Liquid biopsy with droplet digital PCR targeted to specific mutations in plasma cell-free tumor DNA can detect ovarian cancer recurrence earlier than CA125

•Droplet digital PCR was applied to detect circulating tumor-derived DNA (ctDNA) in ovarian cancer (OC) follow-up.•Five out of six recurrent OC cases were positive in ddPCR analyses before increased plasma CA125.•Increased allele frequency of the ctDNA is associated with increased tumor volume after OC recurrence.•No ctDNA was detected in the plasma of recurrence-free OC cases (maximum, 439 PFS days)