PET imaging with [¹⁸F]3′-deoxy-3′-fluorothymidine for prediction of response to neoadjuvant treatment in patients with rectal cancer

Purpose: Positron emission tomography (PET) using ¹⁸F-labelled 3′-deoxy-3′-fluorothymidine (FLT) was assessed for therapy monitoring in patients with rectal cancer undergoing neoadjuvant chemoradiotherapy. Methods: Ten patients with locally advanced rectal cancer were included and underwent long-course preoperative chemoradiotherapy (total dose 45 Gy, 1.8 Gy/day, concomitant 250 mg/m² 5-fluorouracil) followed by surgery. FLT-PET was performed prior to chemoradiotherapy, 2 weeks after initiation of chemoradiotherapy and preoperatively (3-4 weeks post chemoradiotherapy). FLT uptake was correlated with histopathological tumour regression and changes in T stage. Results: Mean tumour FLT uptake was 4.2±1.0 SUV before therapy and decreased significantly to 2.9 ± 0.6 SUV 14 days after initiation of chemoradiotherapy (-28.6% ± 10.7%, p = 0.005). The preoperative scan showed a further decrease to 1.9 ± 0.4 SUV (-54.7% ± 7.6%, p = 0.005). However, the degree of change in FLT uptake 2 weeks after initiation and after completion of neoadjuvant therapy did not correlate with histopathological tumour regression. Conclusion: FLT-PET did not seem to be a promising method for assessment of tumour response in the studied chemoradiotherapy regimen in patients with rectal cancer.