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C.Rossi et al.
patients). Here again, alterations of epigenetic modifiers were the most frequent (41.7% of total variants occurred in 89% of patients) (Figure 6; Online Supplementary Table S3). Altogether, FOXO1 mutations occurred in five FL patients (two from the training cohort and three from the validation cohort), four of which had SUVmax>14.5.
Discussion
Our multi-parametric analysis of two independent cohorts of FL patients shows that baseline SUVmax values above 14.5 identified those with early unfavorable out- comes, defined here as poorer PFS and higher rate of POD24 events. Although this threshold is an absolute value which needs to be confirmed in larger cohorts, our study suggests that the overall high level of SUVmax at diag- nosis could potentially reflect specific biological behav- iors in FL tumors. Our results build on those recently obtained by Strati et al.8, who demonstrated the prognos- tic impact of baseline SUVmax>18 in FL patients. Their use of a higher threshold could be explained by i) the more favorable dissemination of disease in the FL patients from our study (17% vs. 0% of Ann Arbor stage I-II)8, ii) the use of R-CHOP or other R-chemotherapy as induction treat- ment in our cohorts, while 15% of patients in Strati et al.8 received rituximab alone; iii) the fact that all of our patients received maintenance therapy while only few patients received Rm in Strati et al.,8 and iv) PET acquisi- tion and measures were performed on different PET/CT devices. These discrepancies suggest a better chance of disease control in our series, with a lower risk of relapse and fewer PFS events.25,28 Moreover, Strati et al.8 demon- strated a significant association between the largest lymph node ≥6 cm and SUVmax>18. In our study, we focused on TMTV to assess the tumor burden seeing as available computed tomography was not mandatory for patient enrollment in the study. Nevertheless, for 119 (91%) patients with available computed tomography, the size of the largest lymph node was strongly related to TMTV but not to SUVmax. We found no correlation between the tumor burden and SUVmax in our series.
Pre-treatment PET staging is more sensitive than CT for identifying FL patients who have a high risk of transfor- mation to aggressive lymphoma.29 In line with our results, several other studies have suggested that SUVmax threshold values ranging from 10 to 13 could be used to identify FL with a higher risk of transformation.4,30,31 However, only tumor biopsy possibly guided by PET can rule out FL transformation into high grade lymphoma, which is de facto associated with reduced PFS and OS. Here, SUVmax>14.5 did not necessarily indicate that FL would develop or transform into aggressive lymphoma. The biopsy sites used for FL diagnosis exhibited the higher SUVmax at PET baseline, while their histological analyses did not unveil any transformation. In our series, high avidity of FDG in FL samples was not associated with a specific infiltration pattern of major immune cell popula- tions and was not correlated with increased of SES for IEGS33 and for T-cell activation gene set. Nevertheless, some FL samples lacked available frozen tissue for RNA sequencing and were unsuitable for further analyses. Our findings should thus be confirmed in a larger prospective cohort with sufficient biopsied material from the most FDG-avid FL tumor.
Moreover, the lack of association between tumor immunological contexture and prognosis could be related to anti-CD20 maintenance therapy, as already suggest- ed.32 While baseline SUVmax appears unrelated to immune T-cell infiltration, it remains related to tumor cell prolifer- ation, as shown by high DNA repair and Ki-67 prolifera- tive indexes. Thus, these results suggest that SUVmax>14.5 at baseline in FL patients with Ki-67 staining ≥10% could define patients with high risk of relapse or progression. While TMTV, a surrogate marker of tumor cell burden, was unrelated to baseline SUVmax or patient outcomes in our series, TMTV >510 cm3 has been associated with a higher risk of treatment failure in FL patients.6 However, this discrepancy could be related to the absence of main- tenance treatment in the study by Meignan et al.6 Considering that the long-term results of the PRIMA study showed improved PFS in FL patients receiving rit- uximab maintenance (10.5 years compared to 4.1 years without maintenance treatment),33 and the GALLIUM study34 showed an additional benefit of obinutuzumab over rituximab for PFS, anti-CD20 maintenance could mitigate the prognostic value of TMTV at baseline. Accordingly, TMTV had no prognostic value in either arm (rituximab or obinutuzumab) of the GALLIUM study. This suggests that FL prognosis could be more related to the proportion of proliferating tumor cells than to the cell burden of the tumor. Ultimately, without evi- dence of transformation, high SUVmax in FL samples may be associated with more aggressive tumor cells. This has been observed in other low-grade lymphomas,35 suggest- ing that the ability of FL tumors to proliferate may affect their avidity to FDG and thus the PET signals observed in FL patients.
The frequency of alterations in the oncogenic pathway did not vary with the SUVmax level, but we identified a FOXO1 mutation in a subset of patients with SUVmax>14.5. The prognostic value of FOXO1 mutations has already been shown in DLBCL36 and FL.12,37 For instance, FOXO1 mutations were more frequently observed in high-risk FL patients with POD24 when compared with the non- POD24 subgroup (25% vs. 10%, P=0.04)12,37 and were associated with shorter failure-free survival (HR 2.74, range, 1.23-6.09; P=0.013) in the m7-FLIPI cohort.12 Moreover, the FOXO1 transcription factor appears to be implicated in the regulation of the abundance of CD20 on the surface of tumor cells, so alterations could influence the response to rituximab-based therapies as previously reported in an in vitro study of cell line assays.38 On the contrary, we found certain mutations such as TP53 and MYC more often in patients with SUVmax<14.5. However, these observations could be related either to the spatial genetic heterogeneity of FL39 or to possible areas of FL transformation.39 In this context, high SUVmax levels could be very helpful to identify those FL sites with suspected transformation. That is why we selected lymph node biopsies with the most FDG-avid sites to ensure lack of transformation. However, single biopsy specimens can- not rule out transformation/genomic alteration or varia- tion in other sites. Finally, baseline SUVmax outperformed FLIPI to predict patient outcomes, in addition to multi- biopsy site assessment of gene mutation profile; it could therefore possibly be used to identify patients who are at risk of poor outcomes.
In conclusion, SUVmax>14.5 at baseline FDG-PET is associ- ated with poor PFS and high risk of POD24 events in FL
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haematologica | 2022; 107(1)