WE‐A‐218‐02: Assessing the Repair of DNA Damage from Multi‐Pass CT Protocols

Abstract

Purpose: To determine whether radiation damage is additive for fractionated doses at diagnostic levels using mouse tumor infiltrating lymphocytes (Til) in an in vitro assay. Methods: Til cells placed in the center chamber of a CDTI head phantom were irradiated using a Multidetector row CT scanner (Siemens Sensation64). 20mGy, 40mGy or 60mGy (reported CTDIvol values) irradiations were performed by either a single pass or multiple passes of 20mGy separated by 2 minutes. The cellular response to DNA double strand breaks (DSBs) was quantified by flow cytometric quantitation of gammaH2AX foci 1hr post‐irradiation at room temperature. Non‐irradiated controls were included for each sample. Collected flow cytometry data, corrected for debris and dead cells using side and forward scatter gating, were analyzed with CellQuest Pro Software. Fluorescence histograms were generated and median fluorescence of each treatment sample was compared to respective controls and excess gammaH2AX signal was reported. Results represent the average from three independent experiments. Results: A linear, dose‐dependent response of excess gammaH2AX signal was observed for single pass doses of 20– 60mGy. However, in cells exposed to multiple passes of 20mGy, a significant decrease (P<0.05) in excess gammaH2AX signal was observed compared to equivalent single pass doses. Conclusion: These preliminary results suggest multi‐pass CT protocols may alter the biological response to DNA DSBs in vitro, with significantly reduced gammaH2AX signal when the same total dose is fractionated, even with very short inter‐fraction intervals (minutes), rather than delivered as a single exposure. Kinetic analysis is necessary to determine whether this reduction is due to increased repair rate or decreased repair capacity. This altered response could have marked implications for current diagnostic procedures and thus underscore the importance of understanding how imaging protocols may affect the biological response in order to accurately assess risk estimates and biological dose. This work is partially supported by a SIEMENS Corporation research grant.