Middelheim Hospital, Da Vinci PET Center (Sint Augustinus), Antwerp 2020, Belgium
Correspondence to current address: Department of Nuclear Medicine, Jules Bordet Cancer Institutue, rue Héger-Bordet I, 1000 Brussels, Belgium. E-mail: tarikbelhocine{at}yahoo.fr
SIR, I read with great interest the recent article of Brodmann et al. [1]. It is an interesting contribution that assesses the exact role of 18FDG PET in the management of patients presenting with giant cell arteritis (GCA). The most important finding was the inability of 18FDG PET to detect a metabolic signal at the level of temporal arteritis, while localizing 100% of extratemporal GCA involvement. Based on personal experience in evaluating GCA with 18FDG PET, published recently [2], I would like to respond to the article of Brodmann et al.
The fact that 18FDG PET is not suitable for the diagnosis of temporal arteritis is not really surprising. In the above-mentioned paper, we emphasized the limited spatial resolution of commercially available PET scanners. This is a technical limitation that prevents the detection of any metabolic signal within anatomical structures smaller than 45 mm in size. In addition, the physiological uptake of 18FDG by the grey matter of the brain certainly obscures the slight uptake within the temporal arteries. As a consequence, 18FDG PET should not be used for the diagnosis of temporal arteritis.
Although the authors rightly conclude that metabolic imaging cannot replace invasive biopsy, we note that only eight of out 22 patients had a temporal biopsy. Therefore, a definitive correlation with a pathological gold standard was provided in only 36% of cases.
Currently, the diagnosis of GCA and/or temporal arteritis is based on the ACR classification. Not only is biopsy-proven GCA just one among five of the ACR criteria (three out of five are needed for diagnosis), but also it is not unusual to have false-negative biopsies in GCA patients because their arterial disease is focal. Indeed, in the present series, 77% of patients (17/22) had GCA affecting the large vessels with concomitant involvement of the temporal arteries.
The interesting finding in this study was that 50% of patients presented with extratemporal GCA (11/22). In all of these cases, 18FDG PET was positive and completely agreed with colour-coded duplex sonography. In particular, five of out 22 (23%) 18FDG-avid patients had no evidence of temporal involvement.
In the study group, the diagnosis of GCA was initially made on clinical criteria, thereby, generating a bias with regard to the appropriate role of the imaging modalities. In a large population of patients who do not necessarily meet the ACR criteria (including the temporal biopsy), a positive whole-body 18FDG PET may provide valuable metabolic arguments for the diagnosis of GCA.
In several series assessing the value of 18FDG PET in large-vessel vasculitides, including a recent review co-authored with Daniël Blockmans, 18FDG PET appears to be particularly useful in cases of GCA with atypical presentation (isolated inflammatory syndromes, fever of unknown origin or negative temporal biopsy). Such circumstances are often the cause of repeated, time-consuming and ultimately fruitless radiological studies. Thus, whole-body metabolic imaging may efficiently guide the diagnosis when extratemporal localizations are detected [35].
In this series, the sensitivity of colour-coded duplex sonography (CDD) in detecting temporal involvement based on the halo sign (17/22 = 77%) is rather greater than that reported in the recent literature. Salvani et al. reported a sensitivity of only 40% in a large series of 86 patients [6], and Schmid et al. obtained a sensitivity of 50% in a series of 20 patients [7]. Besides the focal arterial involvement, the operator dependency and the learning curve characterizing the use of CDD probably explain the limited sensitivity of the technique.
In conclusion, until now there has been no perfect modality for the diagnosis of temporal GCA. Because of the particular pathological pattern of the arterial disease, neither CDD nor temporal biopsy has a sensitivity that is clinically suitable for detecting temporal GCA (i.e. >90%). 18FDG PET may play a role in the management of atypical GCA, when the CDD and the temporal biopsy are non-contributive, by demonstrating extratemporal localizations. Metabolic imaging may be also of interest for the assessment of the inflammatory process after corticoid therapy.
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