Circulating tumor DNA (ctDNA) consists of genomic DNA fragments shed by tumor cells into the bloodstream that contain the same mutations present in the primary tumor. In human medicine, ctDNA has been utilized as a liquid biopsy tool and a noninvasive biomarker for the early diagnosis, prognosis, and management of several tumors. This study explored the utility of ctDNA monitoring using digital polymerase chain reaction (dPCR) during follow-up in two dogs with oral malignant melanoma. Whole-exome sequencing was performed to identify patient-specific tumor mutations, and dPCR probes designed for these mutations were then used to measure ctDNA levels in plasma samples obtained during follow-up. In two patients, ctDNAs were detected at the time of diagnosis, and in one patient, complete remission was attained via radiation; the disappearance of ctDNA was observed, followed by a re-elevation, indicating lymph node metastasis. In another patient in whom radiation therapy was ineffective, persistent ctDNA was noted during follow-up. To the best of our knowledge, this is the first study to identify patient-specific somatic mutations and monitor ctDNA using dPCR.
