Prognostic role of lymph node micrometastasis in oral and oropharyngeal cancer: A systematic review

Background: An estimated 20% of patients with oral and oropharyngeal squamous cell carcinoma (OOSCC) have micrometastases (Mi) or isolated tumor cells (ITC) in the cervical lymph nodes that evade detection by standard histological evaluation of lymph node sections. Lymph node Mi and ITC could be one reason for regional recurrence after neck dissection. The aim of this study was to review the existing data regarding the impact of Mi on the survival of patients with OOSCC. Methods: PubMed and the Cochrane Library were searched for articles reporting the impact of Mi and ITC on patient survival. Two authors independently assessed the methodological quality of retrieved studies using the Downs and Black index. Data were also extracted on study type, number of included patients, mode of histological analysis, statistical analysis, and prognostic impact. Results: Sixteen articles with a total of 2064 patients were included in the review. Among the 16 included studies, eight revealed a statistically significant impact of Mi on at least one endpoint in the Kaplan-Meier and/or multivariate analysis. Three studies regarded Mi as Ma, while five studies found no impact of Mi on survival. Only one study demonstrated an impact of ITC on patient's prognosis in the univariate but not in the multivariate analysis. Conclusion: The majority of cases included in the review were patients with oral cancer. The findings provide lowcertainty evidence that Mi negatively impacts survival. Data on ITC were scarcer, so no conclusions can be drawn about their effect on survival. The lower threshold to discriminate between Mi and ITC should be defined for OOSCC since the existing thresholds are based on data from different tumors. The histological, immunohistological, and anatomical characteristics of Mi and ITC in OOSCC as well as the effect of radiotherapy on Mi should be further investigated separately for oral and oropharyngeal carcinomas.