The state of LN in patients with AEOC affects the choice of lymphadenectomy and prognosis32. To the best of our knowledge, this was the first clinical study to assess occult pelvic and para-aortic LNM in patients with AEOC after cytoreductive surgery. In our study, the proportion of clinical OLNM was high, accounting for 51.3% of all LNM and 48.3% of negative LN on PET/CT scans. The primary tumor location and metastatic TLG index extracted from TLG of metastatic and all lesions were significantly associated with OLNM. The final logistic model could preoperatively predict OLNM in AEOC.
The use of systematic lymphadenectomy is controversial for patients with EOC. Although the positive rate of lymph nodes in patients with presumed early-stage EOC was low, appropriately 3% − 14% detected by systematic lymphadenectomy, LNM in AEOC was high4,5,33. LNM was found in 60.9% of all patients in our study, similar to the previous study 9. Occult nodal involvements were frequently found in AEOC. In our study, only 48.75% of the cases with LNM had positive lymph nodes on PET/CT scans. The other 51.25% of cases were negative in PET/CT scans. 18F-PET/CT is the most accurate modality for preoperative tumor staging and detection of metastatic lymph nodes. A high proportion of LNM is negative in PET/CT scans due to the low spatial resolution of PET-CT images and the low tissue uptake of the 18FDG. Most of the metastatic lesions, including LNM, tents to uptake less fluorodeoxyglucose than the primary lesions. Furthermore, tumor FDG binding depends on histological type and has a low affinity in mucus and clear cell adenocarcinoma34. In addition, the micro-metastases will not be detected because of the low reconstructed spatial resolution of PET. The previous study demonstrated sensitivity for detecting metastatic lesions 4 mm or less in short-axis diameter was 12.5%, that for between 5 and 9 mm was 66.7%35. Finally, massive ascites and extensive peritoneal metastasis also impact the assessment of LNM. In summary, OLNM is frequently observed in AEOC, which leads to the low sensitivity of 18F-PET/CT for LNM evaluation.
Many previous studies demonstrate that it was feasible to improve the diagnostic efficiency in LNM by clinical factors, radiological findings, and radiomics features extracted from PET or CT images. A new diagnostic tool based on multivariate analysis including three variables: pelvic and/or para-aortic LNM on CT PET/CT, and initial PCI ≥ 10, initial CA125 ≥ 500 was proposed27. Another study used preoperative radiological scores to predict pelvic and/or para-aortic LNM28. Although the ROC-AUC of those prediction models was moderate, the preoperative assessment of PCI / or colon involvement was difficult and ambiguous. Multiple studies have demonstrated the value of radiomics features in predicting the LNM of EOC28,36. The radiomics signatures extracted from CT images incorporating LN reports by radiologists could improve the diagnostic efficiency in LNM36. However, the calculation of the radiomics features is very complicated and time-consuming. We also found that the evaluation of LNM mainly relied on the subjective assessment of radiologists in previous studies. It is easy to identify positive LN on PET/CT scans because of hypermetabolism of metastatic LN. Identification of OLNM is critical for improving sensitivity in detecting LNM.
Previous studies have investigated the predictive value of PET metabolic parameters in AEOC25,27,37. MTV and TLG were independent prognostic factors for disease progression and overall survival after cytoreductive surgery in patients with EOC23,25. In our study, MTV, TLG, metastatic TLG index, and metastatic MTV index were highly associated with OLNM. The metastatic TLG and MTV indexes can quantify EOC's metastatic capacity, which positively correlates with metastatic tumor burden. OLNM, as part of metastatic lesions, was closely associated with soakage and metastasis of malignant ovarian tumors. Therefore, the metastatic TLG index was the only independent predictive factor in both univariate and multivariate and had a higher predictive value than other PET-related parameters. Bilateral ovarian cancers were associated with a high risk of OLNM because bilateral tumors showed more tumor heterogeneity and invasiveness. The final multivariate model incorporated clinical factors, radiological findings, and composite Index extracted from PET metabolic parameters.
There are several limitations to the present study. Firstly, this study is a single center retrospective analysis, potentially suffering from its inherent selection bias and limiting data generalizability. All patients underwent PET/CT scans in our hospitals using the same devices. The resolution of the image, administration of a carbohydrate free diet before scanning, and time for uptake were inevitably different, which might result in heterogeneous results. Further multicenter study is necessary to prove the efficiency of the result of this study. Secondly, the patients with neoadjuvant chemotherapy before surgery were not included in our study because NAC affects the pathological evaluation of postoperative LN status. NACT was considered for patients with advanced-stage ovarian cancer who are not good candidates for primary debulking surgery due to advanced age, frailty, poor performance status, comorbidities, or who have disease unlikely to be optimally cytoreduced38,39. Further study will be needed to explore OLNM in patients who received NACT before primary cytoreductive surgery. Thirdly, our study focused on patients with negative LN on PET/CT scans. Only the patients with negative reports in PET/CT scans were included in our study. Comparison between OLNM and pathologically confirmed no LNM, regardless of positive LN on PET/CT, will be needed in further study.