CT measurements of psoas muscle density (PMD) and area (PMA) (with or without indexing to height-squared or body-surface-area) are used interchangeably as sarcopenia measures - it is unknown which best correlates with surgical risk.

1.Determine the correlation between psoas muscle density, area, and indexed area

2.Identify which psoas measures most strongly associated with surgical outcomes

The University Hospital Geelong Colorectal database included all patients who underwent elective colorectal surgery from 2007-2014 (minimum five-years follow-up). Pre-operative CT scans were reviewed, psoas measures correlated with each other and with outcomes.

University Hospital Geelong is a regional referral hospital in Victoria, Australia.

This database listed 552 patients, 120 were excluded as pre-operative CT-films were not accessible, leaving 432 patients included.

Psoas muscle density, area, and area indexed by height-squared and body-surface-area.

Pearson correlations investigated correlations between psoas muscle measures. Logistic regression and ROC-analysis investigated each psoas measures association with peri-operative morbidity. Kaplan-Meier survival-analysis investigated the association of each psoas measure with long-term survival.

Mean age was 70.4 years, 41% were female.

Psoas muscle density correlated poorly with area (R²=0.15). Unindexed psoas muscle area correlated well with area indexed by height-squared (R²=0.950) and body-surface-area (R²=0.938).

Long-term survival was associated with psoas muscle density (HR1.515(95%CI 1.062-2.161)) and area (HR1.886(95%CI 1.322-2.692)).

Increasing psoas muscle density (reduced sarcopenia) was associated with decreased major-complications (OR0.963(95%CI 0.938-0.989)) and peri-operative mortality (OR0.903(95%CI 0.847-0.962)), with ROC-curve AUC=0.829 indicating an accurate test. There was no association between psoas muscle area and major-complications (OR1.000(95%CI 1.000-1.000)), nor peri-operative mortality (OR1.000(95%CI 0.999-1.001)), with ROC-curves AUC=0.507-0.521.

Indexed area measures were not associated with outcomes.

Psoas muscle density and area did not correlate. Both were associated with long-term survival, but only density was associated major-complications and mortality. Indexing removed the correlation of area with long-term survival.