To do this, says Costes, "I simulated the optical transformation when imaging events at the nanoscale are viewed with a fluorescent microscope." One optical transformation is blurring, with the result that, in a real microscope, two nearby radiation-induced foci may blend together and look like one.
When such factors were taken into account, Costes's initial model, following the NASA model, confirmed random distribution of double-strand breaks at the micron scale. The frequency of DSBs was radiation dependent, however: in the case of cosmic rays (NASA's main concern), which lead to complex double-strand breaks, there was good agreement with the RIF frequencies seen in the microscope. But in the case of gamma rays, which lead primarily to sparse and noncomplex DSBs, measured frequencies in real microscope images were lower.
This suggests that RIF correspond to sites of complex double-strand breaks in DNA. Moreover focusing on cosmic rays for the NASA project although the DSB frequency was the same as predicted by the models, the RIF in real microscope images were distributed very differently.
Model physics, real biology
"For one thing, there is a time effect," says Costes. "Just five minutes after cells are exposed to high-energy particles, microscope images already show a nonrandom distribution of RIF." The RIF occur along straight lines, as expected for a particle track, but are not randomly distributed along the lines. "Even though we have the right foci frequency along a track, many foci appear to repulse each other within the first 30 minutes after irradiation" a suggestion that they might be moving to specific regions of the nucleus.