Comparison of circulating tumor cells and cell-free DNA in the molecular characterization of patients with head and neck cancer

e18521 Background: The role of liquid biopsy in diagnosis and therapy monitoring in patients with head and neck cancer has been much less studied compared to other cancers. Our aim was to evaluate the perfomance in the isolation and recovery for molecular characterization of circulating tumour cells (CTC) of a new immunoafinity-based method and to compare it with the molecular diagnostic yield of plasma cell-free DNA. Methods: Patients with recurrent/metastatic (RM) head and neck cancer (HNC) were enrolled prospectively. Forty mililiters (ml) of plasma were collected at one or several time-points. First blood draw was always collected before starting a new therapeutic intervention or at the time of radiologic progression. For CTC detection and isolation, either anti-EpCAM or both anti-EpCAM + anti-EGFR antibodies were used. Digital PCR and castPCR were used to study KRAS and PI3KCA mutations in non-squamous HNC. A 15-gene customized NGS panel was used to characterized both CTC and cfDNA in patients with squamous HNC. Results: Between February 2016 and October 2018, 14 patients with R/M HNC were included (n = 1 local-only disease, n = 10 local and distant disease, n = 3 distant-only disease). Squamous histology (S): n = 9. Non-squamous (NS): n = 5 (1 naso-ethmoidal intestinal-type adenocarcinoma, 1 parotid gland exadenoma pleomorfic carcinoma, 2 parotid-gland salivary duct carcinomas (SDC), 1 parotid-gland high-grade neuroendocrine carcinoma). Twenty-five CTC determinations were performed. In 5 patients serial CTC determinations were performed. Median CTC was 4 (min-max: 0-49). Median CTC among 11 CTC determinations in S-HNC was 4 (min-max: 0-49). Median CTC was 3 CTC (min-max: 0-26) among the 14 determinations performed in NS-HNC. Digital PCR unveiled mutations in CTC and in cfDNA in 2 of 4 patients tested with NS histology (KRAS, PIK3CA), with one of them being concordant for the specific mutation. NGS unveiled mutations in CTC in 7/9 patients and in cfDNA in 6/9 patients, with only one loci-concordant case between CTC and plasma. Conclusions: IsoFlux detected CTC in the majority of patients with R/M HNC, regardless of the histologic type, and allowed for molecular characterization of CTC using different techniques for mutational analysis. Both NGS and digital PCR allowed for the detection in cell-free DNA of commonly mutated genes in HNC. Liquid biopsy should be more actively studied in this disease in order to better define its role in diagnosis and therapeutic monitoring.