6063 Background: The prognosis of early oral squamous cell carcinomas (OSCC) is strongly correlated with local disease control. However, patients with T1-T2 OSCC operated with clear margins have a 5-year local relapse rate of 10 to 15%. An accurate evaluation of the microscopic residual disease (MRD) is therefore essential. Interestingly, innovative techniques have recently been developed to detect circulating tumor DNA. Methods: A prospective multicenter trial based on patients with T1-T2 OSCC treated surgically with clear margins, was designed to assess the feasibility of a molecular analysis (tetranucleotide instability) of these margins to evaluate a potential MRD while adapting the postoperative strategy (NCT00232960). We, then, evaluate the feasibility of detecting MRD in the margins by identifying specific molecular abnormalities in the primary tumor with Whole Exome Sequencing (WES) and explore them in margins with deep target NGS panel (TGS) and digital PCR (dPCR) for hotspot mutations. Results: 310 patients were included initially with 216 tumor/margins samples available for molecular analysis. After a standard pathologic analysis, all surgical margins were negative.Median follow-up was 58 months [30;83]. Tumor was informative for tetranucleotide instability analysis in 63% of cases. Positive molecular margins were observed in 17.3% of cases, leading to a postoperative treatment (surgery or radiotherapy). In informative tumors, molecularly driven treatment seemed to lower the 5-year local recurrence rate from 14.1% to 6.4% (p=0.15). Among this initial cohort, 108 primary tumors were secondarily screened by WES to find mutations of interest, with an informativity rate of 75%. Positive molecular results (TGS or dPCR), with at least one positive margin, were observed in 19.4% of patients. Then, analyzing retrospectively the oncologic outcomes, we observed a significant benefit for local control at 5 years for patients with TGS-dPCR negative results in all margins compared to pathologic negative margins (91.7% versus 64.1%, p=0.02). Conclusions: To our knowledge, this is the first report of local MRD assessment after surgery using NGS.Molecular analysis of resection margins for early OSCC lead to a better evaluation of MRD and personalized postoperative decision making to improve local control. Clinical trial information: NCT00232960 .
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