Interim 4'-[methyl-C]-thiothymidine PET for predicting the chemoradiotherapeutic response in head and neck squamous cell carcinoma: comparison with [F]FDG PET.

Purpose: We investigated the potential of interim 4'-[methyl-C]thiothymidine ([C]4DST) PET for predicting the chemoradiotherapeutic response for head and neck squamous cell carcinoma (HNSCC), in comparison with 2-deoxy-2-[F]fluoro-D-glucose ([F]FDG) PET.

Methods: A total of 32 patients with HNSCC who underwent both [C]4DST and [F]FDG PET/CT before therapy (baseline) and at approximately 40 Gy point during chemoradiotherapy (interim) were available for a retrospective analysis of prospectively collected data. The baseline was treatment-naïve PET/CT scan as part of staging. The maximum standardized uptake value (SUVmax), metabolic tumor volume (MTV) from [F]FDG PET or proliferative tumor volume (PTV) from [C]4DST PET, and total lesion glycolysis (TLG) from [F]FDG PET or total lesion proliferation (TLP) from [C]4DST PET were measured. MTV or PTV was defined as the volume with an SUVmax greater than 2.5. The differences in SUVmax (ΔSUVmax), MTV (ΔMTV) or PTV (ΔPTV) and TLG (ΔTLG) or TLP (ΔTLP) from baseline to interim PET scans were calculated. Patients without or with evidence of residual or recurrent disease at 3 months after completion of chemoradiotherapy were classified as showing a complete response (CR) and non-CR, respectively.

Results: All patients showed increased uptake in primary tumor on baseline [C]4DST and [F]FDG PET studies. All patients showed increased uptake on interim [F]FDG PET, whereas 18 patients showed no increased uptake on interim [C]4DST PET. After chemoradiotherapy, 25 patients were found to be in CR group and 7 to be in non-CR group. [C]4DST ΔSUVmax, ΔPTV, and ΔTLP for CR group showed significantly greater reductions than the corresponding values for non-CR group (P = 0.044, < 0.001, < 0.001, respectively). However, there were no significant differences in [F]FDG ΔSUVmax, ΔMTV, or ΔTLG between CR group and non-CR group. [C]4DST ΔMTV of -90 was the best cutoff value for the early identification of patients with non-CR.

Conclusion: These preliminary results suggest that interim [C]4DST PET might be useful for predicting the chemoradiotherapeutic response in patients with HNSCC, in comparison with [F]FDG PET.