2.3. Definition and follow-up of SPM

The primary outcome of this study was the development of an SPM, which was defined as any type of malignant tumor occurring more than 6 months after the lymphoma. The SEER program adheres to the International Classification of Diseases for Oncology, Third Edition guidelines to distinguish SPMs from recurrent disease. To obtain comprehensive estimations for the risk of SPMs, we first estimated the risk for all types of combined SPMs and then separately estimated the risk in different types of malignant tumors, including the lung (respiratory system, lung and bronchus, pleura, trachea, mediastinum and other respiratory organs), central nervous system (CNS) (brain and other nervous system, brain, cranial nerves other nervous system), hepatobiliary pancreas (liver and Intrahepatic bile duct, liver, intrahepatic bile duct, gallbladder, other biliary, pancreas), urinary system (Urinary) (Urinary, urinary bladder, kidney, and renal pelvis, ureter, other urinary organs), leukemia (leukemia, lymphocytic leukemia, myeloid and monocytic leukemia, other leukemia), and digestive system (digestive system, esophagus, stomach, small intestine, colon, and rectum, rectum and rectosigmoid junction, anus, anal canal, and anorectum). The follow-up for SPMs began 6 months after lymphoma diagnosis and ended at the date of diagnosis of any SPM, all-cause death, or after 30 years of follow-up, whichever occurred first. The cutoff point for follow-up was defined as December 31, 2019 (November 2021 SEER data release).