Micrometastasis in lymph nodes of colorectal cancer

Colorectal cancer (CRC) is one of the most common cancers worldwide. Postoperative adjuvant chemotherapy is recommended for node‐positive stage III patients. A systematic meta‐analysis reported that the presence of micrometastases in regional lymph nodes (LNs) was associated with poor survival in patients with node‐negative CRC. Because most data employed in the meta‐analysis were based on retrospective studies, we conducted a prospective clinical trial and concluded that stage II is a transitional zone between stage I and stage III, where CRC tumors continuously increase the micrometastasis volume in LNs and proportionally raise the risk for tumor recurrence. The one‐step nucleic acid amplification (OSNA) assay is a simple and rapid technique to detect CK19 mRNA using the reverse‐transcription loop‐mediated isothermal amplification (RT‐LAMP) method. Using the OSNA assay, we and colleagues reported that the upstaging rates of pStages I, IIA, IIB, and IIC were 2.0%, 17.7%, 12.5%, and 25%, respectively, in 124 node‐negative patients. Survival analysis indicated that OSNA positive stage II CRC patients had a shorter 3‐y disease‐free survival rate than OSNA negative stage II CRC patients. In 2017, AJCC TNM staging (the 8th version) revised the definition of LN metastasis in colon cancer and it is stated that micrometastasis should be considered as a standard LN metastasis. To our surprise, this revision was based on a meta‐analysis to which our previous study on micrometastasis largely contributed. The remaining questions to be addressed are how to find micrometastases efficiently and whether postadjuvant chemotherapy is effective to prevent disease recurrence and to contribute to longer survival.