5536 Background: Standard guidelines recommend adjuvant chemotherapy for stage I clear cell ovarian cancer (CCOC), despite data demonstrating excellent outcomes. Since 2012, the BC Cancer provincial treatment guidelines for surgically staged stage IA/B and IC1 (defined by intraoperative rupture only) CCOC has been to offer observation only. We reviewed the clinical outcomes of stage I CCOC patients since policy implementation. Methods: A retrospective, population-based cohort study of all stage I CCOC patients operated on between April 2012 and December 2017 was conducted. Patient, tumor, surgical and clinical outcome data were collected. Survival analysis was conducted using Kaplan-Meier methods. Results: 78 patients with stage I disease were identified (see Table). Among stage IC1 patients, 9 received adjuvant therapy despite provincial policy, 6 of which were due to sharp dissection. 40 patients with stages IA/B and IC1, who underwent post-operative observation, were included in the analysis. Median duration of follow-up was 36 months. Median age at diagnosis was 55 years and >50% patients had a Charlson Comorbidity Index of 0 (N= 26) and an Eastern Cooperative Oncology Group performance status of 0 (N=28) prior to diagnosis. Lymph node dissection was not performed in 20 patients. All 16 cases tested immunohistochemically for mismatch repair were intact, and 2 of 6 cases with tumour genomic sequencing had an AURKA aberration. There were 4 recurrences (10%), 3 of which were metastatic. 5-year disease-free survival is 90%, and 5-year overall survival is 95% for stage IA/B and 90% for stage IC1 (p=0.645). In comparison, 5-year overall survival for stage IC2 (surface involvement) and IC1 with sharp dissection (all received chemotherapy) is 82% and for stage IC3 (positive washings) is 23% (p<0.001). Conclusions: Outcomes of patients with stage I A/B and C1 CCOC remain excellent. Adjuvant therapy can be safely omitted, with low recurrence rates and survival over 90% at 5 years. Consideration of disease substage is valuable in predicting the clinical outcomes of stage I CCOC. [Table: see text]
PV-0548: The role of adjuvant therapy in stage IA serous and clear cell uterine cancer: a pooled analysis