In the present study, interim [18F]-FDG-PET evaluation by qPET and ∆SUVmax yielded closely related measurements. Outcome prediction was similar, suggesting that the two methods convey comparable information. Importantly and in contrast to the visual Deauville scale, qPET clearly defines DS5, which is crucial for the identification of PTCL patients at high risk of treatment failure.
The correlation between qPET and ∆SUVmax was similar to the findings in DLBCL [7]. qPET was correlated with baseline SUVmax in PTCL, but not in DLBCL, reflecting differences in the responsiveness to chemotherapy. In most DLBCL patients, response is rapid and accompanied by a marked reduction in [18F]-FDG uptake. In PTCL, chemotherapy is much less effective. Persistently high [18F]-FDG uptake translates into high qPET values.
The large sample size (449–898 patients) of our previous qPET studies allowed us to define precise borders between the five Deauville categories [5,6,7]. Irrespective of disease, the borders identified were identical. Because the relationship between the uptake in a residual lesion and the reference region, as perceived by the reader, should not be disease-specific, we adopted the previously defined quantitative Deauville scale also for PTCL. The small size of our PTCL study precluded a formal confirmation of the thresholds.
Interim assessment in ALK-negative PTCL was characterized by higher positive and lower negative predictive values than in DLBCL [7], reflecting differences in the frequency of treatment failure [2]. qDS5 identified 18 of 43 patients (41.9%) to be at risk of treatment failure, all but one of whom progressed or died within 30 months. An SUVmax reduction < 50% allocated 17 patients (39.5%) to the high-risk group, with similarly poor outcome. Both quantitative methods appear suitable to select patients for an early treatment change. qPET may be preferable, because it does not require a baseline [18F]-FDG-PET/CT, thus minimizing the influence of factors interfering with the evaluation [7]. However, it is important to mention that the qPET method as described here was developed 10 years ago, based on scanner systems available at that time. With the new PET scanner generations voxel size became markedly smaller so that an adapted qPET calculation is recommendable which is independent of voxel size. In the EuroNet-PHL-C2 and the GPOH-HD2020 registry trials, the SUVpeak is calculated as the SUVmean of the hottest connected voxels forming a volume of 0.2 ml (instead of the SUVmean of the 4 hottest voxels) [9].
For patients failing conventional chemotherapy, the most promising treatment is allogeneic transplantation. In a recent first-line trial, transplantation after five treatment cycles proved impossible in almost a third of patients, mainly because of progression beyond cycle 2 [10]. The window for allogeneic transplantation may be narrow in high-risk PTCL. Interim [18F]-FDG-PET could help detect impending progression before it is too late.
The major limitation of our study is its small size, inherent in all investigations in rare diseases. The results should be confirmed in an independent cohort, preferably of larger size.
In conclusion, qPET requiring a single [18F]-FDG-PET scan identifies similarly large fractions of PTCL patients at risk of treatment failure as does ∆SUVmax relying on a comparison of two scans. qPET stringently defines DS5 which is crucial for risk allocation.