How is the lumbar skeletal muscle index (LSMI) calculated in an adult cancer patient?

How to Calculate Lumbar Skeletal Muscle Index (LSMI)

The lumbar skeletal muscle index (LSMI) is calculated by measuring the total cross-sectional skeletal muscle area at the third lumbar vertebra (L3) on CT imaging, then dividing this area (in cm²) by the patient's height squared (in m²) to yield cm²/m². 1

Technical Measurement Process

Image Acquisition and Analysis Location

• Use CT imaging at the L3 vertebral level as this location has been validated to correlate with whole-body skeletal muscle volume 1
• The scan should be a standard 5-mm-thick axial slice at L3 1
• CT-based analysis at L3 is the gold standard for assessment of muscle mass in cancer patients 1

Muscle Area Segmentation

• Measure the total cross-sectional area (cm²) of all abdominal skeletal muscles at L3 using image analysis software 1, 2
• This includes psoas, paraspinal, and abdominal wall muscles 1
• Use Hounsfield Unit (HU) thresholds to identify skeletal muscle tissue (typically -29 to +150 HU for muscle) 1, 2
• Most radiotherapy contouring software or specialized body composition analysis programs can perform this segmentation automatically 2

Index Calculation

• LSMI (cm²/m²) = Total skeletal muscle area at L3 (cm²) ÷ Height² (m²) 1, 3
• Height must be measured in meters and squared before division 1
• The resulting value represents the skeletal muscle index normalized for patient stature 1

Interpretation: Sarcopenia Cut-off Values

For Cancer Patients (Most Validated)

• Men: LSMI < 55 cm²/m² indicates severe muscle depletion 1
• Women: LSMI < 39 cm²/m² indicates severe muscle depletion 1
• These are well-validated, BMI and gender-specific cut-offs available in the oncology literature 1

For Liver Disease Patients

• Men: LSMI < 50 cm²/m² indicates sarcopenia 1
• Women: LSMI < 39 cm²/m² indicates sarcopenia 1
• These cut-offs were derived from cirrhotic patients on liver transplant lists based on clinical outcomes 1

For Polycystic Liver Disease

• Men: LSMI < 52.4 cm²/m² indicates severe skeletal muscle depletion 1
• Women: LSMI < 38.5 cm²/m² indicates severe skeletal muscle depletion 1
• These values were defined in a cancer population but applied to PLD assessment 1

Important Technical Considerations

Measurement Precision and Quality

• Inter-observer variability and agreement for CT-based LSMI are excellent 1
• The technique provides reproducibility of approximately 2% 1
• Both initial and follow-up scans must use identical protocols, same software, and same patient positioning for valid comparisons 1

Additional Body Composition Data Available

• The same L3 CT slice can simultaneously provide skeletal muscle radiodensity (mean HU), which indicates muscle quality and presence of myosteatosis 1
• Visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) cross-sectional areas can also be calculated 1, 2
• Intermuscular adipose tissue (IMAT) can be quantified 2

Common Pitfalls to Avoid

• Do not use psoas muscle area alone as it correlates less strongly with total body protein than total skeletal muscle area and leads to greater misclassification of mortality risk 1
• Do not use alternate vertebral levels without validation as current evidence is inadequate to provide definitive recommendations for vertebral slices other than L3 4
• Do not rely on BMI or weight alone as sarcopenia frequently occurs in overweight or obese cancer patients (sarcopenic obesity), and excess body weight masks muscle depletion 1, 2
• Ensure the CT scan actually includes L3 as not all diagnostic scans based on tumor location will include this level 4

Clinical Significance

• LSMI values below the established cut-offs are strongly associated with mortality, surgical complications, and dose-limiting chemotherapy toxicity in cancer patients 1, 3
• Low LSMI independently predicts overall survival even after controlling for performance status and disease stage 5
• LSMI assessment enables identification of patients requiring intensive nutrition and exercise rehabilitation 1