Oncologic Status of Obturator Lymph Node Metastases in Locally Advanced Low Rectal Cancer: A Japanese Multi-Institutional Study of 3487 Patients

Background The American Joint Committee on Cancer tumor-node-metastasis staging system for rectal cancer defines lateral pelvic lymph nodes (LPLNs) only in the internal iliac region as regional. However, the Japanese Society for Cancer of the Colon and Rectum (JSCCR) staging system, also considers obturator lymph nodes (LNs) as regional. This retrospective cohort study evaluated the oncologic status of obturator LNs in low rectal cancer. Methods The study identified 3487 patients with pT3-T4 low rectal cancer who had undergone curative resections without preoperative radiotherapy or chemotherapy between 2003 and 2011 in the JSCCR database and divided them into six groups. Overall survival (OS) and recurrence-free survival (RFS) were analyzed by groups. Results Histologic LPLN metastases were identified in 8% (279/3487) of all the patients and in 18.2% (279/1530) of the patients who underwent lateral pelvic node dissection. The 5-year OS and RFS rates of the obturator-LPLN group ( P = 0.095) were worse than those of the internal-LPLN group ( P = 0.075), but the difference was not significant. The OS of the obturator-LPLN group was similar to that of the resectable liver metastasis group ( P = 0.731), and the RFS of the obturator-LPLN group was significantly better than that of the other-LPLN group ( P = 0.016). Conclusion The prognosis for obturator LN metastases in low rectal cancer was not significantly worse than for internal iliac LN metastases, defined as regional by the current American Joint Committee on Cancer staging system, and the oncologic status of obturator LNs warrants more studies.