Ponente
Descripción
In radiotherapy, one of the basic pillars against cancer, the aim is to max- imize the damage to tumor cells while sparing the normal tissues surrounding the tumor as much as possible from irradiation. A great effort has been made in this direction, with the development of increasingly precise clinical devices producing the radiation beams and irradiating the patients. Nowadays, main treatments are based on megavoltage X-rays devices, though other forms of radiation (protons or heavy ions) are growing in clinical use. In many of them secondary neutrons are produced and a considerable dose deposition on tissues outside the target volume may occur, thus compromising patient health in the long term.
Apart from these radiological protection effects, neutrons could be also considered from the therapy point of view. In all cases, the absorbed dose is the fundamental magnitude. In principle, it is used to determine the effects produced by ionizing radiation on living tissues. However, there is no a di- rect, one-to-one, relationship between absorbed dose and biological effects. The reason is that such effects depend on many different factors (dose frac- tionation, absorbed dose rate, radiation quality, specific biological systems that are irradiated, etc.) A crucial aspect concerns the end-point considered to quantify the radiation effects. Specifically, cell survival, chromosomal aberrations, molecular damage to DNA, and other have been considered and the common approach has been to try to define general, weighting factors that, together with the absorbed doses, allow the estimation of the biological effects. One of these factors, maybe the most used in radiobiology, is the relative biological effectiveness (RBE), which is defined as the ratio between the dose of a reference radiation and the dose of the radiation under con- sideration, both producing the same biological effect. Consequently, RBE is strongly affected by the end-point considered to measure this biological effect [1].
In the case of neutrons, the RBE is know to be high, making small neu-
tron doses to be non negligible at all. But apart from the end-point problem, current uncertainties, still too high, and the strong energy dependence [2], mainly due to the way in which the “non-charged” neutrons interact with matter, impose the necessity of gaining more knowledge on the neutron RBE.
This situation has produced changes in neutron radiation protection stan- dards over time. The most significant, in what refers to the weighting factors, can be found in ICRP publications [3,4]. Radiation protection standards de- pend on the data adopted for the assessment [5] and thus recommendations in radiation protection require new quality data to reduce the existing dif- ferences between the values adopted by various international organizations. It is therefore necessary to continue the research in this field in order to im- prove our knowledge of the effect of neutrons on living tissues, a knowledge of great importance in radiological protection and, eventually, in the possible therapeutic applications of neutron beams.
In this work, the state-of-the-art of the neutron radiobiology is updated, summarizing the most relevant achievements in recent years in this research line.
[1] IAEA, TRS-461. Relative biological effectiveness in ion beam therapy. International Atomic Energy Agency. Vienna, 2008.
[2] G Baiocco, S Barbieri, G Babini, et al. The origin of neutron biological effectiveness as a function of energy. Sci Rep 2016;6:34033.
[3] ICRP. Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Ann ICRP 1991;21 (1-3).
[4] ICRP. Relative biological effectiveness (RBE), quality factor (Q), and radiation weighting factor (wR). ICRP Publication 92. Ann ICRP 2003;33 (4).
[5] NRC Regulations Title 10, Code of Federal Regulations: 10 CFR. 20.1004, 2014.
