Prognostic Ceiling Effect of the Peritoneal Cancer Index in Super-Extended Pseudomyxoma Peritonei of Appendiceal Origin: Impact of Anatomical Disease Distribution and Surgical Complexity.

BACKGROUND: Cytoreductive surgery with hyperthermic intraperitoneal chemotherapy is widely adopted as the principal treatment for selected patients with pseudomyxoma peritonei from low-grade appendiceal mucinous neoplasms. Prognostic uncertainty persists in patients with extreme peritoneal tumor burden. This study assessed predictors of survival in the overall cohort and explored whether established prognostic factors remain applicable in patients with super-extended disease and very high PCI. METHODS: Consecutive patients with pseudomyxoma peritonei from low-grade appendiceal mucinous neoplasms undergoing cytoreductive surgery with or without hyperthermic intraperitoneal chemotherapy were retrospectively analyzed. Survival analyses were performed in the overall cohort. An exploratory threshold analysis identified the most discriminative cut-off for the Peritoneal Cancer Index. In the super-extended PCI subgroup, compartmental disease saturation and additional complexity-enhancing resections were assessed; restricted mean survival time at 36 months was estimated with bootstrap confidence intervals. RESULTS: Seventy-three patients were included. In the overall cohort, higher PCI and incomplete cytoreduction independently predicted poorer overall survival; higher PCI, incomplete cytoreduction, and intraoperative complications predicted worse disease-free/progression-free survival. The optimal prognostic threshold was PCI 30. Among 20 patients with PCI greater than 30, the prognostic discrimination of PCI and completeness of cytoreduction attenuated, whereas supramesocolic complexity-enhancing resections were associated with poorer overall survival and reduced restricted mean survival time at 36 months. CONCLUSION: Beyond a Peritoneal Cancer Index of 30, global burden metrics become less informative. Outcomes appear more closely linked to disease topography, particularly in cases of extensive supramesocolic compartment involvement. This may identify a distinct high-risk phenotype.