Development of a new radiobiology beam line for the study of proton RBE at the 18 MeV proton cyclotron facility at CNA

In the last decades, particle radiation therapy, mainly performed with protons and carbon ions, has gained great interest in the medical community thanks to its excellent clinical results. The bases of this success are: (1) the physical advantages of protons and heavier ions with respect to conventional radiation therapy with photons, due to their more selective energy deposition in depth; and (2) their increased radiobiological effectiveness, since protons and heavier ions offer a higher cell-killing effectiveness if compared to photons for a same level of absorbed dose, property usually referred to as Relative Biological Effectiveness or RBE. The RBE depends on different factors, both physical and biological, such as radiation quality, track structure, linear energy transfer (LET), tissue type, biological endpoint, etc. However, in the case of protons, there is still place for improvements in what concerns the understanding of the behaviour of the RBE on both cancer and healthy cells, and its role in treatment planning and delivery. Even if it is agreed that proton RBE varies towards the distal Bragg peak region, increasing with LET, nowadays in clinical proton therapy treatments, a uniform RBE value of 1.1 is generally used, leading possibly to suboptimal proton treatment plans, as stated in Wedenberg et al. Therefore, studies of RBE at low proton energies are necessary in order to optimize proton therapy treatment plans and reach a consensus on the RBE variations near the Bragg peak. However, not many proton facilities can provide a monochromatic beam for radiobiological measurements at low energies (roughly below 20 MeV). With this purpose, we are preparing an experimental setup at the external beam line of the 18MeV proton cyclotron facility installed at the CNA (Seville, Spain) to cover this gap. [Methods and Results] In this work, we present our first feasibility studies. At present, the experimental beam line installed at the cyclotron facility counts on tw