Quantifying skeletal muscle wasting during chemoradiotherapy with Jacobian calculations for the prediction of survival and toxicity in patients with gastric cancer

Background and purpose This study aims to investigate changes in body composition during adjuvant chemoradiotherapy (CRT) quantitatively and visually in patients with gastric cancer and evaluate the correlation of the changes with survival and toxicity. Materials and methods A total of 182 gastric cancer patients who underwent surgery and adjuvant CRT were included. CT images at the level of the third lumbar vertebra (L3) were analyzed to assess the areas of muscles. We further proposed a framework to quantify muscle changes using Jacobian calculations. Jacobian value (Jac) was calculated for each voxel of muscle by measuring the ratio of muscle changes, where Jac <1 indicated shrinkage and Jac >1 indicated expansion. Variances between the pre- and post-CRT body composition were quantitatively computed, and their impact on toxicity and long-term outcomes were studied. Results The skeletal muscle index decreased from 44.4 ± 7.4 cm2/m2 to 43.6 ± 7.0 cm2/m2 after CRT. Skeletal muscle wasting and Jac 0.35 ≥ 5% were negatively associated with overall survival (p = 0.036 and p = 0.008, respectively). A new prediction model combining clinical variables and Jacobian features was constructed, and yielded a c statistic of 0.78 (95% CI 0.68–0.87). Jac 0.75 to Jac 1.15 (−25%