A simplified acquisition protocol for dynamic [18F]-FMISO PET studies of colorectal tumours

1886 Objectives Tumour hypoxia can be quantified using [18F]-fluoromisonidazole (FMISO) PET studies. FMISO binds irreversibly in viable hypoxic cells, and the tracer molecules typically need a long time to diffuse from the blood vessels to the hypoxic regions. For identification of hypoxic tissue, it is important to characterise both the early and late phases of the tracer uptake. For this purpose, long dynamic PET studies, with three separate scanning sessions over a period of several hours, have been used in the past. We have investigated the possibility of using a simplified acquisition protocol for FMISO studies in colorectal tumours. Methods Nine patients with colorectal cancer were scanned after i.v. injection of 18F-FMISO in a PET/CT scanner. Each patient had three dynamic PET scans, starting at the time of injection and ~100 and ~200 min p.i., respectively. The data from the 3 scans were co-registered based on the CT images. Image derived arterial input functions were obtained and both voxel-based and VOI-based kinetic analyses were performed using compartmental modelling, spectral analysis and Patlak analysis. The results obtained with two simplified acquisition protocols, were compared to those from the complete data sets. The two simplified protocols consisted of the data from the 1st scan combined with that from either the 2nd or the 3rd scan. We refer to these as S12 and S13, respectively, and to the full data set as S123. The comparisons were done using correlation analysis and Bland-Altman plots. The output parameters of interest were the blood-to-tissue uptake rate-constant K1, the irreversible binding rate-constant k3, the irreversible uptake rate-constant Ki, and the volume of distribution for reversible tracer uptake Vd. Results Results from the correlation analysis are presented in the table below, showing that good correlation was obtained for all parameters, with slopes close to 1, small intercepts and high R2 values (in many cases > 0.99). For K1 the best correlation was obtained for S12, while for the other parameters, S13 gave better results. This is because K1 reflects blood flow, which determines the early tracer uptake. On the other hand k3 and Ki reflect tracer binding, which mainly influences the late part of the tracer kinetics. Conclusions Our results suggest that a 2-scan acquisition protocol could be used instead of a 3-scan protocol for FMISO studies of colorectal tumours, providing equivalent information with good accuracy. The best timing for the 2nd scan is not entirely clear. However, results suggest that optimal timing of the second scan should be between 100 and 200 minutes. This work was supported by NIHR UCLH-BRC, CRUK KCL/UCL-CIC and GE Healthcare.