Twelve-Month Metastatic State as a Landmark-Based Prognostic Stratifier in Metastatic Sarcoma

Background: Prognostic assessment in metastatic sarcoma remains challenging, and baseline variables may not adequately reflect outcome after metastatic onset. We investigated whether the 12-month landmark provides clinically relevant prognostic stratification for post-metastasis survival, while institution was examined as a contextual and adjustment variable rather than as the primary analytic focus. Methods: This retrospective observational study was based on prospectively collected real-world-time clinical data from two sarcoma centers. Overall survival (OS) and post-metastasis survival (PMS) were first analyzed descriptively across the two contributing centers using Kaplan–Meier methods. The principal analysis was a landmark-based prognostic stratification of PMS from 12 months after first metastatic documentation, with patients classified at the landmark as oligometastatic or polymetastatic. Univariable and multivariable Cox regression models were used to assess prognostic associations. Results: A total of 236 patients were included (135 at Institution A and 101 at Institution B). Crude OS was broadly comparable between institutions, whereas PMS showed a numerical separation. In the landmark analysis, metastatic state at 12 months clearly stratified subsequent PMS, with persistent oligometastatic disease associated with more favorable outcome than polymetastatic disease. In the reduced multivariable landmark model, polymetastatic state at 12 months remained the strongest independent predictor of inferior PMS (HR 3.09, 95% CI 1.87–5.09; p = 0.00001). High histologic grade also retained independent adverse prognostic significance (G3 vs. G1: HR 5.16, 95% CI 1.59–16.79; p = 0.006), whereas institution showed only a non-significant trend after adjustment (HR 0.67, 95% CI 0.42–1.07; p = 0.094). Conclusions: In metastatic sarcoma, realized metastatic state at 12 months provides stronger landmark-based prognostic stratification for post-metastasis survival than crude institutional comparison. These findings support a longitudinal state-based prognostic framework in which survival after metastatic onset is more informatively stratified by disease state at the 12-month landmark than by crude center attribution.