Introduction Primary mucinous ovarian cancer (PMOC) is a unique and rare subtype of ovarian cancer. In 2014, the World Health Organization introduced a new histologic classification by dividing PMOC into two subtypes: expansile or infiltrative. In this study, we investigated the clinical implications of their histological subtypes on survival outcomes.

Methods We identified patients with PMOC who had undergone primary surgery between 2003 and 2021. Patients with other types of ovarian cancer or severe comorbidities were excluded. We collected patients’ baseline characteristics, surgical details, and pathological information. Progression-free survival and overall survival were calculated, while prognostic factors were also investigated.

Results We included 131 patients in total. The median age was 50 years, and 103(78.6%) patients had stage I disease. During 55.9 months of median follow-up, there were 27 recurrences and 20 deaths. Among them, 113 patients were classified into 87(77%) expansile and 26(23%) infiltrative subtypes after the slide review. Advanced stage, lymph node involvement, and residual tumors after surgery were more common in the infiltrative subtype. The infiltrative group showed worse 5-year progression-free and overall survival rates (figure 1). In multivariate analyses, advanced stage and residual tumor after surgery were associated with worse prognosis, while the infiltrative subtype showed no statistical significance (table 1). In the subgroup analysis of stage I disease, there was no difference in survival between the two groups.

Conclusion/Implications In PMOC, the infiltrative histological subtype showed worse prognosis than the expansile subtype, with a higher proportion of advanced-stage tumors. However, it remains uncertain whether infiltrative subtype is an independent prognostic factor.

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Abstract EP276/#634 Figure 1

(A) PFS (B) OS according to histologic classification among all patients