Individuals diagnosed with primary colorectal adenocarcinoma (CRA) face an elevated likelihood of developing second primary malignancies (SPMs). This study aimed to develop and validate prognostic nomograms for these patients to aid personalized follow-up and treatment planning.

We retrieved clinical data of patients diagnosed with primary CRA between 2004 and 2020 from the Surveillance, Epidemiology, and End Results (SEER) database. Patients were divided into only one primary malignancy (OOPM) and SPM group. The SPM cohort was randomly divided into training (70%) and validation (30%) sets. A Cox regression model identified independent prognostic factors, which were incorporated into a nomogram to predict 3-, 5-, and 10-year overall survival (OS). The model’s performance was evaluated using the concordance index (C-index), calibration plots, ROC curves, and decision curve analysis (DCA).

Among 187,204 patients with CRA, 18,668 (9.97%) developed SPMs. The most common SPM sites were the prostate, lung/bronchi, and colon in males and the breast, colon, and lung/bronchi in females. The final nomogram incorporated ten variables: age, sex, marital status, tumor size, primary tumor surgery, chemotherapy, and disease stage information (SEER stage, T, and N stages). The model demonstrated moderate discriminative ability, with C-indices of 0.635 and 0.634 in the training and validation sets, respectively. Calibration and DCA indicated good agreement between predicted and observed outcomes and clinical utility.

We present a nomogram that offers individualized survival estimates for CRA patients with SPMs. While further validation is warranted, this tool shows potential for improving risk assessment and supporting clinical decision-making.