Page 112 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

Data Analysis All scans were controlled for image-quality (16) and visually interpreted by an experienced nuclear medicine physician (DO), who identified suspicious PCa metastases in bone and/or lymph nodes. Lesions were semi-automatically delineated using in-house developed software (ACCURATE-tool, previously benchmarked against commercially available image-analysis tools (17)) using a 50% isocontour of SUVpeak (sphere of 1.2 cm diameter, positioned to maximize its mean value) with correction for local background uptake to obtain volumes- of-interest (VOIs) (18). Blood activity concentrations (for TBR calculation) were measured in the ascending aorta using: i) a single image slice 3x3 voxel (12x12 mm) VOI, and ii) a 3x3 voxel VOI in 5 consecutive slices (10). VOIs were created on both original (EARL1) and PSF-reconstructions (EARL2). From each VOI, the following metrics were recorded on a lesion-level: 5 tumour volume (mL), TBR (tumour VOI activity concentration/blood activity concentration), SUV, and total lesion uptake (TLU). TBR was calculated using both the mean, peak, and max activity within the VOI, yielding TBRmean, TBRpeak, and TBRmax, respectively. SUV variants included SUVmax (maximum SUV within the VOI), SUVpeak (mean SUV within a 12mm diameter sphere positioned within the VOI to yield the highest value), and SUVmean (mean SUV within the VOI). SUV was normalized to body weight. TLU was defined as lesion SUVmean or TBRmean multiplied by lesion volume, yielding TLUSUV and TLUTBR, respectively. Additionally, two patient-level metrics were derived: Total PSMA-positive Tumour Volume (TTV) and PSMA Total Tumour Burden (TTB). TTV was defined as the sum of the delineated tumour volumes within a patient. TTB was defined as the sum of the TLUSUV and TLUTBR within a patient, yielding TTBSUV and TTBTBR, respectively. As recommended by PERCIST guidelines for PET response assessment(19), and to balance the number of analysed lesions between patients, for lesion-based analyses we selected the 5 hottest lesions in case of >5 PSMA-avid lesions. For patient-level analysis, all suspicious PSMA-avid lesions were included. Statistical Analysis To assess difference in uptake intervals and injected dosages between test and retest scans we used the Wilcoxon singed-rank test for paired data. Repeatability of quantitative PET metrics was quantified using repeatability coefficients (RC; in percentages). The RCs were calculated as 1.96 times the standard deviation of relative test-retest differences d, that were calculated as follows: [18F]DCFPyL PET repeatability 111