Supplementary Material for: The Impact of Treatment Preferences in Second-Line Chemotherapy on the Prognosis of HER2-Negative Metastatic Breast Cancer

<i>Objective:</i> We assessed the impact of treatment preferences in second-line chemotherapy on breast cancer prognosis using the SELECT BC study. <i>Methods:</i> The SELECT BC study was performed in patients with HER2-negative metastatic breast cancer treated with initial chemotherapy. From these patients, 618 were assigned to 2 groups (S-1 group, 309; taxane group, 309). The S-1 and taxane groups were each subdivided into 3 groups: crossover group, protocol-recommended group, and other group, and the analysis of overall survival (OS) was performed using Cox regression with inverse probability weighting, to adjust for postrandomization confounding. <i>Results:</i> In the taxane group, the OS of the crossover group (39.6 months) was better than that of the protocol-recommended group (35.7 months) and the other chemotherapy group (36.9 months) (vs. the protocol-recommended group, HR 0.72 [95% CI 0.52-0.98], <i>p </i>= 0.037; vs. the other chemotherapy group, HR 0.71 [95% CI 0.43-1.18], <i>p = </i>0.183). In the S-1 group, there was no statistically significant difference in OS between the 3 groups. <i>Conclusion:</i> The study of the combination of first-line chemotherapy and second-line chemotherapy showed that S-1 might be recommended as a second-line chemotherapy in patients in whom taxane was the primary chemotherapy.