Skeletal Muscle Radiation Attenuation at C3 Predicts Survival in Head and Neck Cancer

<b>Background</b>: Sarcopenia assessed by skeletal muscle area (SMA) at the third lumbar vertebra (L3) is an established prognostic marker in many malignancies, including head and neck cancer (HNC). However, in HNC, L3 is rarely assessed. The prognostic value of myosteatosis, measured by skeletal muscle radiation attenuation (SMRA) remains largely unexplored. This study evaluated both muscle metrics at the third cervical vertebra (C3) for locoregional control (LRC) and overall survival (OS) in HNC. <b>Methods</b>: SMA and SMRA at C3 were quantified in CT scans of 904 HNC cases by a deep learning-based segmentation pipeline with manual verification. Cox proportional hazards models assessed associations with LRC and OS. <b>Results</b>: Median SMA was 36.64 cm² (IQR: 30.12-42.44). Median SMRA was 50.77 HU (IQR: 43.04-57.39). In multivariable analysis, lower SMA (HR 1.85, 95% CI: 1.19-2.88, <i>p</i> ≤ 0.001) and lower SMRA (HR 1.76, 95% CI: 1.22-2.54, <i>p</i> < 0.001) were associated with lower LRC. For OS, lower SMA (HR 1.53, 95% CI:1.06-2.20, <i>p</i> = 0.02) and lower SMRA (HR 2.13, 95% CI: 1.58-2.88, <i>p</i> < 0.001) were associated with a worse outcome in multivariable analysis. <b>Conclusions</b>: Both SMRA and SMA assessed at C3 correlate with worse LRC and OS in HNC.