aortic aneurysm metabolic activity and growth rate, however this has only been demonstrated in the size range mm. Although the larger lesions within this group have higher SUV max values predicting more rapid growth as hypothesised, increased metabolic activity in aneurysms of diameter mm appears to predict slower growth. There are insufficient numbers in the current dataset to allow clinically useful prediction of aneurysm growth from SUV max. The data suggest that the relationship between metabolic activity as measured by FDG PET-CT and aneurysm growth is not straightforward. One possible explanation is that the inflammatory mechanisms at play vary as the aneurysm grows with resultant changes in glucose uptake. Also, most of the patients had PET-CT undertaken for assessment of malignancy and it is possible that systemic effects of their underlying disease in some way altered aneurysm pathobiology. The use of FDG PET-CT for vascular assessment has been described previously in relation to aortitis [3], aortic dissection [4] and atherosclerotic plaque [5]. One study established that abdominal aortic aneurysms exhibited greater FDG uptake than undilated aortas [6]. Another performed FDG PET-CT to examine mural uptake in aneurysms pre-operatively, however the factor that most altered management was the high incidence of unexpected malignant lesions demonstrated [7]. It has also been suggested elsewhere that symptomatic and inflammatory aneurysms are more likely to take up FDG [8]. Our study is the first of which we are aware which has correlated aneurysm metabolic activity against growth rate in an unselected population. Although raising new pathophysiological questions, our results give some encouragement that PET-CT may have a useful future role in the assessment of abdominal aortic aneurysms, however the relationship between metabolic activity and aneurysm behaviour would appear to be complex and should be further studied. Possible directions for future research include prospectively investigating a non-cancer population and considering whether novel non-fdg isotopes may have a role in better imaging aneurysm inflammation. References 1. Chaikof EL, Brewster DC, Dalman RL et al. (2009) The care of patients with an abdominal aortic aneurysm: The Society for Vascular Surgery practice guidelines. Journal of Vascular Surgery 80(8S):2S-49S. Page 9 of 11

10 2. Alexander JJ (2004) The Pathobiology of Aortic Aneurysms. Journal of Surgical Research 117: Schmidt WA, Blockmansb D (2005) Use of ultrasonography and positron emission tomography in the diagnosis and assessment of large-vessel vasculitis. Current Opinion in Rheumatology 17: Rudd HF (2010) The Role of 18F-FDG PET in Aortic Dissection. Journal of Nuclear Medicine 51(5): Sheikine Y, Akramc K (2010) FDG-PET imaging of atherosclerosis: Do we know what we see? Atherosclerosis 211: Truijers M, Kurvers HAJM, Bredie SJH et al. (2008) In Vivo Imaging of Abdominal Aortic Aneurysms. Journal of Endovascular Therapy 15: Truijers M, Pol JA, Kurvers H et al. (2009) Incidental finding of malignancy in patients preoperatively evaluated for aneurysm wall pathology using PET/ CT. Journal of Vascular Surgery 49(5): Reeps C, Essler M, Pelisek J et al. (2008) Increased 18Ffluorodeoxyglucose uptake in abdominal aortic aneurysms in positron emission/computed tomography in association with inflammation, aortic wall instability, and acute symptoms. Journal of Vascular Surgery 48(2): Personal Information Dr D B Stobo Dr F W Poon West of Scotland PET-CT Centre Gartnavel General Hospital 1053 Great Western Road Glasgow, G12 0YN Scotland, UK Dr A W Reid Radiology Department Glasgow Royal Infirmary 84 Castle Street Glasgow, G4 0SF Scotland, UK Page 10 of 11

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