We assessed the impact and prognostic significance of alterations in muscle quality and quantity (myosteatosis and sarcopenia, respectively) in patients with esophageal cancer treated with radiotherapy (RT).We retrospectively pooled 258 patients with esophageal squamous cell cancer who underwent RT. Myosteatosis and sarcopenia were determined based on the skeletal muscle index derived from the muscle area and attenuation at the L3 level from computed tomography images. Subgroups were formed as 2 subgroups of non-sarcopenia/myosteatosis and sarcopenia/myosteatosis (with or without other muscle status) at either timepoint of RT, 3 subgroups of only-sarcopenia, only myosteatosis (without other muscle status), and the co-presence of sarcopenia and myosteatosis at either timepoint of RT, as well as 4 subgroups of continuous sarcopenia/myosteatosis, developed sarcopenia/myosteatosis, reduced sarcopenia/myosteatosis and non-sarcopenia/myosteatosis according to alterations of muscle status at both timepoints of RT. Overall survival (OS) was compared. Univariate and multivariate analyses based on Cox regression identified independent risk factors for prognosis.Either pre- or post-RT, patients with sarcopenia and myosteatosis (with or without other muscle status) had poor OS. Patients with only myosteatosis (without other muscle status) showed the best OS (1352 days pre-RT vs. 1648 days post-RT), while patients with concurrent myosteatosis and sarcopenia had the worst OS (907 days pre-RT vs. 706 days post-RT). The ascending order of OS for sarcopenia alterations was as follows: continuous sarcopenia (1093 days), non-sarcopenia (1740 days), developed sarcopenia (2187 days), and reduced sarcopenia (2208 days) (P = 0.002). The ascending order of OS for myosteatosis alterations was ranked as follows: continuous myosteatosis (1165 days), reduced myosteatosis (1275 days), developed myosteatosis (1783 days), and non-myosteatosis (1942 days) (P = 0.061). Univariate and multivariate Cox regression analyses revealed that increased age, longer tumor length, developed myosteatosis, and continuous myosteatosis were independent prognostic factors for OS.Muscle mass status at presentation and alterations in patients with esophageal cancer before and after RT should be considered prognostic indicators.Copyright © 2024. Published by Elsevier Inc.