Background: Lateral lymph node dissection (LLND) is performed for advanced rectal cancers in Japan; however, it can cause sexual and urinary dysfunction. The incidence of lateral LN metastasis is estimated at 7–13.9%; therefore, excessive rectal surgery with LLND should be avoided, especially for prophylactic purposes. To identify the patients who require LLND, we examined metastases in perirectal LNs by using a one-step nucleic acid amplification (OSNA) assay to predict lateral LN metastases. Methods: Twenty-five patients who underwent surgery with bilateral LN dissection due to T3–T4 rectal cancers were prospectively included in this study. Twenty-two patients (88.0%) received preoperative chemotherapy. Among 1052 LNs from 25 patients (median 40 per case), 135 perirectal LNs (median 6 per patient) were divided into three pieces and analyzed by OSNA, reverse transcriptase-polymerase chain reaction for carcinoembryonic antigen mRNA, and pathological examination after surgery. These results were compared with the pathological diagnosis of lateral LNs. Results: Lateral LN metastases were present in 4 of 25 patients (16.0%). All of these patients were positive by OSNA for perirectal LN metastases. The OSNA assay had a sensitivity of 100%, specificity of 86%, positive predictive value of 57%, and negative predictive value (NPV) of 100% for predicting lateral LN metastases. Conclusions: The findings from this prospective study suggest that the OSNA assay of perirectal LNs may be useful for determining when LLND is necessary because of its high NPV, even in patients treated with preoperative chemotherapy.
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