Inverse dose protraction effects of high-LET radiation; Evidence and significance

Hamada, Nobuyuki*; Matsuya, Yusuke  ; Zablotska, L. B.*; Little, M. P.*

Radiation-induced biological effects vary depending on the linear energy transfer (LET), which represents the quality of radiation. When exposed to high-LET radiation acutely, the biological effects are in general greater than those of low-LET radiation; however, the effects of dose protraction remain unclear. Here, we review the current knowledge on inverse dose protraction effects of high-LET radiations. To the best of our knowledge, we identified 79 biological or epidemiological papers published since 1967 on high-LET radiation, such as neutrons, deuterons, -particles, light ions, and heavy ions. These papers include biochemical changes in cell-free macromolecules, neoplastic transformation, cell death, DNA damage responses and gene expression changes in mammalian cell cultures of human or rodent origin, gene mutations, cytogenetic changes, cancer, non-cancer diseases (e.g., testicular effects, cataracts, cardiovascular diseases) and life shortening in non-human mammals, and induction of lung cancer and bone tumors in humans. Meanwhile, the number of the papers (i.e., 79 papers) is about half of that on low-LET radiation (i.e., 154 papers). Manifestations and mechanisms of IDPEs of high-LET radiation are far less understood than those of low-LET radiation, warranting further studies that will be pivotal to assess the implications for radiation protection.