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 cm², (SD = 20.3 cm²). 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.