2.3. Body composition analysis

Axial images from CT scans were converted into the Digital Imaging and Communications in Medicine (DICOM) format for analysis. Two axial images at the L3 vertebral level were collected per patient: the first image was obtained when both transverse processes of L3 were visible, while the second image was the image immediately inferior to the first (usually 5 mm inferior). Slice-O-Matic software (version 5.0; TomoVision, Montreal, Quebec, Canada) was used to perform body composition analysis into the SM area, subcutaneous and intramuscular adipose tissue (SAT) area, and visceral adipose tissue (VAT) area, measured in mm² [18]. CT Hounsfield unit thresholds ranged from −29 to 150 for SM, −190 to −30 for SAT, and −150 to – 50 for VAT. Values for each component were averaged over the two axial images. The skeletal muscle index (SMI), subcutaneous adipose tissue index (SATI), and visceral adipose tissue index (VATI) were calculated by dividing the component areas by height (m²) and reported as cm²/m². Sarcopenia is defined as an SMI of ≤52.4 cm²/m² for men or ≤38.5 cm²/m² for women [6], excess subcutaneous adipose tissue as a SATI of ≥50 cm²/m² for men or ≥42 cm²/m² for women [19], and excess VAT as a VATI of ≥52.9 cm²/m² for men or ≥51.5 cm²/m² for women [19]. Two raters (one medical student and one urology resident) were trained to read Slice-OMatic images by an expert genitourinary radiologist, and each was randomly assigned to read half of the images. To allow for interrater reliability measurement, both raters and the expert radiologist read an overlap of 30 cases independently, selected by random number generation.