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[Abstract] ICRP Publication 131, “Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection,” examines stem/progenitor cell response to ionizing radiation, specifically in view of the development of stochastic effects, as these cells can “accumulate the protracted sequence of mutations necessary to result in malignancy”As discussed in Publication 131, carcinogenesis depends on mechanistic factors associated with tissue stem cell populations, e.g., the number and sensitivity of stem cells to radiation-induced mutation; the retention of mutated stem cells in tissue; and the population of stem cells with a sufficient number of predisposing mutations. In addition, it is thought that the stem cell competition model will help explain the varying risks among tissues from the effects of chronic irradiation, age dependence of radiation carcinogenesis, and potentially the sparing effects of risks at low doses. ICRP Publication 131 examines several biological processes that could act to protect tissues from the accumulation of mutated stem cells (and consequent disease or malignancy):○ Accurate DNA repair (a relatively quick-acting defense); ○ Rapidly-induced death (apoptosis) of injured stem cells; ○ Passing of mutations to daughter differentiating cells (i.e., so they are not retained in parental cells); and/or ○ Stem cell competition, where undamaged stem cells “outcompete” damaged stem cells for niche residence. As the body of knowledge with regard to stem cells and radiation carcinogenesis grows, it will be of use to radiobiologists to review work conducted in parallel by stem cell biologists, particularly in areas related to the regulation of mutated stem cells within tissue. In these several years, tissue stem cell biology with related to senescence and carcinogenesis has developed very rapidly. In this context, some works have discussed the aspects of stem cell competition within tissues for reducing the mutated complement of stem cells. If the similar mechanism will be found in many different tissue stem cells, the risk of low dose/low dose rate radiation will be discussed from the different perspective of the current radiological protection systems.[要約]放射線は、医療の診断、治療などを通して人間の健康に大きく貢献している。また、ラドンなどの自然放射線によって、日常生活などでも深く関わっている。人間生活を考えると、放射線、特に低線量・低線量率放射線が最も関係が深く、その生物学的解釈を通して、リスクの有無、大きさをより科学的に捉え、更にはそれを放射線防護体系として一般化する努力が不断に必要とされる。放射線防護体系に係るごく最近の動きだけでも、極めて重要な事項が幾つもある。例えば、福島事故に係るICRP（国際放射線防護委員会International Commission on Radiological Protection: 以下ICRP）文書ドラフト（ICRP、201X）の公表は、ICRP勧告の解釈を、これまでより、より分かりやすく、現実的な内容として発展させるものとして重要と考えられる。また、低線量・低線量率放射線影響については、ICRP Pub131 の公表（2015）により、これまで依拠してきた疫学データ、特に広島・長崎の原爆被爆者疫学データから離れて、生物学的な観点から、これまでとは異なる低線量・低線量率放射線影響を見るきっかけができた可能性が考えられる。その新たな観点を踏まえて、組織幹細胞（競合）に関する最近の生物研究成果を見ると、ICRP Pub131による新たな生物学的解釈を支持する報告が出ていると考えられる。低線量・低線量率放射線影響の解釈を発展させるために、今後も増えると期待できる新しい生物研究成果を、今後の放射線防護体系の議論に取り入れていくことが重要と考えられる。 |
