The effectiveness of skeletal muscle evaluation at the third cervical vertebral level for computed tomography‐defined sarcopenia assessment in patients with head and neck cancer

BACKGROUND: Computed tomography (CT)‐defined sarcopenia is a prognostic indicator in head and neck cancer (HNC), with the gold standard for muscle evaluation using cross‐sectional area (CSA) at the third lumbar vertebra (L3). We compared methods using CSA at the third cervical vertebra (C3). METHODS: Muscle CSA was measured at L3, and CSA at C3 was used to estimate L3 CSA using a prediction model. Agreement and sarcopenia diagnosis were evaluated. RESULTS: Good correlation was found between measured and estimated CSA (101 scans; r = 0.86, p < 0.001). CSA mean difference (bias) 9.99 cm(2), (SD = 20.3 cm(2)). Skeletal muscle index bias 5.85% (SD = 13.4%), 95% limits of agreement (LoA) (−20.4 to 32.1%, r = 0.29), exceeded clinically accepted limits of 5%. Sarcopenia was diagnosed in 26%‐(L3), 45%‐(C3), with weak agreement (ƙ = 0.368, 95% confidence interval, 0.192–0.544, p < 0.001) (sensitivity 79.2%, specificity 66.7%). CONCLUSION: Agreement between measures was weak. Widespread LoA, proportional bias, and sarcopenia misclassification indicates that estimates using C3 cannot replace actual measures at L3.