Pouget, J-P., Frelon, S., Ravanat, J-L., Testard, I., Odin, F. and Cadet, J. Formation of Modified DNA Bases in Cells Exposed either to Gamma Radiation or to High-LET Particles. Radiat. Res. 157, 589–595 (2002).The aim of the present study was to measure the formation of eight base modifications in the DNA of cells exposed to either low-LET (60Co γ rays) or high-LET (12C6 particles) radiation. For this purpose, a recently optimized HPLC-MS/MS method was used subsequent to DNA extraction and hydrolysis. The background level of the measured modified bases and nucleosides was shown to vary between 0.2 and 2 lesions/106 bases. Interestingly, thymidine glycols constitute the main radiation-induced base modifications, with an overall yield of 0.097 and 0.062 lesion/106 bases per gray for γ rays and carbon heavy ions, respectively. Both types of radiations generate four other major degradation products, in the following order of decreasing importance: FapyGua > 5-HmdUrd > 5-FordUrd > 8-oxodGuo. The yields of formation of FapyAde and 8-oxoAde are one order of magnitude lower than those of the related guanine modifications, whereas the radiation-induced generation of 5-OHdUrd was below the limit of detection of the assay. The efficiency for both types of radiation to generate base damage in cellular DNA is low because the highest yield per gray was 0.097 thymine glycols per 106 DNA bases. As a striking observation, the yield of formation of the measured DNA lesions was found to be, on average, twofold lower after exposure to high-LET radiation (12C6 ) than after exposure to low-LET γ radiation. These studies show that the HPLC-MS/MS assay provides an accurate, reliable and sensitive method for measuring cellular DNA base damage.