Histological Characteristics and Early-Stage Diagnosis Are Associated With Better Survival in Young Patients With Epithelial Ovarian Cancer: A Retrospective Analysis Based on Surveillance Epidemiology and End Results Database

Purpose To analyze the potential prognostic factors of epithelial ovarian cancer (EOC) in women aged under 35 compared to those aged 60–79. Methods Cases were retrospectively obtained from SEER database. Clinical characteristics, such as race, histological type, AJCC stage, laterality of tumors, CA125 results, and surgical strategies, were analyzed in < 35 years group and 60–79 years group. Kaplan-Meier survival curves were used to evaluate overall survival (OS) and cause-specific survival (CSS). Cox proportional hazard model was used to identify the predictors for CSS. Results Sixteen thousand eight hundred forty-seven EOC patients diagnosed in 2004–2015 were identified from SEER database, with 1,015 aged under 35 and 15,833 aged 60–79. In < 35 years group, mucinous (32.2%) was the most common histological type, followed by high-grade serous (26.6%) and endometrioid (18.3%), while in 60–79 years group, high-grade serous (68.3%) represented the leading histological type. Most young women were diagnosed at stage I (57.7%), while most old women were diagnosed at stage (48.1%). Both 5-year OS and 5-year CSS were higher in < 35 years group (5-year OS: 76.00% vs 40.18%, p < 0.001; 5-year CSS: 83.56% vs 55.18%, p < 0.001). The multivariate analysis identified histological type and stage as prognostic factors for CSS in both groups. Endometrioid represented a positive predictor for CSS, while carcinosarcoma and malignant Brenner were related to a worse CSS. (< 35 years group: carcinosarcoma vs endometrioid: HR 5.630, p=0.024; malignant Brenner vs endometrioid: HR 4.005, p < 0.001; 60–79 years group: carcinosarcoma vs endometrioid: HR 3.606, p < 0.001; malignant Brenner vs endometrioid: HR 2.291, p < 0.001). Tumors laterality, CA125 levels, surgery and lymphadenectomy failed to be associated with the CSS in < 35 years group, while found to be independent risk factors in 60–79 years group. Conclusion EOC women aged under 35 had a better survival outcome over EOC women aged 60–79, owing to high proportion of endometrioid and mucinous types in histology, as well as early-stage diagnosis. Identification of histological types and gene profiles should be underscored in young EOC patients.


INTRODUCTION
Ovarian cancer (OC), one of the most lethal gynecological cancers, caused 295,414 new cases and 184,799 deaths across the world in 2018 (1). Although the mortality has decreased due to the improvement of treatment strategies in the past 50 years, the 5year survival rate is still under 50% because of late diagnosis (2). Epithelial ovarian cancer (EOC), which includes several subtypes with distinctive pathological and clinical features, takes up the majority of OC patients (3). EOC has the highest incidence rate in women in their 6 th to 7 th decades (4), while is rarely seen in young women, with 1.1% in aged under 25, and 4.1% in aged under 30 (5). Several studies (5)(6)(7)(8)(9)(10)(11) have focused on this small group of patients and found that they were associated with distinct patterns of clinicopathological characteristics and a higher overall survival rate. However, few studies included a large population, and some did not analyze the surgical strategies and the associated outcomes in the patient group. Women's fertility rate sharply decreases after age of 35 (12). Thus, women aged under 35 represent a special group to be considered in many GYN/OB diseases. By retrospectively analyzing the data obtained from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) program, the current study aims to conduct a thorough analysis including clinicopathologic characteristics, surgical strategies and their relationship with the survival outcome in women aged under 35 compared to women aged 60-79, hoping to provide some helpful evidence for young EOC patients' counseling and treatment strategies selection.

Study Population
Women with EOC under the age of 35 were included in < 35 years group, while women with EOC aged between 60 and 79 were identified as 60-79 years group for comparison.

Data Extraction
Data in this study was extracted from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) program released in November 2016. Women diagnosed with EOC (primary site: C56.9 Ovary) from 2004 to 2015 with the age range of 0-34 or 60-79 were identified. Parameters extracted included demographic, clinicopathological, treatment and survival information. The histological classification was identified according to 2014 WHO EOC histological types and a prior population-based study (13). The histological results were collected from pathology laboratories according to the standards issued by the North American Association of Central Cancer Registries (NAACR). Exclusion criteria were cases with missing information on histological types, AJCC stage, surgery intervention, lymph nodes removement during operation or surgery on remote sites ( Figure 1). Surgical methods were classified into no surgery performed, unilateral salpingooophorectomy (USO) or bilateral salpingo-oophorectomy (BSO) (without hysterectomy), USO or BSO (with hysterectomy), USO or BSO (NOS), debulking or cytoreductive surgery (NOS), pelvic exenteration, and others.

Statistical Analysis
Median age of each group was calculated by independent samples t-test, and the rest variables were evaluated by chisquared test, Fisher exact test, or post hoc test. Kaplan-Meier survival curves and log-rank test with 95% confidence intervals (CIs) were applied to evaluate the outcomes between the two groups. Risk factors were analyzed using Cox proportional hazards regression model with 95%CI. We considered p < 0.05 as statistically significant. For the post hoc test, the adjusted standardized value > 3 was regarded statistically significant. Data extraction was completed in SEER*Stat, and all analyses were performed using the SPSS Statics software, version 25.0 (IBM Corporation, NY, USA).

Included Patient Characteristics
As shown in Table 1 young women underwent lymphadenectomy than old women (68.4% vs 55.2%, p < 0.001). There was no significance in the tendency of surgery on distant sites in the two groups ( Table 2).

Risk Factors of Cause-Specific Survival
In the < 35 years group, histological type, laterality of tumors, AJCC stage, level of CA125 before therapies, surgery, and surgery on other sites (all p < 0.001) were risk factors for CSS in univariate survival analysis ( In the 60-79 years group, race, histological type, laterality of tumors, AJCC stage, level of CA125 before therapies, surgery,      (14). However, other retrospective studies (6,(8)(9)(10)13) found that serous histological type took up the largest portion in EOC women aged under 35 or 40. The difference could be explained by the following reasons. Firstly, most studies did not apply 2014  (17). It is agreed that with improved histopathology techniques and a greater understanding of mucinous carcinomas, the incidence of mucinous EOC drops to 3% (18). Therefore, the percentage of mucinous EOC in both < 35 years and 60-79 years group would be lower than the current results.
In both < 35 years group and 60-79 years group, endometrioid, mucinous, low-grade serous, and clear cell tumors indicated a higher 5-year CSS, while carcinomas, malignant Brenner tumors, and high-grade serous indicated lower 5-year CSS in the univariate survival analysis. The results are similar to the study of Aihua Lan (19), which is based on a SEER database without age stratification. In the multivariate analysis, however, mucinous was related to a poorer survival rate in 60-79 years group. In < 35 years group, although there was no statistical significance, the hazard rate of mucinous histological type was 1.505. The difference of mucinous-associated survival outcomes could result from several factors. Firstly, as we mentioned above, patients with mucinous carcinomas metastasized from the gastrointestinal tract were also included in our study and this group of patients would have much worse survival outcomes. Secondly, mucinous EOC diagnosed at an early stage had favorable prognosis, while those diagnosed at a late stage and recurrent tumors had poor survival outcomes because of the insensitivity to chemotherapy (13,(20)(21)(22)(23). Increasing studies focused on unfolding the genetic secret of mucinous EOC in recent years. The mutation of KRAS protein, which is associated with the RAS/RAF/MARK pathway, might play an important role in the beginning event of mucinous EOC (24,25). The amplification of HER2 and p53 genes mutation were also reported in patients with mucinous EOC, and both of them were associated with the malignant transformation in the stepwise progression of mucinous EOC (15,22). Considering the high incidence of mucinous EOC in young women, gene diagnosis for prognosis and therapy consultation could be arranged in this patient group in the future.
The early-stage diagnosis of young women with EOC has been demonstrated in several studies (10,26,27). Mucinous and endometrioid histological types, which were commonly observed in young women, often present as localized masses, while high-grade serous tumors, commonly diagnosed in old women, often spread beyond pelvis at diagnosis (27,28). Moreover, endometriosis-associated ovarian cancer (EAOC) including endometrioid and clear-cell histological types, are more often reported in young women. EAOC usually presents chronic pelvic pain, dyspareunia, dysmenorrhea and infertility, as well as significantly elevated CA125 levels, making them easier to be detected at an early stage (29)(30)(31). With the development of advanced molecular techniques, mutations in several genes such as ARID1A, PIK3CA, and CTCF have been found involved in the progression from benign endometriosis to EAOC (32). As a result, it is important for physicians to screen for EAOC in patients with endometriosis, and to initiate management as well as regular monitoring for those with a high risk of EAOC. It is known that positive family history and BRCA1/2 mutation could be detected especially in patients with early-onset EOC, indicating genetics as a key risk factor for young EOC patients (33). Therefore, combing the results of gene profiles with prediction models such as Risk of ovarian malignancy algorithm (ROMA), Copenhagen index (CPH-I), Risk of ovarian cancer algorithm (ROCA), LR2, and the Assessment of Different NEoplasias in the AdneXa (ADNEX) model, would be helpful in early recognition of young women with high risk of EOC and in the further monitoring process (34,35).
The laterality of tumors was found to be a risk factor for the prognosis of old women, with bilateral tumors indicating a poorer outcome. This could be explained by the findings of Ditto et al. that bilaterality of EOC tumors is associated with lymph node metastases (36). Laterality failed to be a risk factor for young women could be due to the relatively small enrolled number. However, since we lacked the information on chemotherapy, which could act as a confounding factor as a number of old women did not receive chemotherapy due to their poor performance status, resulting in a false-positive result. More literature about the laterality of EOC tumors is thus warranted for further study.
In this study, we have demonstrated that young women did not benefit from lymphadenectomy, while old women with lymphadenectomy had a higher CSS compared to those without lymph nodes removal. The difference might be explained by our finding that young women with EOC were mostly at stage I, while the old were mostly at stage III, which suggests that lymph nodes were more likely to metastasize in old patients; thus those undetected lymph nodes containing metastatic cancer cells in old women would be removed. It could also be explained by the possible selective bias that women underwent lymphadenectomy in the control group might be relatively younger, and with a better health status compared to those without lymphadenectomy. Lymphadenectomy still represents a controversial issue in the surgery for ovarian cancer. The recent LION trial (37) focused on advanced ovarian patients with R0 cytoreductive surgery and negative lymph nodes detected both preoperatively and during the operation. In this patient group, the systemic pelvic and paraaortic lymphadenectomy were not associated with a better OS or progression-free survival (PFS). Similar results were observed in patients with ovarian cancer at early stages (38) and those who underwent lymphadenectomy after primary surgery (39). Due to the limited data on lymph nodes removal such as number, location, postoperative complication, and recurrence rate, we could not get further results. According to the evidence we have, we suggest that for young women there is no need for lymphadenectomy unless lymph nodes detected by radiologic examination or tested positive during the surgery, or fertility-sparing is required.
Some limitations could not be overlooked in the current study. Missing data on histology, stage and surgery might cause selective bias. Secondly, information on residual disease, cytoreduction surgery and neoadjuvant chemotherapy was included since 2010, which restricted us to analyze their roles in survival outcomes in the young patient group. Experience of surgeons, the specific type of lymphadenectomy, the recurrence incidence of EOC, and fertility outcomes of those who underwent uterine preserving surgeries were not included in the SEER database, thus we could not analyze their relationship with the outcomes of EOC patients. Currently, most researchers hold the view that combined biomarkers, instead of single one, are encouraging in the screening process (35,40).For example, human epididymis protein 4 (HE4) and CA125 are included in ROMA with a sensitivity of 71% and specificity of 88% (41). However, only CA125 results were available in the SEER database.
In summary, women with EOC aged under 35 have higher OS and CSS compared to women aged 60-79, which could be due to a large percentage of mucinous and endometrioid histological types and early-diagnosis in the young EOC women group. Physicians can provide more positive prognosis information for young EOC patients, and the identification of histological types should be underscored in the diagnosis of this patient group. Moreover, gene diagnosis might play an important role in further prognosis assessments and clinical decisions.

DATA AVAILABILITY STATEMENT
Publicly available datasets were analyzed in this study. This data can be found here: https://seer.cancer.gov/.

ETHICS STATEMENT
Ethical review and approval was not required for the study on human participants in accordance with the local legislation and institutional requirements. Written informed consent for participation was not provided by the participants' legal guardians/next of kin because the data in the study was extracted from the SEER database, and the authors have obtained the approval to use the database.

AUTHOR CONTRIBUTIONS
YH, XM, and ZyL designed the study. YH, BjL, and XM extracted and analyzed the data. YH and BjL wrote and edited the manuscript. All authors contributed to the article and approved the submitted version.