18 F-fluorothymidine (FLT)-PET and diffusion-weighted MRI for early response evaluation in patients with small cell lung cancer: a pilot study

Background Small cell lung cancer (SCLC) is an aggressive cancer often presenting in an advanced stage and prognosis is poor. Early response evaluation may have impact on the treatment strategy. Aim We evaluated 18 F-fluorothymidine-(FLT)-PET/diffusion-weighted-(DW)-MRI early after treatment start to describe biological changes during therapy, the potential of early response evaluation, and the added value of FLT-PET/DW-MRI. Methods Patients with SCLC referred for standard chemotherapy were eligible. FLT-PET/DW-MRI of the chest and brain was acquired within 14 days after treatment start. FLT-PET/DW-MRI was compared with pretreatment FDG-PET/CT. Standardized uptake value (SUV), apparent diffusion coefficient (ADC), and functional tumor volumes were measured. FDG-SUV peak , FLT-SUV peak , and ADC median ; spatial distribution of aggressive areas; and voxel-by-voxel analyses were evaluated to compare the biological information derived from the three functional imaging modalities. FDG-SUV peak , FLT-SUV peak , and ADC median were also analyzed for ability to predict final treatment response. Results Twelve patients with SCLC completed FLT-PET/MRI 1–9 days after treatment start. In nine patients, pretreatment FDG-PET/CT was available for comparison. A total of 16 T-sites and 12 N-sites were identified. No brain metastases were detected. FDG-SUV peak was 2.0–22.7 in T-sites and 5.5–17.3 in N-sites. FLT-SUV peak was 0.6–11.5 in T-sites and 1.2–2.4 in N-sites. ADC median was 0.76–1.74 × 10 − 3 mm 2 /s in T-sites and 0.88–2.09 × 10 −3 mm 2 /s in N-sites. FLT-SUV peak correlated with FDG-SUV peak , and voxel-by-voxel correlation was positive, though the hottest regions were dissimilarly distributed in FLT-PET compared to FDG-PET. FLT-SUV peak was not correlated with ADC median , and voxel-by-voxel analyses and spatial distribution of aggressive areas varied with no systematic relation. LT-SUV peak was significantly lower in responding lesions than non-responding lesions (mean FLT-SUV peak in T-sites: 1.5 vs. 5.7; p = 0.007, mean FLT-SUV peak in N-sites: 1.6 vs. 2.2; p = 0.013). Conclusions FLT-PET and DW-MRI performed early after treatment start may add biological information in patients with SCLC. Proliferation early after treatment start measured by FLT-PET is a promising predictor for final treatment response that warrants further investigation. Trial registration Clinicaltrials.gov, NCT02995902 . Registered 11 December 2014 - Retrospectively registered.