Motion-based tissue ex vivo (MOTEX) assay to assess proton and X-ray irradiation responses in head and neck squamous cell carcinoma

Background and purpose: Proton radiotherapy is applied for various tumor sites, offering better dose distribution resulting in decreased excess radiation of healthy tissue. Furthermore, the biological effects of proton irradiation may be different from X-ray irradiation, depending on tumor characteristics. This is particularly relevant for head and neck squamous cell carcinoma (HNSCC), which displays high biological heterogeneity. However, this heterogeneous response to various radiation modalities is currently not included in clinical decision making due to lack of response prediction models. Materials and methods: for up to five days. We subsequently analyzed proliferation, apoptosis, DNA repair and immune cell infiltration. Results: Most tumors (five out of nine) showed similar response to proton and X-ray irradiation. However, in three tumors a significantly larger decrease in viability was measured upon proton irradiation. One of those tumors was homologous recombination deficient (HRD). The other two proton-sensitive tumors showed low numbers of infiltrating immune cells, including tumor infiltrating lymphocytes, while the most X-ray sensitive tumor had a particularly high immune cell infiltration. Conclusion: assay revealed heterogeneous response of HNSCC tumors to proton and X-ray irradiation. The tumors with increased sensitivity to proton irradiation either showed HR deficiency or low immune cell infiltration. However, not all immune-deserted tumors showed this skewing towards X-ray resistance. These findings require validation in a larger cohort of patients.