Background: The Peritoneal Surface Oncology Group International (PSOGI) recommends pathologic reporting of tumor cellularity in patients with pseudomyxoma peritonei (PMP) undergoing cytoreductive surgery and hyperthermic intraperitoneal chemoperfusion (CRS-HIPEC). We investigated the prognostic significance of PMP cellularity, or lack thereof (acellular mucin), following CRS-HIPEC. Methods: We reviewed clinical data for 310 CRS-HIPEC procedures in low-grade (American Joint Committee on Cancer grade G1) PMP with acellular mucin (n = 19), scant cellularity (n = 30), or moderate cellularity (n = 242). Kaplan–Meier survival curves and multivariate Cox regression models identified prognostic factors affecting oncologic outcomes. Results: Compared with patients with acellular mucin, those with scant and moderate cellularity had higher PCI and less-frequent complete macroscopic resection. After an estimated median follow-up of 49 months, 4 patients (14%) with scant cellularity and 127 patients (56%) with moderate cellularity progressed, while none of the patients with acellular mucin progressed. While the median progression-free survival (PFS) was not reached for patients with acellular mucin or scant cellularity (estimated 5-year PFS probability of 100 and 83%, respectively), patients with moderate cellularity demonstrated a median PFS of 32 months (estimated 5-year PFS probability of 27%). In a multivariate model, degree of disease cellularity, or lack thereof (acellular mucin), was an independent predictor of PFS but not overall survival. Conclusions: Early disease progression is unlikely in patients with acellular mucin undergoing CRS-HIPEC, as opposed to a 14% recurrence rate with scant cellularity. Thorough pathologic assessment for cellularity, or lack thereof (acellular mucin), is vital for accurate prognostication of disease progression for patients with low-grade PMP undergoing CRS-HIPEC.
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