It suggested that serum MMP-9, serum SAA, double lung V5, and V20 were independent risk factors for sCAP caused by radiotherapy for esophageal cancer; V10 (P=0.103) V30 (P=0.115) in both the lungs and MLD (P=0.437) were related to the occurrence of sCAP caused by radiotherapy for esophageal cancer, but they were not independent factors related to the occurrence of sCAP caused by radiotherapy for esophageal cancer, as shown in Table 4. This evidence concerns the gene MMP9 and esophageal cancer.