Second Primary Malignancies in Patients with Solid Tumors: The Prognostic Role of Magnesium Depletion Score

Background: Second primary malignancies (SPMs) have become an increasingly recognized clinical entity in modern oncology as advances in treatment prolong survival. This study aimed to investigate the incidence of SPMs in patients with solid tumors, to evaluate associated demographic and clinical factors, and to examine the potential prognostic role of the Magnesium Depletion Score (MDS). Methods: This retrospective cohort study included 1,027 patients diagnosed with solid tumors between 2010 and 2023, with complete follow-up data available. Patients were stratified according to the presence of SPM. Demographic, clinical, and lifestyle variables were compared. Survival analyses were performed using the Kaplan–Meier method, and risk factors associated with SPM were assessed using univariable logistic regression. Results: SPMs were identified in 7.1% of patients. Patients with SPMs were more frequently diagnosed at earlier stages (28.8% stage I) and had significantly longer overall survival compared to those without SPMs (median OS: 7.89 vs. 2.52 years; p < 0.05). Logistic regression analysis revealed that former smoking (OR = 1.99), presence of diabetes (OR = 1.78), and each one-unit increase in MDS (OR = 1.59) were significantly associated with SPM development. Advanced stage at initial diagnosis was inversely associated with the likelihood of SPM occurrence. Conclusion: SPMs are more commonly observed in patients diagnosed at an earlier stage and with longer survival. MDS may serve as a potential biomarker for predicting SPM risk. These findings highlight the importance of long-term follow-up and metabolic assessment in cancer survivors.