CT body composition and sarcopenia measurement

What it measures. On a single axial CT slice at the third lumbar vertebra, muscle and fat are separated by their density in Hounsfield units. Skeletal muscle area divided by height squared gives the skeletal muscle index; mean muscle density gives myosteatosis (fat within muscle); and visceral and subcutaneous fat areas come from the same slice. Low muscle by sex-specific thresholds is called sarcopenia even when the patient is not thin, and obese patients with low muscle (sarcopenic obesity) have the worst outcomes.

Evidence. Prado and colleagues showed in 2008 (Lancet Oncology) that sarcopenic obesity in patients with lung and gastrointestinal cancers independently predicted survival and that muscle mass, not body surface area, tracked chemotherapy toxicity. Since then hundreds of studies have linked low muscle and low muscle density to complications after surgery, dose-limiting toxicity from chemotherapy and targeted drugs, and shorter survival in most solid tumours. Deep-learning tools now segment the slice in seconds, so the measurement can be produced for every staging scan without extra imaging or radiation.

Who should have it and what changes. It is not yet a guideline-mandated test, but the cachexia and nutrition guidelines already treat body composition as a core assessment, and it is the objective counterpart to nutrition screening questionnaires and hand-grip strength. A finding of sarcopenia should trigger dietitian referral, resistance exercise and, in frail older patients, a full geriatric assessment; some centres use it in surgical risk discussions and in choosing between regimens. Dosing chemotherapy by lean mass rather than body surface area is being tested, not yet standard. The cost is software and reporting time; the scan is free because it has already been done.