Impact of rigid and nonrigid registration on the determination of 18F-FDG PET-based tumour volume and standardized uptake value in patients with lung cancer

Purpose: Assessment of the metabolically active tumour tissue by FDG PET is evolving for use in the diagnosis of non-small-cell lung cancer (NSCLC), in the planning of radiotherapy, and in follow-up and response evaluation. For exact evaluation accurate registration of PET and CT data is required. The registration process is usually based on rigid algorithms; however, nonrigid algorithms are increasingly being used. The influence of the registration method on FDG PET-based standardized uptake value (SUVmax) and metabolic tumour volume (MTV) definition has not yet been evaluated. We compared intra- and interindividual differences in SUV and MTV between rigid- and nonrigid-registered PET and CT acquired during different breathing manoeuvres. Methods: The study group comprised 28 radiotherapy candidates with histologically proven NSCLC who underwent FDG PET acquisition and three CT acquisitions (expiration - EXP, inspiration - INS, mid-breath-hold - MID). All scans were re gistered with both a rigid (R) and a nonrigid (NR) procedure resulting in six fused datasets: R-INS, R-EXP, R-MID, NR-INS, NR-EXP and NR-MID. For the delineation of MTVs a contrast-oriented contouring algorithm developed in-house was used. To accelerate the delineation a semiautomatic software prototype was utilized. Results: Tumour mean SUVmax did not differ for R and NR registration (R 17.5±7, NR 17.4±7; p=0.2). The mean MTV was higher by 3±12 ml (p=0.02) in the NR group than in the R group, as was the mean tumour diameter (by 0. 1±0.2 cm; p<0.01). With respect to the three different breathing manoeuvres, there were no differences in MTV in the R group (p>0.7). In intraindividual comparison there were no significant differences in MTVs concerning the registration pairs R-EXP (68± 88 ml) vs. NR-EXP (69±85 ml) und R-MID (68±86 ml) vs. NR-MID (69±83 ml) (both p>0.4). However, the MTVs were larger after NR registration during inspiration (R-INS 68±82 vs. NR-INS 78±93 ml; p=0.02). Conclusion: The use o