ANDANTE: The project: a multidisciplinary approach to neutron RBE

There is growing concern about the possible long-term risks to radiotherapy patients from neutrons, which are generated as a byproduct either in high-energy photon beams, or proton beams. The usual approach to estimating the risk from neutrons is to use ICRP risk factors and radiation weighting factors for neutrons. There are a number of problems with this, and indeed the ICRP state clearly that their numbers are approximations, which should only be used for radiation protection guidelines, and not for individuals. According to the ICRP scheme, the risk from neutrons is determined by means of the relative biological effectiveness (RBE) of neutrons compared to photons for which the risks are better known. However there is considerable uncertainty about how RBE for neutrons varies with dose and neutron energy, or whether the RBE model is even appropriate. Two major reasons for this are that it is very difficult to obtain risk data in exposure situations where the neutron energy is confined to a narrow spectrum (or even well known), and the occurrence of risk events at the low doses of interest is very low giving poor statistics. Previous research on risks from neutron risk has mainly been singlediscipline. That is, the results have been dependent on a single experimental or epidemiological approach. By contrast, the ANDANTE project uses three different disciplines in parallel. This is analogous to taking images of an object using three different imaging modalities and combining them to produce an image much sharper than any of the individual images. The three parallel approaches are: Physics: a track structure model is used to contrast the patterns of damage to cellular macro-molecules from neutrons compared to photons. The simulations reproduce the same energy spectra as are used in the other two approaches. Stem cell radiobiology: stem cells from thyroid, salivary gland, and breast tissue are given well characterised exposures to neutrons and photons. A number of endpoints are used to estimate the relative risk of damage from neutrons compared to photons. As well, irradiated cells will be transplanted into mice to investigate the incidence of progression into tumours. Epidemiology: the relative incidence rates of second cancers of the thyroid, salivary gland, and breast following paediatric radiotherapy (conventional radiotherapy for photons and proton therapy for neutrons) are investigated in a pilot single-institution study, leading to a multi-institution prospective study. The project has completed the first of its four years. Progress on characterising the exposure beams, isolation and initial exposures of stem cells, and data collection for the epidemiological studies will be presented.