Low skeletal muscle mass is a well-known prognostic factor for patients treated for a non-small-cell lung cancer by surgery or chemotherapy. However, its impact in patients treated by exclusive radiochemotherapy has never been explored. Our study tries to evaluate the prognostic value of low skeletal muscle mass and other antropometric parameters on this population. Clinical, nutritional and anthropometric date were collected for 93 patients treated by radiochemotherapy for a NSCLC. Anthropometric parameters were measured on the PET/CT by two methods. The first method was a manual segmentation at level L3, used to define Muscle Body Area (MBAL3), Visceral Fat Area (VFAL3) and Subcutaneous Fat Area (SCFAL3). The second method was an software (Anthropometer3D), allowing an automatic multislice measurement of Lean Body Mass (LBMAnthro3D), Fat Body Mass (FBMAnthro3D), Muscle Body Mass (MBMAnthro3D), Visceral Fat Mass (VFMAnthro3D), and Sub-Cutaneous Fat Mass (SCFMAnthro3D) on the PET/CT. All anthropometrics parameters were normalised by the patient's height. The primary end point was overall survival time. Univariate and then stepwise multivariate cox analysis were performed for significant parameters. Finally, Spearman's correlation between MBAL3 and MBMAnthro3D was assessed. Forty-one (44%) patients had low skeletal muscle mass. The median overall survival was 18 months for low skeletal muscle mass patients versus 36 months for non-low skeletal muscle mass patients (p = 0.019). Low skeletal muscle mass (HR = 1.806, IC95% [1.09-2.98]), serums albumin level < 35 g/l (HR = 2.203 [1.19-4.09]), Buzby Index < 97.5 (HR = 2.31 [1.23-4.33]), WHO score = 0 (HR = 0.59 [0.31-0.86] and MBMAnthro3D < 8.56 kg/m2 (HR = 2.36 [1.41-3.90]) were the only significant features in univariates analysis. In the stepwise multivariate Cox analysis, only MBMAnthro3D < 8.56 kg/m2 (HR = 2.16, p = 0.003) and WHO score = 0 (HR = 0.59, p = 0.04) were significant. Finally, muscle quantified by MBAL3 and MBMAnthro3D were found to be highly correlated (Spearman = 0.9). Low skeletal muscle mass, assessed on the pre-treatment PET/CT is a powerful prognostic factor in patient treated by radiochemotherapy for a NSCLC. The automatic software Anthropometer3D can easily identify patients a risk that could benefit an adapted therapy.