PM5 - Defining ‘Complete Cytoreduction’ after Cytoreductive Surgery and Hyperthermic Intraperitoneal Chemotherapy (CRS/HIPEC) for the Histopathologic Spectrum of Appendiceal Carcinomatosis

INTRODUCTION The completeness of cytoreduction (CC) score, which quantifies residual tumor, is a major prognostic factor when treating appendiceal carcinomatosis with cytoreductive surgery and hyperthermic intraperitoneal chemotherapy (CRS/HIPEC). CC-0 and CC-1 are considered complete cytoreductions (CC-0/1) and associated with the best outcomes. We analyzed if the CC-0/1 definition is reliable across appendiceal histopathologic subtypes. METHODS A prospective database of CRS/HIPEC patients with appendiceal carcinomatosis from 1998-2019 was reviewed to identify patients with CC-0/1. Kaplan-Meier overall survival (OS) and progression-free survival (PFS) by CC-score for each histopathology were calculated. RESULTS Overall, 297 patients had CC-0/1 scores. Mean age was 54±12 years with 67% females. Histopathologic subtypes were 45% low-grade mucinous carcinoma peritonei (LGMCP), 27% high-grade MCP (HGMCP), 20% HGMCP with signet ring cells (HGMCP-S), and 8% goblet cell adenocarcinoma (GCAC). CC-0 and CC-1 occurred in 57% and 43% of LGMCP, 65% and 35% of HGMCP, 68% and 32% of HGMCP-S, and 79% and 21% of GCAC, respectively. CC-1 had significantly higher disease burden than CC-0 across all subtypes. OS and PFS were statistically longer for CC-0 versus CC-1 in HGMCP-S (p=0.001 & p=0.012, respectively) and GCAC (p<0.001 & p<0.001), but not for LGMCP (p=0.098 & p=0.398) or HGMCP (p=0.167 & p=0.356). CONCLUSIONS Survival outcomes for CC-0 and CC-1 after CRS/HIPEC are different for HGMCP-S and GCAC, but not for LGMCP and HGMCP. In HGCMP-S and GCAC, only CC-0 should be considered a ‘complete cytoreduction’ and analyzed separately from CC-1. This distinction is key for surgeons to improve outcomes in these patients.