PURPOSE: the main purpose of this study was to diagnose pre-sarcopenia in cancer patients who had lack of computed tomography (CT) abdominal images, with a newly discovered method based on cervical images. MATERIAL AND METHODS: a sample of 37 patients with either lung cancer or a cancer that affected the upper digestive system underwent radiotherapy computed simulation which included measurements at C3 and L3 regions. Skeletal muscle mass (SMM) and skeletal muscle index (SMI) were determined by Hounsfield units and compared in both regions. Pre-sarcopenia was identified according to the cut-points currently established: ≤ 41 cm2/m2 in females, ≤ 43 cm2/m2 in males with a BMI ≤ 25 kg/m2, and ≤ 53 cm2/m2 in males with a BMI > 25 kg/m2. RESULTS: the correlation of SMM and SMI between the C3 and L3 regions was R2 = 0.876 and R2 = 0.805, respectively. Moreover, there was a positive association (86.49%) in terms of the diagnosis of pre-sarcopenia according to both regions. In total, eleven pre-sarcopenic patients (29.37%) were identified; three of them being overweight (27.27%) and two of them being obese (18.18%). CONCLUSION: a single sectional cross at the level of C3 can be used for the diagnosis of pre-sarcopenia. This new method avoids unnecessary irradiation, saves hospital costs and detects malnutrition before starting radiotherapy treatment in cancer patients who have lack of CT abdominal imaging.