29P Exploring the risk of second primary malignancies in head and neck versus non-head and neck small cell carcinoma survivors

Background: Patient-derived tumor slices offer the opportunity to study individual patient responses to (targeted) radiotherapy, with the potential to tailor radiotherapy protocols in future.This is particularly valuable for heterogeneous and rare cancers, where clinical trials and standardized protocols are lacking.Methods: Fresh patient-derived HNSCC samples were sectioned into 400 mm slices and cultured on cell culture inserts.The slice cultures were then irradiated, either alone or in combination with inhibitors (potential radiosensitizer).After 2 and 24 hours, the samples were fixed and frozen.DNA double strand breaks (DSBs) were analyzed by quantifying 53BP1 foci in nuclei co-stained with the SCC marker p63 using immuno-fluorescence microscopy.Radiation induced DNA damage was correlated with patient's clinical outcome to radiation therapy, if possible.Results: Tumor slices from over 50 patients were successfully cultivated (success rate >95%, stable oxygenation and proliferation >72 hours).1: HPV-positive oropharyngeal squamous cell carcinoma (OPSCC) samples (n14) had significantly higher residual DSBs than HPV-negative ones (n12) post-radiation (3 Gy: 4.9 vs. 1.2 foci/ nucleus; p < 0.0001).Patients with high residual DSB (> 5 foci/nucleus) showed significant higher 3-year-tumor-relapse-free-survival than patients with little radiation induced DNA damage (< 5 foci/nucleus).2: Sinonasal squamous cell carcinoma (SNSCC) samples (n16) showed variable residual DSBs correlated to p16/HPV and smoking status.Higher ex vivo residual DSBs corresponded with favorable clinical radiotherapy response.3: WEE1/PARP inhibition significantly increased residual DSBs in SNSCC (n4/6), indicating selective radiosensitization potential.Conclusions: Ex vivo tumor slices offer a promising model for refining radiotherapy in head and neck cancer, e.g.patient selection in HPV de-escalation trials.SNSCC tissue slices show varied DNA damage repair capacity and could guide therapy decisions in this challenging entity.Dual WEE1/PARP inhibition could boost radiosensitivity in a SNSCC subgroup, meriting further study.