Challenges Surrounding Relative Biological Effectiveness for Particle Therapy

Particle therapy delivered either with protons, carbon, or other ions has unique biological effects when compared to photon therapy. To optimise the clinical outcomes of particle therapy, it is necessary to understand the biological characteristics of particle beams (PB) including how these relate to physical factors and introduce this knowledge into the treatment planning process. The enhanced biological effects produced by PB are quantified by their relative biological effectiveness (RBE) which defines the dose modification necessary to achieve the same biological endpoint. Linear energy transfer (LET) and RBE values are highest around the distal ends of a beam and especially at the distal fall-off of the Bragg curves. Numerous models to predict the RBE of proton beams currently exist. Tumour control monitoring is also mandatory especially when the tumour types are known to be highly radiosensitive to standard megavoltage radiotherapy where the RBE may be very close to unity.