Second Primary Malignancies in Gastrointestinal Cancer Survivors: Epidemiology, Risk Factors, and Future Prediction Models.

Advances in gastrointestinal (GI) cancer care have produced a growing population of long-term survivors at risk of second primary malignancies (SPMs). SPM definitions vary across studies, and SPMs remain underrepresented in cancer forecasting and prediction tools for GI cancer survivors. We conducted a narrative review across major GI cancer subtypes, summarizing site-specific SPM risk, key etiological pathways, and available prediction tools. Approximately one in ten colorectal cancer (CRC) survivors develops an SPM at ten years (pooled SIR ~1.15), a figure specific to CRC that does not generalize to other GI sites. Gastric cancer survivors carry excess risk of thyroid (O/E 2.00), esophageal, small intestinal, and pancreatic (O/E 1.60) malignancy, amplified by radiotherapy. Esophageal squamous cell carcinoma survivors show markedly elevated risk of upper aerodigestive tract tumors, consistent with field cancerization. Lynch syndrome (~16% second CRC risk at 10 years) and familial adenomatous polyposis (near-100% lifetime CRC risk without colectomy) confer some of the highest lifetime risks of metachronous GI malignancy reported to date. Most forecasting frameworks treat cancer incidence as a single event and do not adequately capture survivor-specific transitions, competing mortality, or germline susceptibility. GI cancer survivors form a distinct group whose risk is often overlooked. Harmonizing SPM definitions, extending multistate and competing-risk modeling, and integrating hereditary-risk stratification are near-term priorities for individualized surveillance.