Comparison of FLT vs. FDG for early therapy monitoring of diffuse large B-cell lymphoma

Objectives Early therapy response assessment with FDG PET/CT in diffuse large B-cell lymphoma (DLBCL) has unsatisfactory positive predictive value. 3'-deoxy-3'-[F-18]-fluorothymidine (FLT), a tumor proliferation agent, might be more specific in this setting. The aim of this preliminary analysis was to evaluate findings on interim FDG and FLT PET/CTs performed after 2 cycles of R-CHOP in DLBCL patients.

Methods In this prospective, non-interventional, multicenter study, patients diagnosed with de novo DLBCL scheduled to receive R-CHOP every 3 weeks for 6 cycles were eligible. Participants were consented for FDG and FLT PET/CT scans performed 16 days or later after the second cycle of chemotherapy. Interim scans were evaluated visually by comparing lesions to liver: any uptake above the liver was defined as positive, similar as equivocal, below as negative. Metabolic tumor volume (MTV), SUV and total volume activity (TLA = MTV x SUVmean) were extracted from PET data. For FDG, TLA>100 was defined as positive, 100≥TLA≥10 as equivocal, and TLA<10 as negative. For FLT, TLA≥20 was defined as positive, and below as negative.

Results From November 2011 to 2012, a total of 23 patients were eligible, but only 16 fulfilled inclusion criteria. Qualitative analysis showed similar FDG/FLT results for 9 patients. In 7 cases, FDG/FLT scans were discordant: 3 FDG +/FLT -, 3 FDG equivocal/FLT -, 1 FDG equivocal/FLT +. Semi-quantitative analysis showed identical proportion of concordant/discordant (9:7) results, but findings are not affecting the same patients as with qualitative analysis.

Conclusions Qualitative and semi-quantitative analysis showed discordant FDG/FLT results in 7 patients. FLT provides definitive answers for equivocal interim FDG scans, giving more confidence to the readers. Long term clinical follow-up and accrual of patients are still on-going, eventually providing more insight to this preliminary analysis.

Research Support NIH Funding #1R01CA 152923-01A1