Psoas muscle index is not representative of skeletal muscle index for evaluating cancer sarcopenia

Abstract

Background: A common method for diagnosing sarcopenia involves estimating the muscle mass by computed tomography (CT) via measurements of the cross-sectional muscle area (CSMA) of all muscles at the third lumbar vertebra (L3) level. Recently, single-muscle measurements of the psoas major muscle at L3 have emerged as a surrogate for sarcopenia detection, but its reliability and accuracy remain to be demonstrated.

Methods: This prospective cross-sectional study involved 29 healthcare establishments and recruited patients with metastatic cancers. The correlation between skeletal muscle index (SMI = CSMA of all muscles at L3/height² , cm² /m² ) and psoas muscle index (PMI = CSMA of psoas at L3/height² , cm² /m² ) was determined (Pearson's r). ROC curves were prepared based on SMI data from a development population (n = 488) to estimate suitable PMI thresholds. International low SMI cut-offs according to gender were studied for males (<55cm² /m² ) and for females (<39 cm² /m² ). Youden's index (J) and Cohen's kappa (κ) were calculated to estimate the test's accuracy and reliability. PMI cut-offs were validated in a validation population (n = 243) by estimating the percentage concordance of sarcopenia diagnoses with the SMI thresholds.

Results: Seven hundred and sixty-six patients were analysed (mean age 65.0 ± 11.8 years, 50.1% female). Low SMI prevalence was 69.1%. Correlation between the SMI and PMI for the entire population was 0.69 (n = 731, P < 0.01). PMI cut-offs for sarcopenia were estimated in the development population at <6.6cm² /m² in males and at <4.8 cm² /m² for females. The J and κ coefficients for PMI diagnostic tests were weak. The PMI cut-offs were tested in the validation population where 33.3% of the PMI measurements were dichotomously discordant.

Conclusions: A diagnostic test employing single-muscle measurements of the psoas major muscle as a surrogate for sarcopenia detection was evaluated but found to be unreliable. The CSMA of all muscles must be considered for evaluating cancer sarcopenia at L3.

Keywords: CT scan; Cancer sarcopenia; L3; Low muscle mass; Psoas; Skeletal muscle index.

Figure 1

Pearson's r coefficients for the correlation between the skeletal muscle index (SMI) and psoas muscle index (PMI) of both (a), right (B) and left (C) psoas muscles.

Figure 2

ROC curves based on SMI values of both psoas muscles together, for (A) males and (B) females with and without sarcopenia as per cut‐off 1.

Figure 3

Abdominal CT scan images taken at mid‐L3 applied to quantify SMI and PMI with discordant evaluation. Panel (A) shows a metastatic lung cancer female patient with a high SMI measured at 39.5 cm²/m² (>39 cm²/m²) but a low PMI at 3.6 cm²/m² (<4.8 cm²/m²). Panel (B) present a metastatic colon cancer male patient with a low SMI measured at 45.6 cm²/m² (<55 cm²/m²) but a high PMI at 8.2 cm²/m² (>6.6 cm²/m²).