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Purpose To show that intrinsic radiosensitivity varies greatly for protons and carbon (C) ions in addition to photons, and that DNA repair capacity remains important in governing this variability. Methods We measured or obtained from the literature clonogenic survival data for a number of human cancer cell lines exposed to photons, protons (9.9 keV/μm), and C‐ions (13.3–77.1 keV/μm). We characterized their intrinsic radiosensitivity by the dose for 10% or 50% survival (D10% or D50%), and quantified the variability at each radiation quality by the coefficient of variation (COV) in D10% and D50%. We also treated cells with DNA repair inhibitors prior to irradiation to assess how DNA repair capacity affects their variability. Results We found no statistically significant differences in the COVs of D10% or D50% between any of the radiation qualities investigated. The same was true regardless of whether the cells were treated with DNA repair inhibitors, or whether they were stratified into histologic subsets. Even within histologic subsets, we found remarkable differences in radiosensitivity for high LET C‐ions that were often greater than the variations in RBE, with brain cancer cells varying in D10% (D50%) up to 100% (131%) for 77.1 keV/μm C‐ions, and non‐small cell lung cancer and pancreatic cancer cell lines varying up to 55% (76%) and 51% (78%), respectively, for 60.5 keV/μm C‐ions. The cell lines with modulated DNA repair capacity had greater variability in intrinsic radiosensitivity across all radiation qualities. Conclusions Even for cell lines of the same histologic type, there are remarkable variations in intrinsic radiosensitivity, and these variations do not differ significantly between photon, proton or C‐ion radiation. The importance of DNA repair capacity in governing the variability in intrinsic radiosensitivity is not significantly diminished for higher LET radiation.

Purpose: To determine the effect of a strong magnetic field on TLD‐100, OSLD (Al2O2:C), and PRESAGE dosimetry devices. This study will help to determine which types of dosimeters can be used for quality assurance and in‐vivo dosimetry measurements in a magnetic resonance imaginglinear accelerator (MRI‐linac) system. Methods: The dosimeters were separated into two categories which were either exposed or not exposed to a strong magnetic field. In each category a set of dosimeters was irradiated with 0, 2, or 6 Gy. To expose the dosimeters to a magnetic field the samples in that category were place in a Bruker small animal magnetic resonance scanner at a field strength slightly greater than 2.5 T for at least 1 hour preirradiation and at least 1 hour post‐irradiation. Irradiations were performed with a 6 MV x‐ray beam from a Varian TrueBeam linac with 10×10 cm2 field at a 600 MU/min dose rate. The samples that received no radiation dose were used as control detectors. Results: The readouts of the dosimeters which were not exposed to a strong magnetic field were compared with the measurements of the dosimetry devices which were exposed to a magnetic field. No significant differences (less than 2% difference) in the performance of TLD, OSLD, or PRESAGE dosimeters due to exposure to a strong magnetic field were observed. Conclusion: Exposure to a strong magnetic field before and after irradiation does not appear to change the dosimetric properties of TLD, OSLD, or PRESAGE which indicates that these dosimeters have potential for use in quality assurance and in‐vivo dosimetry in a MRI‐linac. We plan to further test the effect of magnetic fields on these devices by irradiating them in the presence of a magnetic fields similar to those produced by a MRI‐linac system. Elekta‐MD Anderson Cancer Center Research Agreement