| Summary: | <h4>Purpose</h4> <p>Understanding the mechanisms behind induced biological response following exposure to ionising radiation is not only important in assessing the risk associated with human exposure, but potentially can help identify ways of improving the efficacy of radiotherapy. Over the decades there has been much discussion on what is the key biological target for radiation action and its associated size. It was already known in the 1930s that microscopic features of radiation significantly influenced biological outcomes. This resulted in the development of classic target theory, leading to field of microdosimetry and subsequently nanodosimetry, studying the inhomogeneity and stochastics of interactions, along with the identification of DNA as a key target.</p> <h4>Conclusions</h4> <p>Ultimately the biological response has been found to be dependent on the radiation track structure (spatial and temporal distribution of ionisation and excitation events). Clustering of energy deposition on the nanometre scale has been shown to play a critical role in determining biological response, producing not just simple isolated DNA lesions but also complex clustered lesions that are more difficult to repair. The frequency and complexity of these clustered damage sites is typically found to increase with increasing LET. However in order to fully understanding the consequences, it is important to look at the relative distribution of these lesions over larger dimensions along the radiation track, up to the micrometre scale. Correlation of energy deposition events and resulting sites of DNA damage can ultimately result in DNA complex gene mutations and complex chromosome rearrangements following repair, with the frequency and spectrum of the resulting rearrangements critically dependent on the spatial and temporal distribution of these sites and therefore the radiation track. Due to limitations in the techniques used to identify these rearrangements it is likely that the full complexity of the genetic rearrangements that occur has yet to be revealed. This paper discusses these issues from a historical perspective, with many of these historical studies still having relevance today. These can not only cast light on current studies but guide future studies, especially with the increasing range of biological techniques available. So let us build on past knowledge to effectively explore the future.</p>
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