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To identify the parameters related to the effective selection of patients who could receive prognostic benefit from lateral pelvic node dissection.Accurate preoperative diagnosis of lateral nodal involvement (LNI) remains difficult, and the indications for lateral lymph node dissection have been controversial.A total of 244 consecutive patients who underwent potentially curative surgery with lateral dissection for advanced lower rectal cancer (1985–2000) were reviewed. Patients were stratified into groups based on various parameters, and the therapeutic value index for survival benefit was compared among groups. The therapeutic index of lateral dissection was calculated by multiplying the frequency of metastasis to the lateral area and the cancer-related 5-year survival rate of patients with metastasis to the lateral area, irrespective of metastasis to other areas (mesorectal, superior rectal artery [SRA], and inferior mesenteric artery [IMA] areas).LNI was observed in 41 patients (17%); and 88% of them had nodal involvement in the region along the internal iliac/pudendal artery or in the obturator region (“vulnerable field”). The cancer-related 5-year survival rate among the patients with LNI was 42%; the therapeutic index for lateral dissection was calculated as 7.0 patients, which was much higher than that of lymphadenectomy of the SRA area (1.6 patients) and the IMA area (0.4 patients), and almost comparable to that of lymphadenectomy of the upward mesorectal area (6.9 patients). Although it was possible to select groups at high and low risk for LNI based on several parameters related to tumor aggressiveness, such as tumor differentiation in biopsy specimens, the therapeutic value index was not significantly different between these groups. Unlike these parameters, the diameter of the largest lymph node in the “vulnerable field,” which was positively correlated with the rate of LNI but irrelevant to the prognosis, was able to successfully stratify patients by therapeutic index.Advanced lower rectal cancer patients having LNI in the lateral pelvic area are likely to receive prognostic benefit from lymphadenectomy. The most efficient means of determining the effectiveness of lateral dissection preoperatively is to estimate the nodal diameter in the “vulnerable” lateral regions by diagnostic imaging.