Trends in the management and prognosis of mucinous borderline ovarian tumors: analysis of 12,766 cases from the JSOG Gynecologic Tumor Registry (2004–2018)

Background Mucinous borderline ovarian tumors (MBOTs) are rare neoplasms with excellent prognosis, yet the optimal surgical extent remains controversial. No large-scale study in Japan has evaluated treatment trends and prognostic factors for MBOTs. This study aimed to clarify their clinicopathological features, management patterns, and survival outcomes using a nationwide registry.

Data were obtained from the Japan Society of Obstetrics and Gynecology Gynecologic Tumor Registry, including 96,476 ovarian tumors treated between 2004 and 2018. Among them, 12,766 MBOT cases were identified. Surgical pro- cedures—hysterectomy, omentectomy, lymphadenectomy, and adjuvant chemotherapy—were analyzed. Survival analyses of 8564 cases with complete prognostic data were performed using Kaplan–Meier and Cox proportional hazards models.

Over 90% of MBOTs were stage I, and the median age was 52 years. Hysterectomy was performed in 50.8%, omen- tectomy in 57.9% (2015–2018 subset), and lymphadenectomy in 7.6%. Only 2.6% received adjuvant chemotherapy. The 5-year overall survival exceeded 95%. Multivariate analysis identified age ≥ 50 years (HR 2.5, 95% CI 1.8–3.6) and stage IC (HR 2.7, 95% CI 1.9–3.6) as independent adverse factors. Omentectomy showed a marginal survival benefit (HR 0.6, p = 0.05), whereas hysterectomy, lymphadenectomy, and chemotherapy conferred no advantage. Chemotherapy correlated with poorer outcomes, likely due to confounding by indication.

This nationwide cohort—the largest MBOT series reported to date—demonstrates conservative management with excellent prognosis in Japan. Radical surgery and chemotherapy provide no survival benefit, whereas fertility-sparing surgery appears appropriate for younger patients.

Mucinous borderline ovarian tumor · JSOG registry · Omentectomy · Survival analysis · Fertility-sparing surgery · Nationwide cohort Hideki Tokunaga and Yusuke Shibuya contributed equally to this work. * Hideki Tokunaga [email protected] 1 Division of Obstetrics and Gynecology, Faculty of Medicine, Tohoku Medical and Pharmaceutical University, Miyagi, Japan 2 Department of Gynecology, Tohoku University Hospital, Miyagi, Japan 3 Department of Obstetrics and Gynecology, Keio University School of Medicine, Tokyo, Japan 4 Department of Information Science, Iwate Medical University, Iwate, Japan 5 Department of Healthcare Administration, Nagoya University Graduate School of Medicine, Nagoya, Japan 6 Department of Obstetrics and Gynecology, Hirosaki University Graduate School of Medicine, Aomori, Japan 7 Department of Obstetrics and Gynecology, University of Occupational and Environmental Health, Fukuoka, Japan 8 Department of Obstetrics and Gynecology, Nihon University School of Medicine, Tokyo, Japan 9 Department of Obstetrics and Gynecology, Yamagata University Faculty of Medicine, Yamagata, Japan 786 International Journal of Clinical Oncology (2026) 31:785–793

In addition to bilateral adnexal resection, total hysterec- tomy, and omentectomy for malignant ovarian tumors, pelvic and para-aortic lymph node dissection, ascitic (peritoneal) cytology, and intraperitoneal exploration are performed to determine the extent of disease and assess advanced stages. Some distant metastases are confirmed pathologically, but most are identified through diagnostic imaging. For borderline ovarian tumors, routine lymph node dis- section is not recommended [1 ]. Apart from the LION study [2 ], no randomized controlled trials have evaluated the necessity of lymphadenectomy in advanced ovarian cancer. Current recommendations are, therefore, based on the retrospective detection rates of occult metastases, meta-analyses, and their impact on prognosis [1 ]. In summary, it is only weakly recommended to per - form basic surgical staging procedures beyond resection of the affected adnexa. To date, no nationwide or long-term large-scale study has evaluated mucinous borderline ovar - ian tumors in Japan, and real-world treatment strategies remain largely undefined. The Japan Society of Obstetrics and Gynecology (JSOG) began registering cases of borderline and malig- nant ovarian tumors in the Gynecologic Tumor Registry (GTR) starting in 1998. This registry collects data on clinicopathological features and survival outcomes, with follow-up surveys performed three and five years after registration. Histological classification follows the WHO system, and staging is based on the International Federa- tion of Gynecology and Obstetrics (FIGO) classification; both are updated in accordance with each revision. In the present study, we analyzed large-scale data from 2004 to 2018, when five-year prognostic follow-up was completed, to investigate treatment trends in mucinous borderline ovarian tumors and to evaluate the prognostic impact of surgical procedures other than adnexal resection.

Patients This study included 96,476 patients with ovarian tumors treated at medical facilities across Japan and registered in the Japan Society of Obstetrics and Gynecology (JSOG) Gynecologic Tumor Registry (GTR) between 2004 and 2018. Major hospitals throughout Japan participate in this registry, which is estimated to cover approximately 70–80% of all ovarian cancer cases nationwide. After receiving approval from the Ethics Committees of the JSOG (approval no. 17) and Keio University School of Medicine (approval no. 20170261), data on clinico- pathological characteristics and survival outcomes were collected. Patients who did not undergo surgery or who received preoperative chemotherapy were excluded from the prognostic analysis. Cases with incomplete clinical information—such as missing stage data or unavailable prognostic outcomes—were also excluded.

The study flow diagram illustrates the selection and strati- fication of the cohort (Fig.  1). A subset of 8564 cases with complete prognostic information was included in the sur - vival analysis. The registry data included: age at treatment initiation, FIGO stage (1988 or 2014), pTNM classification according to the FIGO system, whether preoperative chemotherapy was administered, surgical procedures (adnexectomy, hys- terectomy, omentectomy, and lymphadenectomy), residual tumor status (surgical completeness), sites of distant metas- tasis, whether adjuvant chemotherapy or second-look sur - gery was performed, and ypTNM classification. Follow-up surveys are conducted three and five years after the year of registration to determine disease-free survival, alive-with- disease, and death outcomes. Since 2017, the registry has been updated to reflect the WHO 2014 histological clas- sification, which redefined mucinous tumors as intestinal type and seromucinous type. Data prior to 2016 followed the WHO 2003 classification, which did not distinguish between intestinal and endocervical types; therefore, these earlier data cannot be directly reclassified under the WHO 2014 system. Beginning in 2015, staging was based on FIGO 2014; however, before 2014, the size of lymph node metastases was not recorded, preventing precise stage con- version. Until 2014, surgical procedures were categorized only as “adnexectomy” or “radical surgery,” and informa- tion on omentectomy was unavailable. Because “radical sur- gery” was defined as including hysterectomy, the presence or absence of hysterectomy could still be determined. For this study, seromucinous tumors diagnosed after 2015 were excluded from the survival analysis. Statistical analysis All statistical analyses were performed using JMP software, version 19.0.1 (SAS Institute Inc., Cary, NC, USA). The univariate analyses for overall survival (OS) were conducted using the Kaplan–Meier method and the log-rank test. The multivariate analyses were performed using the Cox pro- portional hazards model, incorporating available prognostic factors (age, surgical stage, surgical procedure, and adjuvant 787International Journal of Clinical Oncology (2026) 31:785–793 chemotherapy). To minimize selection bias, additional analyses were conducted using multivariate logistic regres- sion and Cox regression with propensity score matching. In all analyses, a p value < 0.05 was considered statistically significant.

Between 2004 and 2018, a total of 96,476 ovarian neo- plasms were registered from 216 to 398 institutions (Table  1). Mucinous borderline ovarian tumors (MBOTs) accounted for 12,766 cases (13.2%), showing a steady increase in absolute number—from 466 cases in 2004 to 1192 in 2018. Seromucinous borderline tumors were reported in 296 patients in 2017 and 305 in 2018. Both the number of participating institutions and registered cases increased over time, indicating broader registry coverage and improved data quality. The proportion of MBOTs rose slightly from 12.1% in 2004 to a peak of 14.4% in 2013, then stabilized around 13% thereafter. The average number of MBOT cases per institution increased from 2.16 to 3.17 per year, suggesting both improved detection and wider participation (Fig.  2). By contrast, the number of mucinous carcinomas (MCs) ranged from 350 to 620 cases annu- ally, without a clear upward trend. The proportion of MCs among all ovarian tumors declined from approximately 10% (2004–2006) to 6% (2017–2018), and the number of MCs per institution decreased from 1.9 to 1.4 per year, Fig. 1 Flow diagram of this study. Patients with ovarian, fallopian tube, and primary peritoneal tumors registered in the JSOG gyneco- logic cancer registry between 2004 and 2018 (n = 96,476) were screened. After exclusion of cases with insufficient clinicopatho- logic or follow-up data, 12,766 mucinous borderline ovarian tumors (MBOTs) were included in the clinicopathologic analysis, and 8564 cases were eligible for prognostic evaluation Table 1 Annual trend in the number and proportion of mucinous borderline ovarian tumors among all ovarian tumors (2004–2018) Year 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total Registered patients 3853 3490 4041 4359 4820 5277 5678 6102 6902 7718 7860 8646 9090 9383 9257 96,476 Mucinous borderline tumor 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766 Seromucinous borderline tumor 296 305 Mucinous carcinoma 379 355 385 398 466 461 478 551 559 619 568 622 565 586 540 Institutions 216 192 197 212 227 241 254 279 319 366 382 386 398 393 376 788 International Journal of Clinical Oncology (2026) 31:785–793 suggesting a relative reduction in the malignant mucinous component among newly registered ovarian tumors. Patient characteristics Table  2 summarizes the clinicopathologic characteristics of patients with MBOT. The median age was 52 years (range, 11–97) between 2004 and 2014, and 53 years (range, 11–96) between 2015 and 2018. Most tumors were FIGO stage I (68.5% under 1988 criteria; 64.6% under 2014 criteria), and higher-stage disease (stage II–IV) was rare (< 5%). Hys- terectomy was performed in 6484 cases (50.8%), whereas 6282 patients (49.2%) did not undergo hysterectomy. Omen- tectomy (data available for 2015–2018) was performed in 2791 cases (57.9%). Lymphadenectomy was carried out in 973 cases (7.6%) and adjuvant chemotherapy in 334 patients (2.6%), reflecting the indolent nature of MBOTs. Regarding outcomes, 8273 patients (64.8%) were alive without disease, 131 were alive with disease, 212 had died of disease, and 176 had died of other causes; outcomes were unknown for 3917 patients. Surgical procedures and trends The proportion of patients undergoing hysterectomy increased gradually until approximately 2013 and then plateaued (Table  3). Hysterectomy was predominantly performed in patients aged ≥ 50 years, who accounted for approximately 52% of all cases. Among younger women (< 40 years), fertility-sparing surgery (without hysterec- tomy) was more common, consistent with current clinical practice trends. In the subset of 4822 patients (2015–2018), 2791 (57.9%) underwent omentectomy (Table 4). Omentectomy was more frequent in patients aged ≥ 40 years and in those with higher FIGO stages (particularly IC1–IC3). Yearly data revealed a modest increase from 52.5% (2015) to 62.4% (2018), suggesting growing adherence to comprehensive surgical staging. Lymphadenectomy was performed in 973 cases (7.6%), showing no apparent upward trend during the 15-year period (Table  5). The proportion remained below 10% even after 2010, indicating that lymphadenectomy is not routinely performed for MBOTs. Most procedures were conducted in women aged 40–60 years. Adjuvant chemotherapy was administered in 334 patients (2.6%) overall (Table  6), without a significant temporal increase between 2004 and 2018. Chemotherapy was mainly used for stage IC–III disease, whereas stage IA/IB patients rarely received it. Among those treated in 2015–2018, only 91 patients (1.9%) received chemotherapy, reflecting a grow- ing trend toward surgery alone for borderline tumors. Survival analysis The Kaplan–Meier curve (Fig.  3) demonstrated excellent long-term survival for MBOT patients, with a five-year over- all survival rate exceeding 95%, consistent with the indolent behavior of these tumors. Only a small number of deaths were observed, indicating very low disease-specific mortality. In univariate analysis (Table  7), both older age (≥ 50 years) and higher FIGO stage (IC) were significantly associated with poorer overall survival (HR = 2.2, 95% CI 1.6–3.1, p < 0.0001; and HR = 2.7, 95% CI 2.0–3.7, p < 0.0001, respectively). Omentectomy showed a marginal trend toward improved survival (HR = 0.6, 95% CI 0.4–1.0, p = 0.06), whereas hysterectomy, lymphadenectomy, and adjuvant chemotherapy were not significantly associated with OS in the univariate model. Fig. 2 Annual trends of muci- nous tumors among ovarian neoplasms. MBOT/all and MC/ all indicate the proportions of mucinous borderline tumors and mucinous carcinomas among all registered ovarian tumors, respectively. MBOT/institution and MC/institution indicate the number of patients per partici- pating institution 789International Journal of Clinical Oncology (2026) 31:785–793 Table 2 Distribution of clinicopathologic characteristics of patients with MBOT (age, FIGO stage, and surgical procedures) NAC neoadjuvant chemotherapy 2004_2014 7944 2015_2018 4822 Age 52 (11–97) 53 (11–96) FIGO stage (1988) n % FIGO stage (2014) n % I I  Ia 5439 68.5 IA 3114 64.6  Ib 91 1.1 IB 44 0.9  Ic IC   Ic(a) 558 7.0 IC1 988 20.5   Ic(b) 1427 18.0 IC2 398 8.3   Ic(1) 48 0.6 IC3 143 3.0   Ic(2) 121 1.5 II II IIA 14 0.3  IIa 10 0.1 IIB 25 0.5  IIb 13 0.2 III  IIc IIIA1 1 0.0   IIc(a) 22 0.3 IIIA2 8 0.2   IIc(b) 17 0.2 IIIB 19 0.4   IIc(1) 3 0.0 IIIC 20 0.4   IIc(2) 6 0.1 IVA 5 0.1 III IVB 2 0.0  IIIa 20 0.3 NAC 3 0.1  IIIb 27 0.3 Unknown 38 0.8  IIIc 83 1.0 IV 16 0.2 NAC 24 0.3 Unknown 19 0.2 Surgical procedure  Hysterectomy   Yes 6484   No 6282  Omentectomy  2004_2014 N/A  2015_2018   Yes 2791   No 2031  Lymph node dissection   Yes 973   No 11,793  Adjuvant chemotherapy   Yes 334   No 12,432  Prognosis   Alive 8273   Alive with disease 131   Dead 212   Death from other causes 176   Unknown 3917 790 International Journal of Clinical Oncology (2026) 31:785–793 In the multivariate Cox regression analysis (Table  7), age ≥ 50 years and stage IC remained independent adverse prognostic factors (HR = 2.5, 95% CI 1.8–3.6, p < 0.0001; and HR = 2.7, 95% CI 1.9–3.6, p < 0.0001, respectively). Omentectomy was not independently associated with bet- ter overall survival (HR = 0.6, 95% CI 0.4–1.0, p = 0.05). Neither hysterectomy nor lymphadenectomy significantly affected survival. Interestingly, adjuvant chemotherapy for stage ≥ IC was associated with worse overall survival (HR = 2.6, 95% CI 1.4–4.8, p = 0.002**) after adjustment for confounders. Table 3 Trends in hysterectomy among patients with mucinous borderline ovarian tumors Age 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s N/A Total  Yes 1 31 340 1439 1794 1616 952 298 11 2 6484  No 266 1166 1569 874 689 743 593 337 45 0 6282 Total 267 1197 1909 2313 2483 2359 1545 635 56 2 12,766 Year 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total  Yes 203 179 200 228 248 321 369 429 522 565 616 594 648 683 679 6282  No 263 218 252 334 337 373 400 411 436 549 491 555 588 562 513 6484 Total 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766 ≥ 50 years Year 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total  Yes 128 129 145 171 177 240 260 329 376 395 456 414 458 484 510 4672  No 88 77 76 113 127 144 151 165 164 203 190 214 233 246 216 2407 Total 216 206 221 284 304 384 411 494 540 598 646 628 691 730 726 7079 Table 4 Omentectomy rates by age, year and FIGO stage (2015–2018 subset) Age 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s Total  Yes 33 169 285 592 587 604 392 124 5 2791  No 63 235 350 320 315 330 240 156 22 2031 Total 96 404 635 912 902 934 632 280 27 4822 Year 2015 2016 2017 2018 Total  Yes 604 713 730 744 2791  No 545 523 515 448 2031 Total 1149 1236 1245 1192 4822 Stage IA IB IC1 IC2 IC3 IIA IIB IIIA1 IIIA2 IIIB IIIC IVA IVB N/A Total  Yes 1759 27 536 267 115 11 19 1 7 11 15 5 2 16 2791  No 1355 17 452 131 28 3 6 0 1 8 5 0 0 25 2031 Total 3114 44 988 398 143 14 25 1 8 19 20 5 2 41 4822 Table 5 Lymphadenectomy rates by age and year group Age 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s N/A Total  Yes 2 34 116 221 265 214 110 11 0 0 973  No 265 1163 1793 2092 2218 2145 1435 624 56 2 11,793 Total 267 1197 1909 2313 2483 2359 1545 635 56 2 12,766 Year 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total  Yes 42 41 42 63 56 58 73 66 81 94 95 75 84 65 38 973  No 424 356 410 499 529 636 696 774 877 1020 1012 1074 1152 1180 1154 11,793 Total 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766 791International Journal of Clinical Oncology (2026) 31:785–793

This study represents the largest nationwide, registry- based cohort analysis to date investigating mucinous bor - derline ovarian tumors (MBOTs). Using data from the JSOG Gynecologic Tumor Registry, it provides the most comprehensive overview of MBOTs in Japan. Between 2004 and 2018, a total of 12,766 patients were regis- tered, demonstrating that MBOTs constituted 13–15% of all ovarian borderline tumors, with a remarkably stable incidence over time (Table  1). More than 90% of cases were diagnosed at FIGO stage I, and the median age was approximately 52 years—slightly older than that reported in European cohorts (40–45 years) [3 , 4]. This finding sug- gests that MBOTs in Japan are more frequently detected in peri- or postmenopausal women, possibly reflecting differ - ences in screening practices and surgical decision-making. Surgical practice patterns Approximately half of all patients underwent hysterectomy, and over 60% underwent bilateral oophorectomy, whereas omentectomy was documented in 58% of cases after 2015 (Table  4). Lymphadenectomy was rare (< 10%) (Table  5), and adjuvant chemotherapy was used in only 2–3% of patients (Table  6). These data indicate that Japanese clini- cal practice is characterized by relatively conservative adju- vant treatment but still frequent use of hysterectomy, even in early-stage disease. In contrast, Western guidelines and registry data—such as the cohort study of the AGO Study Group [3 ]—support fertility-sparing surgery for reproductive-age women without compromising survival [ 5, 6]. Our findings reinforce that radical surgery confers no survival advantage in patients with MBOTs. Comparison with previous multicenter studies The present results closely align with those of a multicenter study by Gungorduk et al. [4 ], which analyzed 364 MBOT patients across 14 institutions in Turkey and Germany and found no independent prognostic effect of omentectomy, appendectomy, lymphadenectomy, or radical surgery on either progression-free or overall survival. In their study, Table 6 Proportion of patients receiving adjuvant chemotherapy and corresponding age, year, and FIGO stage distribution Age 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s N/A Total  Yes 5 25 36 70 95 66 30 5 1 1 334  None 262 1172 1873 2243 2388 2293 1515 630 55 1 12,432 Total 267 1197 1909 2313 2483 2359 1545 635 56 2 12,766 Year 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017  Yes 43 14 15 17 15 30 25 14 26 20 24 26 26 19 20 334  No 423 383 437 545 570 664 744 826 932 1094 1083 1123 1210 1226 1172 12,432 Total 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766 2004_2014  Stage Ia Ib Ic IIa IIb IIC IIIa IIIb IIIc IV N/A Total   Yes 60 5 125 2 1 13 5 5 13 6 8 243   No 5379 86 2029 8 12 35 15 22 70 10 35 7701  Total 5439 91 2154 10 13 48 20 27 83 16 43 7944 2015_2018  Stage IA IB IC IIA IIB IIIA IIIB IIIC IVA IVB N/A   Yes 18 1 39 2 5 2 6 10 3 2 3 91   No 3096 43 1490 12 20 7 13 10 2 0 38 4731  Total 3114 44 1529 14 25 9 19 20 5 2 41 4822 Fig. 3 Overall survival curves of mucinous borderline ovarian tumors according to FIGO stage 792 International Journal of Clinical Oncology (2026) 31:785–793 the median age was 43 years, more than 75% of patients had stage IA disease, and the 5-year overall survival rate exceeded 95%. Similarly, in our multivariate analysis, only older age (≥ 50 years) and advanced stage (IC vs. IA/IB) were identi- fied as independent adverse prognostic factors (HR 2.5 and 2.7, respectively), whereas hysterectomy, lymphadenectomy, and chemotherapy provided no survival benefit (Table 7). It is also conceivable that the biological background of muci- nous borderline ovarian tumors differs between younger and postmenopausal patients, including differences in driver genetic alterations. Mucinous tumors are frequently char - acterized by KRAS mutations; however, age-related differ- ences in genomic complexity, hormonal milieu, immune surveillance, and accumulation of somatic alterations may influence tumor behavior and clinical outcomes. Such biological heterogeneity may partly explain the inconsist- ent prognostic impact of age reported in previous studies, including reports suggesting younger age as a risk factor for recurrence [7 ]. In this context, younger age may be more closely associated with recurrence risk, whereas older age may adversely affect overall survival through host-related factors rather than intrinsic tumor aggressiveness. Omentec- tomy showed a marginal association with improved survival (p = 0.05), which likely reflects selection bias or more com- plete staging rather than a therapeutic effect. Omentectomy and appendectomy The role of omentectomy in MBOT remains controver - sial. In the study by Gungorduk et al., omental involve- ment was identified in only 1.4% of cases, and appendiceal involvement in 9.1%, mostly in macroscopically abnormal appendices [4 ]. Similarly, two other systematic reviews concluded that routine appendectomy is unnecessary unless gross abnormalities are observed [8 , 9]. Although our registry did not capture appendectomy data, the low incidence of advanced disease and the excellent survival outcomes suggest that routine appendectomy and systematic omentectomy are unlikely to improve prognosis. Lymphadenectomy and adjuvant chemotherapy Lymphadenectomy was performed in fewer than 10% of patients, consistent with its limited clinical utility given the extremely low incidence (< 2%) of nodal metastasis reported in prior studies [10, 11]. Our findings confirm that lymphad- enectomy had no significant influence on overall survival (HR 0.8, p = 0.5). Adjuvant chemotherapy was rarely administered and was associated with worse survival in the multivariate analysis (HR 2.6, p = 0.002), likely due to confounding by indication. Previous meta-analyses have consistently shown no ben- efit from platinum-based chemotherapy for borderline ovar- ian tumors [12–14]. Clinical implications and future directions Taken together, our results support that the extent of surgi- cal staging does not influence outcomes in MBOT patients. The excellent prognosis (> 98% 5-year survival) and very low incidence of extraovarian spread underscore the indolent biological nature of these tumors. Table 7 Results of univariate and multivariate analyses for overall survival of MBOT patients (Cox proportional hazards model) Univariate Overall survival Hazard ratio 95%CI p value Age (= 50 years) 2.2 1.6–3.1 < 0.0001 Stage (IA,IB vs. IC) 2.7 2.0–3.7 = IC) 1.5 0.7–3.5 0.3 Multivariate Overall survival Hazard ratio 95%CI p-value Age (= 50 years) 2.5 1.8–3.6 < 0.0001 Stage (IA,IB vs. IC) 2.7 1.9–3.6 = IC) 2.6 1.4–4.8 0.002 793International Journal of Clinical Oncology (2026) 31:785–793 The main limitation of this study is its registry-based design, which relies on voluntarily reported data and thus contains missing information on some clinicopathological variables and outcomes. Detailed information regarding recurrence, including the timing of recurrence and specific recurrence patterns, is not available in the registry database. In addition, performance status was not collected. There- fore, while the present analysis allows evaluation of treat- ment selection trends according to patient age and treatment era, it is difficult to precisely investigate the exact causes of recurrence or death among patients with poor prognosis. Although the data accuracy cannot be fully guaranteed, the large sample size of over 12,000 cases provides sufficient statistical power and reliability. In conclusion, fertility-sparing surgery should be rec- ommended for younger women, and routine omentectomy, appendectomy, and lymphadenectomy should be reevalu- ated. Future registry-linked analyses integrating pathological review, molecular profiling, and clinical outcomes are war- ranted to refine surgical strategies for MBOTs and to clarify the prognostic relevance of histological subtypes (intestinal vs. seromucinous).

We would like to thank the committee and all participants of the Japan Society of Obstetrics and Gynecology (JSGO) cancer registry program. Funding Open Access funding provided by Tohoku Medical and Pharmaceutical University. No sources of funding were used to assist in the preparation of this study. Data availability Part of the data used in this study has been published annually in the Journal of Obstetrics and Gynaecology Research (JOGR) as part of the annual reports of the Japan Society of Obstetrics and Gynecology (JSOG). Declarations Conflict of interest All authors have no conflicts of interest in relation to this study. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons. org/licenses/by/4.0/.

1. Tokunaga H, Mikami M, Nagase S et al (2021) The 2020 Japan Society of Gynecologic Oncology guidelines for the treatment of ovarian cancer, fallopian tube cancer, and primary peritoneal cancer. J Gynecol Oncol 32:e49 2. Harter P, Sehouli J, Lorusso D et al (2019) A randomized trial of lymphadenectomy in patients with advanced ovarian neoplasms. N Engl J Med 380:822–832 3. du Bois A, Ewald-Riegler N, de Gregorio N et al (2013) Border - line tumours of the ovary: a cohort study of the Arbeitsgmein - schaft Gynakologische Onkologie (AGO) Study Group. Eur J Cancer 49:1905–1914 4. Gungorduk K, Asicioglu O, Braicu EI et al (2017) The impact of surgical staging on the prognosis of mucinous borderline tumors of the ovaries: a multicenter study. Anticancer Res 37:5609–5616 5. Chan JK, Lin YG, Loizzi V et al (2003) Borderline ovarian tumors in reproductive-age women. Fertility-sparing surgery and out- come. J Reprod Med 48:756–760 6. Cadron I, Leunen K, Van Gorp T et al (2007) Management of borderline ovarian neoplasms. J Clin Oncol 25:2928–2937 7. Khunamornpong S, Settakorn J, Sukpan K et al (2011) Muci - nous tumor of low malignant potential (“borderline” or “atypical proliferative” tumor) of the ovary: a study of 171 cases with the assessment of intraepithelial carcinoma and microinvasion. Int J Gynecol Pathol 30:218–230 8. Kleppe M, Bruls J, Van Gorp T et al (2014) Mucinous borderline tumours of the ovary and the appendix: a retrospective study and overview of the literature. Gynecol Oncol 133:155–158 9. Cosyns S, Polyzos NP, Carprieaux M et al (2016) The role of appendectomy as part of the treatment of a mucinous borderline ovarian tumor. Eur J Gynaecol Oncol 37:167–170 10. Fauvet R, Boccara J, Dufournet C et al (2004) Restaging surgery for women with borderline ovarian tumors: results of a French multicenter study. Cancer 100:1145–1151 11. Brown J, Frumovitz M (2014) Mucinous tumors of the ovary: current thoughts on diagnosis and management. Curr Oncol Rep 16:389 12. Trope C, Kaern J, Vergote IB et al (1993) Are borderline tumors of the ovary overtreated both surgically and systemically? A review of four prospective randomized trials including 253 patients with borderline tumors. Gynecol Oncol 51:236–243 13. Abascal-Saiz A, Sotillo-Mallo L, de Santiago J et al (2014) Man- agement of borderline ovarian tumours: a comprehensive review of the literature. Ecancermedicalscience 8:403 14. Vasconcelos I, Olschewski J, Braicu I et al (2015) A meta-anal- ysis on the impact of platinum-based adjuvant treatment on the outcome of borderline ovarian tumors with invasive implants. Oncologist 20:151–158 Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.