Ovarian clear cell carcinoma (OCCC) is a distinct histologic subtype of epithelial ovarian cancer, comprising approximately 4 % to 15 % of cases [1,2]. The incidence of OCCC is notably higher among Asian women compared to Caucasian women, with a rising trend observed in South Korea since 1999 [3]. OCCC is characterized by unique clinical, histopathological, and molecular features that differentiate it from other epithelial ovarian cancer subtypes [4]. Patients with OCCC are generally younger and more likely to present with early-stage disease. However, advanced-stage OCCC exhibits aggressive behavior and marked resistance to conventional chemotherapy, resulting in significantly worse outcomes compared with other subtypes [[5], [6], [7]]. Therefore, OCCC is a distinct subtype with limited treatment options, representing a significant unmet clinical need and warranting the development of effective therapeutic strategies.

In the KEYNOTE-100 trial, pembrolizumab monotherapy demonstrated limited efficacy in recurrent ovarian cancer [overall response rate (ORR) 8.0 %], whereas a higher response rate was observed in the OCCC subgroup (15.8 %) [8]. Similarly, nivolumab and avelumab have shown greater activity in OCCC compared to other histologic subtypes [9,10]. In the NRG-GY003 trial comparing combination therapy with ipilimumab and nivolumab versus nivolumab alone in epithelial ovarian cancer, patients with OCCC demonstrated a 5-fold higher likelihood of response compared to other histologic subtypes [11]. However, given the limited number of patients with OCCC enrolled in the KEYNOTE-100 and NRG-GY003 trials, the response rates reported for this subgroup should be interpreted with caution. In the recent PEACOCC trial, pembrolizumab demonstrated clinical benefit in previously treated patients with advanced clear cell gynecologic cancers, with a 12-week progression-free survival (PFS) rate of 42 % (95 % CI, 28–57) and the best ORR rate of 25 % (95 % CI, 14–40), including 12 partial responses [12]. These findings suggest a potential role for immune checkpoint inhibitors (ICIs) as a therapeutic strategy in OCCC. Recent genomic studies have also suggested that mismatch repair (MMR) deficiency may be relatively enriched in the OCCC population, and because most earlier immunotherapy studies did not control for or assess MMR or other molecular features, these factors may have contributed to variability in treatment response.

Collectively, these findings suggest that ICIs may represent a promising therapeutic approach in OCCC. Building on this evidence, the present study was designed to evaluate the real-world efficacy and safety of ICIs in patients with recurrent OCCC in South Korea.