Characteristics and classification of first primary cancer patients with second primary cancer: a population-based cohort study.

Cancer survivors have an increased risk of developing subsequent primary tumors. However, the characteristics of first primary cancers (FPCs) with various types of second primary cancers (SPCs) are poorly understood, which hinders screening strategies. We analyzed data from 1,893,258 patients from the Surveillance, Epidemiology, and End Results (SEER) database to characterize and classify of FPC patients with subsequent SPCs at the pan-cancer level. In total, 3% of patients had SPC, with varied incidence rates observed depending on the types of FPC. Their onset patterns of SPC and diversity of SPC varied. Based on the diversity of the high-incidence sites of SPC, we classified FPCs into two categories: FPCs that require whole-body screening and those that need screening of particular body parts. Moreover, according to the different timing of high incidence of SPCs, our system classifies FPCs into two subtypes: FPCs that require long-term monitoring for the occurrence of SPCs and those that require screening at specific time points for SPCs. Furthermore, we identified 11 anatomical sites where over half of FPC types are prone to SPC occurrence at these locations. The risk factors for SPC occurrence in different FPC types and prognostic factors were also elucidated. Overall, we characterize and classify of FPC patients with subsequent SPCs at the pan-cancer level, which can guide the development of distinct screening strategies for each FPC type.