Chemokine receptor–targeted PET/CT provides superior diagnostic performance in newly diagnosed marginal zone lymphoma patients: a head-to-head comparison with [18F]FDG

Background In patients with marginal zone lymphoma (MZL), [18F]FDG PET/CT provided inconsistent diagnostic accuracy. C-X-C motif chemokine receptor 4 (CXCR4) is overexpressed in MZL and thus, may emerge as novel theranostic target. We aimed to evaluate the diagnostic performance of CXCR4-targeting [68Ga]Ga-PentixaFor when compared to [18F]FDG PET/CT in MZL. Methods Thirty-two untreated MZL patients (nodal, n = 17; extranodal, n = 13; splenic, n = 2) received [68Ga]Ga-PentixaFor and [18F]FDG PET/CT within median 2 days. We performed a visual and quantitative analysis of the total lymphoma volume by measuring maximum/peak standardized uptake values (SUVmax/peak), and calculating target-to-background ratios (TBR, defined as lesion-based SUVpeak divided by SUVmean from blood pool). Visual comparisons for both radiotracers were carried out for all target lesions (TL), and quantitative analysis of concordant TL evident on both scans. Last, MZL subtype analyses were also conducted. Results On a patient-based level, [68Ga]Ga-PentixaFor identified MZL manifestations in 32 (100%) subjects (vs. [18F]FDG, 25/32 [78.1%]). Of the 256 identified TL, 127/256 (49.6%) manifestations were evident only on CXCR4-directed imaging, while only 7/256 (2.7%) were identified on [18F]FDG but missed by [68Ga]Ga-PentixaFor. In the remaining 122/256 (47.7%) concordant TL, [68Ga]Ga-PentixaFor consistently provided increased metrics when compared to [18F]FDG: SUVmax, 10.3 (range, 2.53–37.2) vs. 5.72 (2.32–37.0); SUVpeak, 6.23 (1.58–25.7) vs. 3.87 (1.54–27.7); P < 0.01, respectively. Concordant TL TBR on [68Ga]Ga-PentixaFor (median, 3.85; range, 1.05–16.0) was also approximately 1.8-fold higher relative to [18F]FDG (median, 2.08; range, 0.81–28.8; P < 0.01). Those findings on image contrast, however, were driven by nodal MZL (P < 0.01), and just missed significance for extranodal MZL (P = 0.06). Conclusions In newly diagnosed MZL patients, [68Ga]Ga-PentixaFor identified more sites of disease when compared to [18F]FDG, irrespective of MZL subtype. Quantitative PET parameters including TBR were also higher on [68Ga]Ga-PentixaFor PET/CT, suggesting improved diagnostic read-out using chemokine receptor-targeted imaging.

Beyond [ 18 F]FDG assessing glucose consumption in sites of disease, PET agents specifically targeting other (sub) cellular structures overexpressed in MZL have been evaluated in recent years [10,11].For instance, the C-X-C motif chemokine receptor 4 (CXCR4)-directed radiotracer [ 68 Ga] Ga-PentixaFor showed improved read-out capabilities when compared to guideline-compatible work-up, including CT.Those previous results, however, did not focus on a comprehensive head-to-head comparison of [ 68 Ga]Ga-PentixaFor vs [ 18 F]FDG in all lymphoma manifestations.As such, we aimed to compare findings on both PET/CTs in untreated MZL patients, including a visual and quantitative assessment on a patient and target lesion (TL)-level specifically focusing on the whole body lymphoma load.

Methods
We performed a retrospective analysis of our institutional PET/CT database and identified 32 newly diagnosed MZL patients who received [ 68 Ga]Ga-PentixaFor and [ 18 F] FDG PET/CT within no more than 30 days.Patients did not receive oncological treatment between PET scans.All patients provided written informed consent.The local institutional review board waived the need for additional approval as this was a retrospective investigation (waiver no.20220414 01).Prior studies have reported on parts of this patient cohort [10,12,13].In [10], only a "hottest lesion" analysis was conducted; i.e., exclusively, the most intense lesion on PET has been further investigated, while in the present study, a lesion-based head-to-head comparison of the entire lymphoma burden has been performed.

Imaging procedure
PET/CT scans were obtained on Biograph mCT64 or 128 scanners (Siemens Healthineers, Erlangen, Germany).The scans covered an area from the top of the head to the upper thigh.For [ 18 F]FDG scans, a minimum fasting period of 6 h was required, to ensure a blood glucose level below 180 mg/dl.For both radiotracers, imaging was initiated one hour after injection, with a median injected activity of 115 MBq (range, 78-186) for [ 68 Ga]Ga-PentixaFor scans, and 299 MBq (range, 239-406) for [ 18 F]FDG scans.Image acquisition in 3D mode was performed with a flow bed velocity of 1.1 mm/s (mCT128) or 2 min/bed position (mCT64).The images were reconstructed by applying a Gaussian filter of 2.0 mm with a 200 × 200 matrix in three iterations (subsets, 24/21 [mCT 64/128]).Attenuation correction was based on CT scans with or without contrast enhancement [10,14].

Image analysis
A dedicated workstation and software package (syngo.via,version VB60A; Siemens Healthineers, Erlangen, Germany) were used to evaluate images.For a readout of the entire lymphoma load, one reader (S.Sc.) placed spherical volumes of interest (VOIs) around all potential target lesions (TL) with tracer uptake above background levels.Another reader (A.K.) verified all VOIs, and inconclusive findings were reviewed in consensus with two expert readers (S.E. S., R. A. W.).All readers were blinded to clinical data and other imaging results, including the respective other PET scan.After having placed a VOI, the software automatically generated a three-dimensional contour at a 40%-threshold, thereby providing maximum/peak standardized uptake values (SUV max , SUV peak ).For target-to-background-ratio (TBR) calculations, SUV peak(TL) was divided by the averaged mean blood pool SUV from three regions of interest on separate slices in the vena cava superior [10,12].For a headto-head comparison of both radiotracers, two readers (S.Sc., A. K.) identified TL that were identical on both PET scans.

Statistical analysis
We used the GraphPad Prism version 9.3.1 (GraphPad Prism Software, La Jolla, CA, USA) for statistical analyses.Continuous normally distributed (as per Shapiro-Wilk test) variables are presented as mean ± standard deviation; otherwise, median and range are provided.To compare concordant TL, we applied the Wilcoxon matched-pairs signed rank test.For other group comparisons, the Mann-Whitney test was used.P < 0.05 was considered statistically significant.

Quantitative assessment of CXCR4-directed and [ 18 F]FDG PET/CT per MZL subtype
Table 2 shows results of a lesion-wise analysis of quantitative PET parameters, grouped by MZL subtype.For both radiotracers, most MZL lesions were evident in NMZL, followed by EMZL and SMZL.However, [ 68 Ga]Ga-PentixaFor For NMZL, CXCR4-directed imaging showed significantly higher SUV max/peak and TBR for all visible MZL and concordant lesions when compared to [ 18 F]FDG PET/CT (P ≤ 0.01; Table 2).In patients with EMZL, there was no significant difference comparing all lesions detected by both radiotracers (P ≥0.17); however, considering concordant MZL lesions only, [ 68 Ga]Ga-PentixaFor PET/CT showed significantly higher SUV max/peak (P = 0.04), but just missed significance for TBR (P = 0.06; Table 2).Due to the small number of MZL lesions on [ 18 F]FDG PET/CT in SMZL (N = 2), no such quantitative head-to-head comparison has been conducted.

Discussion
In the present visual and quantitative analysis of the total lymphoma load on [ 18 F]FDG and [ 68 Ga]Ga-PentixaFor PET/ CT, we observed that the latter radiotracer revealed MZL manifestations in a higher proportion of patients, along with substantially increased detection rate on a TL level.In a read-out of quantitative PET parameters for concordant lesions on both scans, [ 68 Ga]Ga-PentixaFor almost consistently provided significantly higher SUV max , SUV peak , and TBR, indicative for improved contrast.In a subgroup analysis of MZL subtypes, visual and quantitative comparison of both radiotracers also provided higher diagnostic performance of [ 68 Ga]Ga-PentixaFor PET/CT for and EMZL.As such, based on our visual and quantitative comparison, the latter PET agent may emerge as the radiotracer of choice when investigating MZL patients upon initial diagnostic work-up.
As the reference radiotracer in nuclear oncology, [ 18 F] FDG is well-established in the vast majority of lymphomas, in particular for treatment monitoring [15].Not surprisingly, this agent has also been extensively validated in patients with MZL, but its diagnostic accuracy seems to vary based on the investigated subtype.For instance, Hofmann et al. reported that [ 18 F]FDG PET is positive in nodal, but not EMZL, which is in line with our findings reporting on limited detection rate for the latter subtype on a TL level (Table 2) [16].Nonetheless, the clinical value of this radiotracer remains a matter of debate, as a recent study investigating all three MZL subtypes reported on consistent upstaging after [ 18 F]FDG administration when compared to CT alone [17].Given those controversial findings, novel radiotracers providing a more reliable read-out of the current disease extent are intensively sought.In this regard, theranostic radiotracers may provide improved staging along with potential therapeutic options using ß-emitting 177Lulabelled counterparts.Recent immunohistochemical analysis, however, revealed limited or even abundant expression of somatostatin receptors (SSTR) 2a/5 [18,19], which are specifically addressed by SSTR-directed radiotracers used for imaging and therapy [20].Not surprisingly, in vivo targeting of this receptor subtype then provided a substantial rate of false-negative findings, thereby rendering SSTR as less relevant in the context of MZL [21].CXCR4, however, was substantially upregulated in more than 90% of samples obtained from extranodal MZL of mucosa-associated lymphoid tissue lymphoma [18].Also exploited as theranostic target, such chemokine receptors have been subject to multiple imaging studies using [ 68 Ga]Ga-PentixaFor [10][11][12].For instance, a recent analysis focusing on the diagnostic performance of CXCR4-directed PET/CT relative to guidelinecompatible diagnostic work-up (CT, bone marrow biopsy and esophagogastroduodenoscopy) also provided preliminary evidence on the superiority of this radiotracer relative to [ 18 F]FDG.However, in this previous study, only one single TL, defined as most intense on uptake, was analyzed [10].In the present investigation, we expanded those preceding findings by examining the entire whole-body lymphoma burden on a visual and quantitative level, which then also provided a comprehensive compartment-and subtype-based analyses.Of note, for both lymphonodal and extranodal compartments, CXCR4-targeted imaging revealed substantially more sites of disease which would have been missed by [ 18 F]FDG, in particular in manifestations located in the soft tissue and bone.Although limited by the retrospective nature and small number of enrolled subjects, our study may provide a hint that [ 68 Ga]Ga-PentixaFor may emerge as the novel diagnostic PET agent of choice in MZL.These considerations are also further fueled by an observed elevated TBR in both NMZL and EMZL.Indicative for improved image contrast, those findings suggest that CXCR4-directed imaging may address the clinical need of a reliable diagnostic PET agent, in particular in EMZL, where [ 18 F]FDG only provides rather limited clinical benefit.In this regard, future studies should determine whether the observed high image contrast on [ 68 Ga]Ga-PentixaFor PET indeed improve reader's confidence, e.g., by conducting an interobserver agreement study [22].Nonetheless, chemokine receptor imaging may be less suitable in patients with splenic involvement, which is partially explained by the physiological biodistribution of this agent [23].
Beyond image contrast, we also recorded SUV max of median >10, with individuals even exhibiting values above 37.Based on those PET metrics, therapeutic applications using [ 177 Lu]Lu-or [ 90 Y]Y-PentixaTher would then provide anti-lymphoma efficacy, along with (desired) eradication of the stem cell niche as an integral component of the treatment plan, e.g., to prepare for subsequent stem cell transplantation [24].As such, relative to [ 18 F]FDG, the theranostic aspect of CXCR4-targeted imaging would then not only allow for better delineation of putative sites of disease, but also identify patients eligible for treatment.
Last, several limitations of this study should be noted.First, this study's retrospective nature implies, that it is not possible to histologically confirm specific lesions seen on molecular imaging.In addition, not all lesions that were conspicuous on imaging can be biopsied and confirmed histologically.However, the fact that several readers verified inconclusive lesions in consensus should contribute to diagnostic confidence.Moreover, previous work has shown that CXCR4-directed imaging can be used as an accurate staging tool in patients with marginal zone lymphoma [10].In this regard, Duell and colleagues [10] demonstrated that 16 out of 18 PET-guided biopsies of suspicious lesions exclusively visualized on [ 68 Ga]Ga-PentixaFor PET compared to [ 18 F] FDG were actually histologically confirmed as MZL manifestations.Therefore, even lacking bioptic confirmation of each lesion documented on imaging, we believe that viewed with due caution, our results are of value.

Conclusions
Compared to [ 18 F]FDG, [ 68 Ga]Ga-PentixaFor PET/CT identified more sites of disease in a higher proportion of patients with newly diagnosed MZL, irrespective of the investigated subtype.PET quantification -including TBR -provided consistently higher values on [ 68 Ga]Ga-PentixaFor PET, thereby also suggesting improved image contrast.As such, in patients affected with MZL, our findings may pave the way for a more widespread adoption of this PET agent in the clinic.

Fig. 1 Table 1
Fig. 1 Bar graph showing numbers of marginal zone lymphoma lesions detected by [ 68 Ga]Ga-PentixaFor PET/ CT and [ 18 F]FDG PET/CT per organ compartment

Table 2
Quantitative analyses of marginal zone lymphoma (MZL) lesions presented by subtype for [ 68 Ga]Ga-PentixaFor and [ 18 F]FDG PET Numbers indicate median with range in parenthesis NMZL nodal marginal zone lymphoma, EMZL extranodal marginal zone lymphoma, SMZL splenic marginal zone lymphoma, SUV standardized uptake value, TBR target to background ratio Significant p-values are set in bold.p-values were considered significant if p was less than 0.05 * Due to the small number of lesions on [ 18 F]FDG PET/CT in SMZL (N = 2), a reliable indication of a P value is not possible