Study population
The patients had signed a consent when they were hospitalized, agreed to use clinical data and specimens for scientific research, and promised that their private information would not be disclosed. This study was approved by the Medical Ethical Committee of Xiangya Hospital (No.202005065) and was registered in the Chinese Clinical Trial Registry (ChiCTR2000033446). Clinical records of patients with BOT (diagnosis confirmed histopathologically) between January 2000 and June 2017 were retrieved from the information systems of two institutions (The First Affiliated Hospital of Hunan University of Medicine and Xiangya Hospital, Central South University). Patients from Xiangya Hospital were included into the training cohort, and patients from The First Affiliated Hospital of Hunan University of Medicine were included into the validation cohort.
Inclusion and exclusion criteria
The inclusion criteria were defined as follows: (1) patients who underwent conservative surgical treatment, consisting of unilateral ovarian tumor resection, bilateral ovarian tumor resection, unilateral adnexal resection, or unilateral adnexal resection + contralateral ovarian tumor resection; (2) BOT patients with a definite postoperative pathological diagnosis; (3) patients with complete clinicopathological and follow-up data; and (4) patients who agreed to receive telephone interviews.
In addition, patients were excluded if they met the following exclusion criteria: (1) had missing clinical data and; (2) refused to follow up.
Variables definition
Disease-free survival (DFS) was defined as the time interval between the date of postoperative pathological diagnosis and the date of the last imaging examination without signs of recurrence, recorded in months. The practice period of the surgeon was recorded in years, and it was equal to the operation date of the patient from the time the surgeon first became an attending physician. Re-operation was defined as a comprehensive staging surgery performed after the first operation before tumor recurrence. The follow-up was terminated if there was any evidence of recurrence that was confirmed on imaging.
Data collection
The collected data included: (1) patient demographics like age, gravidity, breast disease history, and ovarian disease history; (2) clinical characteristics, including the FIGO stage (according to the International Federation of Gynecology and Obstetrics 2014 criteria [17]), tumor size (maximum diameter of tumor reported by CT or MRI or ultrasound), type of primary surgery, and postoperative pathological diagnosis; (3) follow-up content: details on treatment, relapse and postoperative fertility, including postoperative adjuvant therapy, time to relapse, natural or assisted pregnancy, delivery way, delivery date, and pregnancy complications; (5) practice period of the primary surgeon.
Model Development
Continuous variables, including age, practice period and tumor size were grouped into graded data at intervals of 10 years, 10 years, and 2 cm, respectively. The patients in the training cohort were divided into the recurrence and non-recurrence groups. We used SPSS 26.0 (IBM Co., Armonk, NY, USA) to compare the difference between the recurrence group and the non-recurrence group, categorical variables were analyzed using the Chi-square test or Fisher’s exact test, and the variables that showed statistically significant results in the univariate log-rank test were subsequently included in the multivariate Cox regression model. Differences were considered significant at a level of P<0.05 and hazard ratios (HR) were represented with their 95% confidence intervals (95% CI). DFS curves were depicted using the Kaplan–Meier method and compared using the log-rank test.
After identifying significant factors related to DFS through multivariate analyses (P<0.05), a nomogram for predicting the 1-, 3-, and 5-year DFS was constructed using the R v4.0 software (R Foundation for Statistical Computing, Vienna, Austria; http://www.r-project.org/) with the rms and survival package.
Model validation
Internal validation was performed using the Bootstrap method for repeated sampling 1000 times, and the calibration of the nomogram was evaluated by the Concordance index (C-index). The calibration curve was analyzed by plotting the nomogram predictions and the actual recurrence rates of BOT patients treated with fertility-preserving surgery. The range of C-index is 0.5-1. Closer the C-index is to 1, better the discrimination between the model prediction results and the reality. The receiver operating characteristic curve (ROC) was drawn, and the precision (sensitivity and specificity) of the model was evaluated by calculating the area under curve (AUC). Closer the AUC value is to 1, better is the model’s discrimination.
External validation was performed using the data of the validation cohort. Time and region of data collection in the validation cohort were different from those in the training cohort, but the inclusion and exclusion criteria were the same.
Delivery methods and recurrence
To investigate the relationship between the methods of delivery and recurrence, we used the Chi-square test of the four-grid to analyze if there was a significant difference in delivery methods between the recurrent and non-recurrent groups in the training cohort. A p-value of <0.05 was considered statistically significant.