Prognosis of stage IV bone cancer based on pattern of metastasis: a retrospective survival analysis of patients from the Surveillance, Epidemiology, and End Results (SEER) database

BackgroundPrognosis in bone cancer patients with metastatic disease is believed to vary based on site/pattern of spread. In 2003, the American Joint Committee on Cancer (AJCC) incorporated this observation into the TNM Staging System by subclassifying metastatic disease into M1a or M1b. We conducted a retrospective survival analysis of patients with primary bone cancer to characterize prognosis and assess outcomes in M1a versus M1b disease. MethodsThe Surveillance, Epidemiology and End Results (SEER) database was searched for cases of primary bone cancer presenting with metastasis from 2010 to 2015. Cases were grouped using AJCC metastatic staging as metastasis to the lung only (M1a) or other pattern of metastasis (M1b). Overall survival and cause-specific survival were assessed using Kaplan-Meier analysis and multivariate cox regression models. Multivariate models adjusted for multiple demographic, tumor characteristic, and treatment covariates. ResultsFive hundred and twenty-six cases met the inclusion criteria for this study. Two hundred and forty-eight were staged as M1a, and 278 were staged as M1b. Mean follow-up time for the cohort was 18.21 months (SD = 16.76). Fifty percent (124 of 248) of M1a and 59.4% (165 of 278) of M1b patients had died by the end of the study. Overall (P = .003) and cause-specific survival (P = .010) times were significantly lower for M1b patients via log-rank test. Adjusted analysis showed that M1b patients had poorer overall survival (HR, 1.505; 95% CI, 1.138-1.989; P = .004) and cause-specific survival (HR, 1.446; 95% CI, 1.091-1.918; P = .010) compared to M1a patients. ConclusionMetastasis pattern is an independent predictor of survival. M1a metastatic disease tends to have a better prognosis compared to M1b. This study supports the decision of the AJCC to subclassify metastatic disease for the purposes of staging and highlights the differences in prognosis between these two patterns of disease.