{"paper_id":"7ff49154-bf9b-48cb-a336-d284eb9a8744","body_text":"Vol.:(0123456789)\nInternational Journal of Clinical Oncology (2026) 31:785–793 \nhttps://doi.org/10.1007/s10147-026-03008-z\nCANCER STATISTICS\nTrends in the management and prognosis of mucinous borderline \novarian tumors: analysis of 12,766 cases from the JSOG Gynecologic \nTumor Registry (2004–2018)\nHideki Tokunaga1  · Yusuke Shibuya2 · Wataru Yamagami3 · Fumiaki Takahashi4 · Eiko Yamamoto5 · \nYoshihito Yokoyama6 · Kiyoshi Yoshino7 · Kei Kawana8 · Satoru Nagase9\nReceived: 29 October 2025 / Accepted: 4 March 2026 / Published online: 19 March 2026 \n© The Author(s) 2026\nAbstract\nBackground Mucinous borderline ovarian tumors (MBOTs) are rare neoplasms with excellent prognosis, yet the optimal \nsurgical extent remains controversial. No large-scale study in Japan has evaluated treatment trends and prognostic factors \nfor MBOTs. This study aimed to clarify their clinicopathological features, management patterns, and survival outcomes \nusing a nationwide registry.\nMethods Data were obtained from the Japan Society of Obstetrics and Gynecology Gynecologic Tumor Registry, including \n96,476 ovarian tumors treated between 2004 and 2018. Among them, 12,766 MBOT cases were identified. Surgical pro-\ncedures—hysterectomy, omentectomy, lymphadenectomy, and adjuvant chemotherapy—were analyzed. Survival analyses \nof 8564 cases with complete prognostic data were performed using Kaplan–Meier and Cox proportional hazards models.\nResults Over 90% of MBOTs were stage I, and the median age was 52 years. Hysterectomy was performed in 50.8%, omen-\ntectomy in 57.9% (2015–2018 subset), and lymphadenectomy in 7.6%. Only 2.6% received adjuvant chemotherapy. The \n5-year overall survival exceeded 95%. Multivariate analysis identified age ≥ 50 years (HR 2.5, 95% CI 1.8–3.6) and stage \nIC (HR 2.7, 95% CI 1.9–3.6) as independent adverse factors. Omentectomy showed a marginal survival benefit (HR 0.6, \np = 0.05), whereas hysterectomy, lymphadenectomy, and chemotherapy conferred no advantage. Chemotherapy correlated \nwith poorer outcomes, likely due to confounding by indication.\nConclusions This nationwide cohort—the largest MBOT series reported to date—demonstrates conservative management \nwith excellent prognosis in Japan. Radical surgery and chemotherapy provide no survival benefit, whereas fertility-sparing \nsurgery appears appropriate for younger patients.\nKeywords Mucinous borderline ovarian tumor · JSOG registry · Omentectomy · Survival analysis · Fertility-sparing \nsurgery · Nationwide cohort\nHideki Tokunaga and Yusuke Shibuya contributed equally to this \nwork.\n * Hideki Tokunaga \n tokunaga.hideki@tohoku-mpu.ac.jp\n1 Division of Obstetrics and Gynecology, Faculty of Medicine, \nTohoku Medical and Pharmaceutical University, Miyagi, \nJapan\n2 Department of Gynecology, Tohoku University Hospital, \nMiyagi, Japan\n3 Department of Obstetrics and Gynecology, Keio University \nSchool of Medicine, Tokyo, Japan\n4 Department of Information Science, Iwate Medical \nUniversity, Iwate, Japan\n5 Department of Healthcare Administration, Nagoya University \nGraduate School of Medicine, Nagoya, Japan\n6 Department of Obstetrics and Gynecology, Hirosaki \nUniversity Graduate School of Medicine, Aomori, Japan\n7 Department of Obstetrics and Gynecology, University \nof Occupational and Environmental Health, Fukuoka, Japan\n8 Department of Obstetrics and Gynecology, Nihon University \nSchool of Medicine, Tokyo, Japan\n9 Department of Obstetrics and Gynecology, Yamagata \nUniversity Faculty of Medicine, Yamagata, Japan\n\n786 International Journal of Clinical Oncology (2026) 31:785–793\nIntroduction\nIn addition to bilateral adnexal resection, total hysterec-\ntomy, and omentectomy for malignant ovarian tumors, \npelvic and para-aortic lymph node dissection, ascitic \n(peritoneal) cytology, and intraperitoneal exploration are \nperformed to determine the extent of disease and assess \nadvanced stages. Some distant metastases are confirmed \npathologically, but most are identified through diagnostic \nimaging.\nFor borderline ovarian tumors, routine lymph node dis-\nsection is not recommended [1 ]. Apart from the LION \nstudy [2 ], no randomized controlled trials have evaluated \nthe necessity of lymphadenectomy in advanced ovarian \ncancer. Current recommendations are, therefore, based \non the retrospective detection rates of occult metastases, \nmeta-analyses, and their impact on prognosis [1 ].\nIn summary, it is only weakly recommended to per -\nform basic surgical staging procedures beyond resection of \nthe affected adnexa. To date, no nationwide or long-term \nlarge-scale study has evaluated mucinous borderline ovar -\nian tumors in Japan, and real-world treatment strategies \nremain largely undefined.\nThe Japan Society of Obstetrics and Gynecology \n(JSOG) began registering cases of borderline and malig-\nnant ovarian tumors in the Gynecologic Tumor Registry \n(GTR) starting in 1998. This registry collects data on \nclinicopathological features and survival outcomes, with \nfollow-up surveys performed three and five years after \nregistration. Histological classification follows the WHO \nsystem, and staging is based on the International Federa-\ntion of Gynecology and Obstetrics (FIGO) classification; \nboth are updated in accordance with each revision.\nIn the present study, we analyzed large-scale data from \n2004 to 2018, when five-year prognostic follow-up was \ncompleted, to investigate treatment trends in mucinous \nborderline ovarian tumors and to evaluate the prognostic \nimpact of surgical procedures other than adnexal resection.\nMaterials and methods\nPatients\nThis study included 96,476 patients with ovarian tumors \ntreated at medical facilities across Japan and registered in \nthe Japan Society of Obstetrics and Gynecology (JSOG) \nGynecologic Tumor Registry (GTR) between 2004 and \n2018. Major hospitals throughout Japan participate in \nthis registry, which is estimated to cover approximately \n70–80% of all ovarian cancer cases nationwide.\nAfter receiving approval from the Ethics Committees \nof the JSOG (approval no. 17) and Keio University School \nof Medicine (approval no. 20170261), data on clinico-\npathological characteristics and survival outcomes were \ncollected. Patients who did not undergo surgery or who \nreceived preoperative chemotherapy were excluded from \nthe prognostic analysis. Cases with incomplete clinical \ninformation—such as missing stage data or unavailable \nprognostic outcomes—were also excluded.\nMethods\nThe study flow diagram illustrates the selection and strati-\nfication of the cohort (Fig.  1). A subset of 8564 cases with \ncomplete prognostic information was included in the sur -\nvival analysis.\nThe registry data included: age at treatment initiation, \nFIGO stage (1988 or 2014), pTNM classification according \nto the FIGO system, whether preoperative chemotherapy \nwas administered, surgical procedures (adnexectomy, hys-\nterectomy, omentectomy, and lymphadenectomy), residual \ntumor status (surgical completeness), sites of distant metas-\ntasis, whether adjuvant chemotherapy or second-look sur -\ngery was performed, and ypTNM classification. Follow-up \nsurveys are conducted three and five years after the year of \nregistration to determine disease-free survival, alive-with-\ndisease, and death outcomes. Since 2017, the registry has \nbeen updated to reflect the WHO 2014 histological clas-\nsification, which redefined mucinous tumors as intestinal \ntype and seromucinous type. Data prior to 2016 followed \nthe WHO 2003 classification, which did not distinguish \nbetween intestinal and endocervical types; therefore, these \nearlier data cannot be directly reclassified under the WHO \n2014 system. Beginning in 2015, staging was based on \nFIGO 2014; however, before 2014, the size of lymph node \nmetastases was not recorded, preventing precise stage con-\nversion. Until 2014, surgical procedures were categorized \nonly as “adnexectomy” or “radical surgery,” and informa-\ntion on omentectomy was unavailable. Because “radical sur-\ngery” was defined as including hysterectomy, the presence \nor absence of hysterectomy could still be determined. For \nthis study, seromucinous tumors diagnosed after 2015 were \nexcluded from the survival analysis.\nStatistical analysis\nAll statistical analyses were performed using JMP software, \nversion 19.0.1 (SAS Institute Inc., Cary, NC, USA). The \nunivariate analyses for overall survival (OS) were conducted \nusing the Kaplan–Meier method and the log-rank test. The \nmultivariate analyses were performed using the Cox pro-\nportional hazards model, incorporating available prognostic \nfactors (age, surgical stage, surgical procedure, and adjuvant \n\n787International Journal of Clinical Oncology (2026) 31:785–793 \nchemotherapy). To minimize selection bias, additional \nanalyses were conducted using multivariate logistic regres-\nsion and Cox regression with propensity score matching. In \nall analyses, a p  value < 0.05 was considered statistically \nsignificant.\nResults\nBetween 2004 and 2018, a total of 96,476 ovarian neo-\nplasms were registered from 216 to 398 institutions \n(Table  1). Mucinous borderline ovarian tumors (MBOTs) \naccounted for 12,766 cases (13.2%), showing a steady \nincrease in absolute number—from 466 cases in 2004 \nto 1192 in 2018. Seromucinous borderline tumors were \nreported in 296 patients in 2017 and 305 in 2018. Both the \nnumber of participating institutions and registered cases \nincreased over time, indicating broader registry coverage \nand improved data quality. The proportion of MBOTs rose \nslightly from 12.1% in 2004 to a peak of 14.4% in 2013, \nthen stabilized around 13% thereafter. The average number \nof MBOT cases per institution increased from 2.16 to 3.17 \nper year, suggesting both improved detection and wider \nparticipation (Fig.  2). By contrast, the number of mucinous \ncarcinomas (MCs) ranged from 350 to 620 cases annu-\nally, without a clear upward trend. The proportion of MCs \namong all ovarian tumors declined from approximately \n10% (2004–2006) to 6% (2017–2018), and the number of \nMCs per institution decreased from 1.9 to 1.4 per year, \nFig. 1  Flow diagram of this study. Patients with ovarian, fallopian \ntube, and primary peritoneal tumors registered in the JSOG gyneco-\nlogic cancer registry between 2004 and 2018 (n = 96,476) were \nscreened. After exclusion of cases with insufficient clinicopatho-\nlogic or follow-up data, 12,766 mucinous borderline ovarian tumors \n(MBOTs) were included in the clinicopathologic analysis, and 8564 \ncases were eligible for prognostic evaluation\nTable 1  Annual trend in the number and proportion of mucinous borderline ovarian tumors among all ovarian tumors (2004–2018)\nYear 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total\nRegistered patients 3853 3490 4041 4359 4820 5277 5678 6102 6902 7718 7860 8646 9090 9383 9257 96,476\nMucinous borderline tumor 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766\nSeromucinous borderline tumor 296 305\nMucinous carcinoma 379 355 385 398 466 461 478 551 559 619 568 622 565 586 540\nInstitutions 216 192 197 212 227 241 254 279 319 366 382 386 398 393 376\n\n788 International Journal of Clinical Oncology (2026) 31:785–793\nsuggesting a relative reduction in the malignant mucinous \ncomponent among newly registered ovarian tumors.\nPatient characteristics\nTable  2 summarizes the clinicopathologic characteristics of \npatients with MBOT. The median age was 52 years (range, \n11–97) between 2004 and 2014, and 53 years (range, 11–96) \nbetween 2015 and 2018. Most tumors were FIGO stage I \n(68.5% under 1988 criteria; 64.6% under 2014 criteria), and \nhigher-stage disease (stage II–IV) was rare (< 5%). Hys-\nterectomy was performed in 6484 cases (50.8%), whereas \n6282 patients (49.2%) did not undergo hysterectomy. Omen-\ntectomy (data available for 2015–2018) was performed in \n2791 cases (57.9%). Lymphadenectomy was carried out in \n973 cases (7.6%) and adjuvant chemotherapy in 334 patients \n(2.6%), reflecting the indolent nature of MBOTs. Regarding \noutcomes, 8273 patients (64.8%) were alive without disease, \n131 were alive with disease, 212 had died of disease, and \n176 had died of other causes; outcomes were unknown for \n3917 patients.\nSurgical procedures and trends\nThe proportion of patients undergoing hysterectomy \nincreased gradually until approximately 2013 and then \nplateaued (Table  3). Hysterectomy was predominantly \nperformed in patients aged ≥ 50 years, who accounted for \napproximately 52% of all cases. Among younger women \n(< 40 years), fertility-sparing surgery (without hysterec-\ntomy) was more common, consistent with current clinical \npractice trends.\nIn the subset of 4822 patients (2015–2018), 2791 (57.9%) \nunderwent omentectomy (Table 4). Omentectomy was more \nfrequent in patients aged ≥ 40 years and in those with higher \nFIGO stages (particularly IC1–IC3). Yearly data revealed \na modest increase from 52.5% (2015) to 62.4% (2018), \nsuggesting growing adherence to comprehensive surgical \nstaging.\nLymphadenectomy was performed in 973 cases (7.6%), \nshowing no apparent upward trend during the 15-year period \n(Table  5). The proportion remained below 10% even after \n2010, indicating that lymphadenectomy is not routinely \nperformed for MBOTs. Most procedures were conducted in \nwomen aged 40–60 years.\nAdjuvant chemotherapy was administered in 334 patients \n(2.6%) overall (Table  6), without a significant temporal \nincrease between 2004 and 2018. Chemotherapy was mainly \nused for stage IC–III disease, whereas stage IA/IB patients \nrarely received it. Among those treated in 2015–2018, only \n91 patients (1.9%) received chemotherapy, reflecting a grow-\ning trend toward surgery alone for borderline tumors.\nSurvival analysis\nThe Kaplan–Meier curve (Fig.  3) demonstrated excellent \nlong-term survival for MBOT patients, with a five-year over-\nall survival rate exceeding 95%, consistent with the indolent \nbehavior of these tumors.\nOnly a small number of deaths were observed, indicating \nvery low disease-specific mortality.\nIn univariate analysis (Table  7), both older age \n(≥ 50 years) and higher FIGO stage (IC) were significantly \nassociated with poorer overall survival (HR = 2.2, 95% \nCI 1.6–3.1, p  < 0.0001; and HR = 2.7, 95% CI 2.0–3.7, \np < 0.0001, respectively). Omentectomy showed a marginal \ntrend toward improved survival (HR = 0.6, 95% CI 0.4–1.0, \np = 0.06), whereas hysterectomy, lymphadenectomy, and \nadjuvant chemotherapy were not significantly associated \nwith OS in the univariate model.\nFig. 2  Annual trends of muci-\nnous tumors among ovarian \nneoplasms. MBOT/all and MC/\nall indicate the proportions of \nmucinous borderline tumors and \nmucinous carcinomas among \nall registered ovarian tumors, \nrespectively. MBOT/institution \nand MC/institution indicate the \nnumber of patients per partici-\npating institution\n\n\n789International Journal of Clinical Oncology (2026) 31:785–793 \nTable 2  Distribution of clinicopathologic characteristics of patients with MBOT (age, FIGO stage, and surgical procedures)\nNAC neoadjuvant chemotherapy\n2004_2014 7944 2015_2018 4822\nAge 52 (11–97) 53 (11–96)\nFIGO stage (1988) n % FIGO stage (2014) n %\nI I\n Ia 5439 68.5 IA 3114 64.6\n Ib 91 1.1 IB 44 0.9\n Ic IC\n  Ic(a) 558 7.0 IC1 988 20.5\n  Ic(b) 1427 18.0 IC2 398 8.3\n  Ic(1) 48 0.6 IC3 143 3.0\n  Ic(2) 121 1.5 II\nII IIA 14 0.3\n IIa 10 0.1 IIB 25 0.5\n IIb 13 0.2 III\n IIc IIIA1 1 0.0\n  IIc(a) 22 0.3 IIIA2 8 0.2\n  IIc(b) 17 0.2 IIIB 19 0.4\n  IIc(1) 3 0.0 IIIC 20 0.4\n  IIc(2) 6 0.1 IVA 5 0.1\nIII IVB 2 0.0\n IIIa 20 0.3 NAC 3 0.1\n IIIb 27 0.3 Unknown 38 0.8\n IIIc 83 1.0\nIV 16 0.2\nNAC 24 0.3\nUnknown 19 0.2\nSurgical procedure\n Hysterectomy\n  Yes 6484\n  No 6282\n Omentectomy\n 2004_2014 N/A\n 2015_2018\n  Yes 2791\n  No 2031\n Lymph node dissection\n  Yes 973\n  No 11,793\n Adjuvant chemotherapy\n  Yes 334\n  No 12,432\n Prognosis\n  Alive 8273\n  Alive with disease 131\n  Dead 212\n  Death from other causes 176\n  Unknown 3917\n\n790 International Journal of Clinical Oncology (2026) 31:785–793\nIn the multivariate Cox regression analysis (Table  7), \nage ≥ 50 years and stage IC remained independent adverse \nprognostic factors (HR = 2.5, 95% CI 1.8–3.6, p  < 0.0001; \nand HR = 2.7, 95% CI 1.9–3.6, p  < 0.0001, respectively). \nOmentectomy was not independently associated with bet-\nter overall survival (HR = 0.6, 95% CI 0.4–1.0, p  = 0.05). \nNeither hysterectomy nor lymphadenectomy significantly \naffected survival. Interestingly, adjuvant chemotherapy \nfor stage ≥ IC was associated with worse overall survival \n(HR = 2.6, 95% CI 1.4–4.8, p  = 0.002**) after adjustment \nfor confounders.\nTable 3  Trends in hysterectomy among patients with mucinous borderline ovarian tumors\nAge 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s N/A Total\n Yes 1 31 340 1439 1794 1616 952 298 11 2 6484\n No 266 1166 1569 874 689 743 593 337 45 0 6282\nTotal 267 1197 1909 2313 2483 2359 1545 635 56 2 12,766\nYear 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total\n Yes 203 179 200 228 248 321 369 429 522 565 616 594 648 683 679 6282\n No 263 218 252 334 337 373 400 411 436 549 491 555 588 562 513 6484\nTotal 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766\n≥ 50 years\nYear 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total\n Yes 128 129 145 171 177 240 260 329 376 395 456 414 458 484 510 4672\n No 88 77 76 113 127 144 151 165 164 203 190 214 233 246 216 2407\nTotal 216 206 221 284 304 384 411 494 540 598 646 628 691 730 726 7079\nTable 4  Omentectomy rates by age, year and FIGO stage (2015–2018 subset)\nAge 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s Total\n Yes 33 169 285 592 587 604 392 124 5 2791\n No 63 235 350 320 315 330 240 156 22 2031\nTotal 96 404 635 912 902 934 632 280 27 4822\nYear 2015 2016 2017 2018 Total\n Yes 604 713 730 744 2791\n No 545 523 515 448 2031\nTotal 1149 1236 1245 1192 4822\nStage IA IB IC1 IC2 IC3 IIA IIB IIIA1 IIIA2 IIIB IIIC IVA IVB N/A Total\n Yes 1759 27 536 267 115 11 19 1 7 11 15 5 2 16 2791\n No 1355 17 452 131 28 3 6 0 1 8 5 0 0 25 2031\nTotal 3114 44 988 398 143 14 25 1 8 19 20 5 2 41 4822\nTable 5  Lymphadenectomy rates by age and year group\nAge 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s N/A Total\n Yes 2 34 116 221 265 214 110 11 0 0 973\n No 265 1163 1793 2092 2218 2145 1435 624 56 2 11,793\nTotal 267 1197 1909 2313 2483 2359 1545 635 56 2 12,766\nYear 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total\n Yes 42 41 42 63 56 58 73 66 81 94 95 75 84 65 38 973\n No 424 356 410 499 529 636 696 774 877 1020 1012 1074 1152 1180 1154 11,793\nTotal 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766\n\n791International Journal of Clinical Oncology (2026) 31:785–793 \nDiscussion\nThis study represents the largest nationwide, registry-\nbased cohort analysis to date investigating mucinous bor -\nderline ovarian tumors (MBOTs). Using data from the \nJSOG Gynecologic Tumor Registry, it provides the most \ncomprehensive overview of MBOTs in Japan. Between \n2004 and 2018, a total of 12,766 patients were regis-\ntered, demonstrating that MBOTs constituted 13–15% of \nall ovarian borderline tumors, with a remarkably stable \nincidence over time (Table  1). More than 90% of cases \nwere diagnosed at FIGO stage I, and the median age was \napproximately 52 years—slightly older than that reported \nin European cohorts (40–45 years) [3 , 4]. This finding sug-\ngests that MBOTs in Japan are more frequently detected in \nperi- or postmenopausal women, possibly reflecting differ -\nences in screening practices and surgical decision-making.\nSurgical practice patterns\nApproximately half of all patients underwent hysterectomy, \nand over 60% underwent bilateral oophorectomy, whereas \nomentectomy was documented in 58% of cases after 2015 \n(Table  4). Lymphadenectomy was rare (< 10%) (Table  5), \nand adjuvant chemotherapy was used in only 2–3% of \npatients (Table  6). These data indicate that Japanese clini-\ncal practice is characterized by relatively conservative adju-\nvant treatment but still frequent use of hysterectomy, even \nin early-stage disease.\nIn contrast, Western guidelines and registry data—such \nas the cohort study of the AGO Study Group [3 ]—support \nfertility-sparing surgery for reproductive-age women without \ncompromising survival [ 5, 6]. Our findings reinforce that \nradical surgery confers no survival advantage in patients \nwith MBOTs.\nComparison with previous multicenter studies\nThe present results closely align with those of a multicenter \nstudy by Gungorduk et al. [4 ], which analyzed 364 MBOT \npatients across 14 institutions in Turkey and Germany and \nfound no independent prognostic effect of omentectomy, \nappendectomy, lymphadenectomy, or radical surgery on \neither progression-free or overall survival. In their study, \nTable 6  Proportion of patients receiving adjuvant chemotherapy and corresponding age, year, and FIGO stage distribution\nAge 10 s 20 s 30 s 40 s 50 s 60 s 70 s 80 s 90 s N/A Total\n Yes 5 25 36 70 95 66 30 5 1 1 334\n None 262 1172 1873 2243 2388 2293 1515 630 55 1 12,432\nTotal 267 1197 1909 2313 2483 2359 1545 635 56 2 12,766\nYear 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017\n Yes 43 14 15 17 15 30 25 14 26 20 24 26 26 19 20 334\n No 423 383 437 545 570 664 744 826 932 1094 1083 1123 1210 1226 1172 12,432\nTotal 466 397 452 562 585 694 769 840 958 1114 1107 1149 1236 1245 1192 12,766\n2004_2014\n Stage Ia Ib Ic IIa IIb IIC IIIa IIIb IIIc IV N/A Total\n  Yes 60 5 125 2 1 13 5 5 13 6 8 243\n  No 5379 86 2029 8 12 35 15 22 70 10 35 7701\n Total 5439 91 2154 10 13 48 20 27 83 16 43 7944\n2015_2018\n Stage IA IB IC IIA IIB IIIA IIIB IIIC IVA IVB N/A\n  Yes 18 1 39 2 5 2 6 10 3 2 3 91\n  No 3096 43 1490 12 20 7 13 10 2 0 38 4731\n Total 3114 44 1529 14 25 9 19 20 5 2 41 4822\nFig. 3  Overall survival curves of mucinous borderline ovarian tumors \naccording to FIGO stage\n\n792 International Journal of Clinical Oncology (2026) 31:785–793\nthe median age was 43 years, more than 75% of patients \nhad stage IA disease, and the 5-year overall survival rate \nexceeded 95%.\nSimilarly, in our multivariate analysis, only older age \n(≥ 50 years) and advanced stage (IC vs. IA/IB) were identi-\nfied as independent adverse prognostic factors (HR 2.5 and \n2.7, respectively), whereas hysterectomy, lymphadenectomy, \nand chemotherapy provided no survival benefit (Table 7). It \nis also conceivable that the biological background of muci-\nnous borderline ovarian tumors differs between younger and \npostmenopausal patients, including differences in driver \ngenetic alterations. Mucinous tumors are frequently char -\nacterized by KRAS mutations; however, age-related differ-\nences in genomic complexity, hormonal milieu, immune \nsurveillance, and accumulation of somatic alterations may \ninfluence tumor behavior and clinical outcomes. Such \nbiological heterogeneity may partly explain the inconsist-\nent prognostic impact of age reported in previous studies, \nincluding reports suggesting younger age as a risk factor for \nrecurrence [7 ]. In this context, younger age may be more \nclosely associated with recurrence risk, whereas older age \nmay adversely affect overall survival through host-related \nfactors rather than intrinsic tumor aggressiveness. Omentec-\ntomy showed a marginal association with improved survival \n(p = 0.05), which likely reflects selection bias or more com-\nplete staging rather than a therapeutic effect.\nOmentectomy and appendectomy\nThe role of omentectomy in MBOT remains controver -\nsial. In the study by Gungorduk et al., omental involve-\nment was identified in only 1.4% of cases, and appendiceal \ninvolvement in 9.1%, mostly in macroscopically abnormal \nappendices [4 ]. Similarly, two other systematic reviews \nconcluded that routine appendectomy is unnecessary unless \ngross abnormalities are observed [8 , 9].\nAlthough our registry did not capture appendectomy data, \nthe low incidence of advanced disease and the excellent \nsurvival outcomes suggest that routine appendectomy and \nsystematic omentectomy are unlikely to improve prognosis.\nLymphadenectomy and adjuvant chemotherapy\nLymphadenectomy was performed in fewer than 10% of \npatients, consistent with its limited clinical utility given the \nextremely low incidence (< 2%) of nodal metastasis reported \nin prior studies [10, 11]. Our findings confirm that lymphad-\nenectomy had no significant influence on overall survival \n(HR 0.8, p = 0.5).\nAdjuvant chemotherapy was rarely administered and was \nassociated with worse survival in the multivariate analysis \n(HR 2.6, p = 0.002), likely due to confounding by indication.\nPrevious meta-analyses have consistently shown no ben-\nefit from platinum-based chemotherapy for borderline ovar-\nian tumors [12–14].\nClinical implications and future directions\nTaken together, our results support that the extent of surgi-\ncal staging does not influence outcomes in MBOT patients. \nThe excellent prognosis (>  98% 5-year survival) and very \nlow incidence of extraovarian spread underscore the indolent \nbiological nature of these tumors.\nTable 7  Results of univariate and multivariate analyses for overall survival of MBOT patients (Cox proportional hazards model)\nUnivariate Overall survival\nHazard ratio 95%CI p value\nAge (< 50 years vs. >= 50 years) 2.2 1.6–3.1 < 0.0001\nStage (IA,IB vs. IC) 2.7 2.0–3.7 < 0.0001\nHysterectomy 0.9 0.7–1.2 0.5\nOmentectomy 0.6 0.4–1.0 0.06\nLymphadenectomy 0.9 0.5–1.6 0.7\nAdjuvant chemotherapy (stage >= IC) 1.5 0.7–3.5 0.3\nMultivariate Overall survival\nHazard ratio 95%CI p-value\nAge (< 50 years vs. >= 50 years) 2.5 1.8–3.6 < 0.0001\nStage (IA,IB vs. IC) 2.7 1.9–3.6 < 0.0001\nHysterectomy 0.8 0.6–1.1 0.1\nOmentectomy 0.6 0.4–1.0 0.05\nLymphadenectomy 0.8 0.4–1.5 0.5\nAdjuvant chemotherapy (stage >= IC) 2.6 1.4–4.8 0.002\n\n793International Journal of Clinical Oncology (2026) 31:785–793 \nThe main limitation of this study is its registry-based \ndesign, which relies on voluntarily reported data and thus \ncontains missing information on some clinicopathological \nvariables and outcomes. Detailed information regarding \nrecurrence, including the timing of recurrence and specific \nrecurrence patterns, is not available in the registry database. \nIn addition, performance status was not collected. There-\nfore, while the present analysis allows evaluation of treat-\nment selection trends according to patient age and treatment \nera, it is difficult to precisely investigate the exact causes of \nrecurrence or death among patients with poor prognosis. \nAlthough the data accuracy cannot be fully guaranteed, the \nlarge sample size of over 12,000 cases provides sufficient \nstatistical power and reliability.\nIn conclusion, fertility-sparing surgery should be rec-\nommended for younger women, and routine omentectomy, \nappendectomy, and lymphadenectomy should be reevalu-\nated. Future registry-linked analyses integrating pathological \nreview, molecular profiling, and clinical outcomes are war-\nranted to refine surgical strategies for MBOTs and to clarify \nthe prognostic relevance of histological subtypes (intestinal \nvs. seromucinous).\nAcknowledgements We would like to thank the committee and all \nparticipants of the Japan Society of Obstetrics and Gynecology (JSGO) \ncancer registry program.\nFunding Open Access funding provided by Tohoku Medical and \nPharmaceutical University. No sources of funding were used to assist \nin the preparation of this study.\nData availability Part of the data used in this study has been published \nannually in the Journal of Obstetrics and Gynaecology Research \n(JOGR) as part of the annual reports of the Japan Society of Obstetrics \nand Gynecology (JSOG).\nDeclarations \nConflict of interest All authors have no conflicts of interest in relation \nto this study.\nOpen Access This article is licensed under a Creative Commons \nAttribution 4.0 International License, which permits use, sharing, \nadaptation, distribution and reproduction in any medium or format, \nas long as you give appropriate credit to the original author(s) and the \nsource, provide a link to the Creative Commons licence, and indicate \nif changes were made. The images or other third party material in this \narticle are included in the article’s Creative Commons licence, unless \nindicated otherwise in a credit line to the material. 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