Compromised detection of short DNA fragments can result in underestimation of radiation-induced clustered DNA damage. The fragments can be detected with atomic force microscopy (AFM), followed by image analysis to compute the length of plasmid molecules. Plasmid molecules imaged with AFM are represented by open or closed curves, possibly with crossings. For the analysis of such objects, a dedicated algorithm was developed, and its usability was demonstrated on the AFM images of plasmid pBR322 irradiated with 60Co gamma rays. The analysis of the set of the acquired AFM images revealed the presence of DNA fragments with lengths shorter than 300 base pairs that would have been neglected by a conventional detection method.
