Comparative Analysis of Primary and Second Primary Multiple Myeloma: A Propensity Score-Matched Study.

Background: With the increasing number of cancer survivors, second primary malignancies (SPMs) are attracting clinical interest. Although SPMs following multiple myeloma (MM) have been studied, data on second primary multiple myeloma (SPMM) remain limited. This study aimed to compare the clinical characteristics and outcomes of SPMM with those of primary MM through a retrospective analysis. Methods: We retrospectively reviewed 183 patients with primary MM and 12 patients with SPMM treated at a single center between 2003 and 2022. To reduce selection bias, propensity score matching (1:3) was performed based on age, sex, year of MM diagnosis, and International Staging System stage. Survival outcomes were assessed using Kaplan-Meier analysis and Cox proportional hazards models. Results: After matching, 48 patients (36 with primary MM and 12 with SPMM) were included in the final analysis. At the time of MM diagnosis, 83.3% of patients with SPMM had achieved complete remission of their primary malignancy. All but one received standard MM treatment. The median overall survival (OS) was 45.1 months for the primary MM group and 41.5 months for the SPMM group. There was no statistically significant difference in OS between the groups (hazard ratio: 0.72; 95% confidence interval: 0.33-1.56). Conclusions: Patients with SPMM, most of whom had well-controlled primary cancers, received active treatment and demonstrated clinical outcomes not significantly different from those with primary MM. These findings support the use of aggressive treatment strategies for SPMM. Larger prospective studies are warranted to establish optimal treatment strategies.