Defining target volumes for stereotactic ablative radiotherapy of early-stage lung tumours: a comparison of three-dimensional 18F-fluorodeoxyglucose positron emission tomography and four-dimensional computed tomography.:PETCT versus 4DCT for SABR in Early Stage NSCLC

AIMS: High local control rates are achieved in stage I lung cancer using<br/>stereotactic ablative radiotherapy. Target delineation is commonly based on<br/>four-dimensional computed tomography (CT) scans. Target volumes defined by<br/>positron emission tomography/computed tomography (PET/CT) are compared with those defined by four-dimensional CT and conventional ('three-dimensional')<br/>(18)F-fluorodeoxyglucose ((18)F-FDG) PET/CT.<br/><br/>MATERIALS AND METHODS: For 16 stage I non-small cell lung cancer tumours, six<br/>approaches for deriving PET target volumes were evaluated: manual contouring,<br/>standardised uptake value (SUV) absolute threshold of 2.5, 35% of maximum SUV<br/>(35%SUV(MAX)), 41% of SUV(MAX) (41%SUV(MAX)) and two different source to<br/>background ratio techniques (SBR-1 and SBR-2). PET-derived target volumes were compared with the internal target volume (ITV) from the modified maximum<br/>intensity projection (MIP(MOD) ITV). Volumetric and positional correlation was<br/>assessed using the Dice similarity coefficient (DSC).<br/><br/>RESULTS: PET-based target volumes did not correspond to four-dimensional CT-based target volumes. The mean DSC relative to MIP(MOD) ITV were: PET manual = 0.64, SUV2.5 = 0.64, 35%SUV(MAX) = 0.63, 41%SUV(MAX) = 0.57. SBR-1 = 0.52, SBR-2 =0.49. PET-based target volumes were smaller than corresponding MIP ITVs.<br/><br/>CONCLUSIONS: Conventional three-dimensional (18)F-FDG PET-derived target volumes for lung stereotactic ablative radiotherapy did not correspond well with those derived from four-dimensional CT, including those in routine clinical use<br/>(MIP(MOD) ITV). Caution is required in using three-dimensional PET for motion<br/>encompassing target volume delineation.