We demonstrate that inhibitors against ATR (AZD6738), and particularly ATM (AZD1390) and DNA-Pkcs (AZD7648), could significantly decrease clonogenic survival of HNSCC cell lines following PBT at both low and relatively high LET (~2 keV/μm and ~8 keV/μm, respectively). The gene discussed is ATM; the disease is head and neck squamous cell carcinoma.