肥満時代のがん悪液質：がん患者の筋肉量減少はBMIと独立した強力な予後予測因子であるという論文を紹介します。
Martin L, Birdsell L, Macdonald N, Reiman T, Clandinin MT, McCargar LJ, Murphy R, Ghosh S, Sawyer MB, Baracos VE. Cancer Cachexia in the Age of Obesity: Skeletal Muscle Depletion Is a Powerful Prognostic Factor, Independent of Body Mass Index. J Clin Oncol. 2013 Mar 25. [Epub ahead of print]
対象はがん患者（肺がんもしくは消化器がん）1473人です。筋肉量はCTでL3レベルの筋面積（腹筋、大腰筋、傍脊柱筋）を身長で2回割った骨格筋指数SMIで評価しています。同時に筋肉減衰muscle attenuation（筋肉内に脂肪が多いとCTで薄い）も評価しています。
結果ですが、生存期間に独立して関連があったのは、高度の体重減少、骨格筋指数低値、筋肉減衰低値（筋肉内に脂肪が多い）の3項目でした。3項目とも該当すると生存期間は8.4カ月でまったく該当しないと生存期間は28.4ヶ月でした。
以上より、CTによる筋肉量と筋肉の質（muscle quality）の評価は有用であり、現体重が肥満かるいそうかに関わらず、高度の体重減少、骨格筋指数低値、筋肉減衰低値で生存期間を予測できるという結論です。
図のように同じ骨格筋指数でもBMIが15.3と40.2の方がいたり、同じBMIでも骨格筋指数が33.7から58.3の方がいたり、同じBMIや骨格筋指数でも筋肉減衰が大きく異なったりしますので、身体計測だけでなく、画像による筋肉量と筋肉の質の評価は有用と言えます。
Abstract
PURPOSE
Emerging evidence suggests muscle depletion predicts survival of patients with cancer.
PATIENTS AND METHODS
At a cancer center in Alberta, Canada, consecutive patients with cancer (lung or GI; N = 1,473) were assessed at presentation for weight loss history, lumbar skeletal muscle index, and mean muscle attenuation (Hounsfield units) by computed tomography (CT). Univariate and multivariate analyses were conducted. Concordance (c) statistics were used to test predictive accuracy of survival models.
Results
Body mass index (BMI) distribution was 17% obese, 35% overweight, 36% normal weight, and 12% underweight. Patients in all BMI categories varied widely in weight loss, muscle index, and muscle attenuation. Thresholds defining associations between these three variables and survival were determined using optimal stratification. High weight loss, low muscle index, and low muscle attenuation were independently prognostic of survival. A survival model containing conventional covariates (cancer diagnosis, stage, age, performance status) gave a c statistic of 0.73 (95% CI, 0.67 to 0.79), whereas a model ignoring conventional variables and including only BMI, weight loss, muscle index, and muscle attenuation gave a c statistic of 0.92 (95% CI, 0.88 to 0.95; P < .001). Patients who possessed all three of these poor prognostic variables survived 8.4 months (95% CI, 6.5 to 10.3), regardless of whether they presented as obese, overweight, normal weight, or underweight, in contrast to patients who had none of these features, who survived 28.4 months (95% CI, 24.2 to 32.6; P < .001).
CONCLUSION
CT images reveal otherwise occult muscle depletion. Patients with cancer who are cachexic by the conventional criterion (involuntary weight loss) and by two additional criteria (muscle depletion and low muscle attenuation) share a poor prognosis, regardless of overall body weight.
Emerging evidence suggests muscle depletion predicts survival of patients with cancer.
PATIENTS AND METHODS
At a cancer center in Alberta, Canada, consecutive patients with cancer (lung or GI; N = 1,473) were assessed at presentation for weight loss history, lumbar skeletal muscle index, and mean muscle attenuation (Hounsfield units) by computed tomography (CT). Univariate and multivariate analyses were conducted. Concordance (c) statistics were used to test predictive accuracy of survival models.
Results
Body mass index (BMI) distribution was 17% obese, 35% overweight, 36% normal weight, and 12% underweight. Patients in all BMI categories varied widely in weight loss, muscle index, and muscle attenuation. Thresholds defining associations between these three variables and survival were determined using optimal stratification. High weight loss, low muscle index, and low muscle attenuation were independently prognostic of survival. A survival model containing conventional covariates (cancer diagnosis, stage, age, performance status) gave a c statistic of 0.73 (95% CI, 0.67 to 0.79), whereas a model ignoring conventional variables and including only BMI, weight loss, muscle index, and muscle attenuation gave a c statistic of 0.92 (95% CI, 0.88 to 0.95; P < .001). Patients who possessed all three of these poor prognostic variables survived 8.4 months (95% CI, 6.5 to 10.3), regardless of whether they presented as obese, overweight, normal weight, or underweight, in contrast to patients who had none of these features, who survived 28.4 months (95% CI, 24.2 to 32.6; P < .001).
CONCLUSION
CT images reveal otherwise occult muscle depletion. Patients with cancer who are cachexic by the conventional criterion (involuntary weight loss) and by two additional criteria (muscle depletion and low muscle attenuation) share a poor prognosis, regardless of overall body weight.
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