Prognostic impact of tumour deposits across N stages in stage III colon cancer: development and validation of a novel N staging system using the SEER database and a Chinese single-centre cohort

OBJECTIVES: To evaluate the prognostic significance of tumour deposit (TD) across different N stages in patients with stage III colon cancer and to develop and validate a novel N staging system that incorporates TD count to improve prediction of cancer-specific survival (CSS). DESIGN: Retrospective cohort study based on population-based data and external validation. SETTING: Secondary and tertiary care settings; data from the SEER database, a population-based cancer registry capturing cancer incidence and survival information (USA, 2010-2017); and a single-centre validation cohort from South China (2015-2019). PARTICIPANTS: A total of 8739 stage III colon cancer patients from the SEER database who underwent curative surgery were included; 1335 (15.3%) had TD. Patients with