151751-Najiba-Chargi

97 Surgery: skeletal muscle mass and free fibula flap reconstruction BODY COMPOSITION MEASUREMENT SMMwas measured as muscle cross-sectional area (CSA) on pre-treatment CT or MRI imaging of the head and neck area at the level of the third cervical vertebrae (C3). The axial slide of the imaging, which showed both transverse processes and the entire vertebral arc, was selected for segmentation of muscle tissue. For CT imaging, muscle area was defined as the pixel area between the radiodensity range of -29 and +150 Hounsfield Units (HU), which is specific for muscle tissue. For MRI, muscle area was manually segmented, and fatty tissue was manually excluded. The CSA was calculated as the sum of the delineated areas of the paravertebral muscles and both sternocleidomastoid muscles. Segmentation of muscle tissue was manu - ally performed using the commercially available software package SliceOmatic (Tomovision, Canada) by a single researcher (EA) who was blinded for patient outcomes. An example of segmentation at the level of C3 is shown in figure 1. CSA at the level of C3 was converted to CSA at the level of L3 using a previously published formula 1. 21 The lumbar skeletal muscle index (LSMI) was calculated by correcting SMM at the level of L3 for squared height as shown in formula 2. Low SMM was defined as a LSMI below 43.2 cm 2 /m 2 , this cutoff value was deter - mined in a separate cohort of head and neck cancer patients. 23 Figure 1. Segmentation of skeletal muscle tissue at the level of the third cervical vertebra (C3) This figure displays two identical axial CT-slides at the level of C3; in the left axial slide muscle tissue is unsegmented. The right CT slide shows both sternocleidomastoid and paravertebral muscles segmented in red. Formula 1: CSA at L3 (cm 2 ) = 27.304 + 1.363 * CSA at C3 (cm 2 ) – 0.671*Age (years) + 0.640 * Weight (kg) + 26.442*Sex (Sex=1 for female and 2 for male) Formula 2: Lumbar SMI (cm 2 /m 2 ) = CSA at L3/length (m 2 ) 6