Introduction: Retroperitoneal sarcomas (RPS) lie in the retroperitoneal space and are covered by a peritoneal layer. However, some RPS have an intraperitoneal component (IPC), which invades into the peritoneal cavity. The significance of such a clinical presentation is unknown. Methods: We retrospectively analyzed our prospectively maintained institutional database of RPS, along with intraoperative photographs taken to document the primary tumor extent at laparotomy. The effects of IPC on overall survival (OS), local recurrence (LR), and distant metastasis (DM) were evaluated. Results: IPC was present in 81 of 493 patients (16.4%). It was significantly associated with older age (64 vs. 59, p = 0.008), gender (67% vs. 33% males, p = 0.005), and multifocality (11.1% vs. 0.5%; p < 0.0001). IPC was not associated with size or any specific histology, while it showed a weak association with high malignancy grade (40.7% vs. 28.6% in G3 tumors; p = 0.076). At a median follow-up of 32 months IPC was associated with worse 5-year OS (54% vs. 74%, p < 0.001) and crude cumulative incidence (CCI) of LR (5-year CCI of LR: 38% vs. 19%, p = 0.001), but not to CCI of DM. However, multivariable models showed that IPC’s effect on OS (HR: 1.52, 95% CI 0.92–2.49, p = 0.1) and LR (HR: 1.34, 95% CI 0.8–2.26, p = 0.27) could be sufficiently explained by other known risk factors. Conclusions: IPC is associated with increased LR and decreased survival. However, the effect of IPC on prognosis is predominantly related to other tumor characteristics already included in published nomograms. IPC should not be a contraindication to a proper surgical resection.
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