High Ki-67 as an independent prognostic factor in extraskeletal osteosarcoma: a comparative cohort study

The objectives of this study are to compare clinicopathological features and outcomes of extraskeletal osteosarcoma (EOS) and conventional osteosarcoma (COS) and identify independent prognostic factors for disease-free survival (DFS) and overall survival (OS).

This study included 72 patients (Group EOS, n = 12; Group COS, n = 60) with a definitive diagnosis. Clinicopathological variables, SATB2 expression, and Ki-67 proliferation index were compared between groups. Outcomes were assessed using the Kaplan-Meier method and the log-rank test, and independent prognostic factors were identified by Cox regression analysis.

Group EOS had significantly higher median age (53.5 vs. 33.4 years, p < 0.001) and Ki-67 proliferation index than Group COS. Primary tumor location differed significantly (p < 0.001), with EOS cases affecting internal organs (kidneys, lungs) and COS cases mainly in long bones around the knee. Metastases were more common in Group EOS (83.3% vs. 13.6%, p < 0.001). Median OS and DFS for Group EOS were 19.3 months (vs. 71.6 months for COS, p = 0.007) and 2.2 months (vs. 21.6 months for COS, p < 0.001), respectively. Multivariate analysis identified age (OS: HR per year 1.04, 95% CI: 1.02-1.06, p < 0.001; DFS: HR 1.03, 95% CI: 1.02-1.05, p < 0.001) and Ki-67 proliferation index (OS: HR per %-point 1.02, 95% CI: 1.00-1.04, p = 0.038; DFS: HR 1.02, 95% CI: 1.01-1.04, p = 0.006) as independent prognostic factors.

Survival outcomes for patients with EOS were significantly worse than for those with COS. High Ki-67 proliferation index and advanced age were associated with a more aggressive clinical course of EOS. Therefore, more intensive follow-up and treatment are prudent for elderly EOS patients with a high Ki-67 proliferation index.