HPV-positive HNSCC cell lines show strongly enhanced radiosensitivity after photon but not after carbon ion irradiation

BACKGROUND AND PURPOSE: HPV positive (pos.) HNSCC cells are significantly more radiosensitive to photon irradiation as compared to HPV negative (neg.) cells. Functionally, this is considered to result from a reduced DSB repair capacity. It was now tested, whether such a difference is also observed when using carbon ion (12C) irradiation.

MATERIAL AND METHODS: Five HPV pos. and five HPV neg. HNSCC cell lines were irradiated with photons or 12C-ions using 2D or 3D cell culture conditions. Clonogenic survival was determined by colony formation assay and DSB repair by immunofluorescence using co-staining of γH2AX and 53BP1 foci.

RESULTS: The pronounced difference in radiosensitivity known for these two entities when exposed to photons in 2D cell culture, was reduced when treated under 3D conditions. Irradiation with 12C-ions strongly enhanced cell killing, whereby increase was more pronounced for the HPV neg. when compared to the HPV pos. cell line (RBE=2.81 vs. 2.14). As a consequence, after 12C-irradiation clonogenic survival was almost identical for the two entities as was demonstrated for all cell lines at a dose of 3 Gy. In line with this, the significant difference in DSB repair capacity between HPV pos. and neg. HNSCC cells, as seen after photon irradiation, was abrogated after 12C-irradiation.

CONCLUSION: While HPV pos. cells are significantly more radiosensitive to photons than HPV neg. cells, no significant difference was seen after 12C-irradiation. This needs to be considered when planning new clinical protocols for the treatment of HPV neg. and pos. tumors with 12C-ions.