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. 2022 Jan 12;12:572. doi: 10.1038/s41598-021-04307-3

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© The Author(s) 2022

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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(a) Pretreatment computed tomography (CT) image shows the tumor (asterisk) at the right lateral wall of the oropharynx. (b) Magnetic resonance imaging scan after initial treatment shows complete response. (c) CT image shows the local recurrence of the tumor (arrow) at the right lateral wall of the oropharynx. (d) The pretreatment circulating tumor DNA (ctDNA) was positive and became negative after initial treatment with surgery. ctDNA was positive again at the time of local recurrence and turned negative again after concomitant chemoradiotherapy with CDDP.