We confirm that analysis of routine CT scans for sarcopenia can predict outcomes for HNSCC patients. Pre-RT sarcopenia is associated with more poor outcomes for nonoperated patients and post-RT sarcopenia for all HNSCC patients. Post-RT sarcopenia, as measured by routine CT, outperformed simple weight loss and BMI-derived cachexia metrics, because loss of lean muscle mass can occur independently of BMI status. These findings suggest the potential benefit for investigating intervention with aggressive nutritional and physiatric methods to prevent sarcopenia during RT and to study how these interventions might affect outcomes in nonoperated, pre-RT sarcopenia patients.