Can SUV_peak derived from PSF ¹⁸F-PET/CT images act as a surrogate for SUV_max derived from EARL compliant reconstructions?

Objectives: We explored the possibility of using SUV_peak (defined as a 1mL sphere with the highest average SUV within a lesion/target) derived from ¹⁸F-FDG PET/CT images reconstructed with point-spread-function resolution modelling as a surrogate for SUV_max derived from images reconstructed with an EARL compliant protocol. SUV_peak was already suggested as being a better alternative than SUV_max when derived from PSF images¹. This assessment was performed using phantom images and clinical data from stage III and IV non-small cell lung cancer (NSCLC) patients. Methods The National Electrical Manufacturers Association (NEMA) image quality phantom was scanned for 5 minutes with a 10:1 sphere-to-background contrast ratio. Additionally, ten stage III and IV NSCLC patients underwent two baseline ¹⁸F-FDG PET/CT scans per day at 60min and 90min post injection, at two separate time-points within one week². All data were reconstructed following two protocols: one compliant with EANM guidelines (EARL multicentre standard) and another with point-spread-function (PSF) resolution modelling. The latter protocol allows for improved lesion detection, but does not provide EARL compliant quantitative results. Patients’ lesions were segmented using a semi-automatic isocontour at SUV=4.0 g/mL. NEMA spheres were manually delineated based on their diameter. The SUV_peak PSF-derived (SUV_peak^PSF) was compared with SUV_max EARL-derived (SUV_max^EARL) with Pearson’s correlations, t-tests and Bland-Altman plots. Results For the phantom scans, the mean relative difference between SUV_peak^PSF and SUV_max^EARL was -11.0% (SD=15.0%) and showed a correlation ρ=0.95. Considering only the three biggest spheres (volumes larger than 5 mL), the mean relative difference was 1.8% (SD=3.5%). The clinical dataset (total of 21 lesions analysed) presented similar results, with a perfect correlation between SUV_peak^PSF and SUV_max^EARL (ρ=1.00) and a mean relative difference of -4.3% (SD=5.2%). This highlights that, overall, SUV_peak^PSF values are slightly lower than SUV_max^EARL values. Only two lesions were identified as smaller than 5 mL using the EARL reconstruction, three with PSF. The relative difference between SUV implementations for those small lesions was -7.1% (SD=4.3%). Moreover, uptake time did not affect the differences seen between SUV_peak^PSF and SUV_max^EARL, which were -4.2% (SD=5.0%) and -4.3% (SD=5.6%) for the 60 and 90 minutes post-injection data (p=0.89), respectively. Conclusion This pilot study suggests that SUV_peak derived from high resolution PSF reconstructed ¹⁸F-FDG PET/CT images might be used as a surrogate for SUV_max^EARL for lesions bigger than 5mL. Conclusions on smaller lesions cannot be substantiated due to limited sample size. Further research using data from different centres and on larger as well as different patient groups is required to fully evaluate the feasibility and clinical impact of using SUV_peak^PSF as a surrogate for SUV_max^EARL. References 1. Mansor, S., Pfaehler, E., Heijtel, D., et al. (2017), Impact of PET/CT system, reconstruction protocol, data analysis method, and repositioning on PET/CT precision: An experimental evaluation using an oncology and brain phantom. Med. Phys., 44, 6413-6424. doi:10.1002/mp.12623 2. Kramer, G. M., Frings, V., Hoetjes, N., et al. (2016). Repeatability of quantitative whole-body 18F-FDG PET/CT uptake measures as function of uptake interval and lesion selection in non-small cell lung cancer patients. Journal of Nuclear Medicine, 57(9), 1343-1349. doi:10.2967/jnumed.115.170225.