Clinical use and future requirements of relative biological effectiveness: Survey among all European proton therapy centres

Background and purpose: The relative biological effectiveness (RBE) varies along the treatment field.
However, in clinical practice, a constant RBE of 1.1 is assumed, which can result in undesirable side
effects. This study provides an accurate overview of current clinical practice for considering proton
RBE in Europe.
Materials and methods: A survey was devised and sent to all proton therapy centres in Europe that treat
patients. The online questionnaire consisted of 39 questions addressing various aspects of RBE consider-
ation in clinical practice, including treatment planning, patient follow-up and future demands.
Results: All 25 proton therapy centres responded. All centres prescribed a constant RBE of 1.1, but also
applied measures (except for one eye treatment centre) to counteract variable RBE effects such as avoid-
ing beams stopping inside or in front of an organ at risk and putting restrictions on the minimum number
and opening angle of incident beams for certain treatment sites. For the future, most centres (16) asked
for more retrospective or prospective outcome studies investigating the potential effect of the effect of a
variable RBE. To perform such studies, 18 centres asked for LET and RBE calculation and visualisation
tools developed by treatment planning system vendors.
Conclusion: All European proton centres are aware of RBE variability but comply with current guidelines
of prescribing a constant RBE. However, they actively mitigate uncertainty and risk of side effects result-
ing from increased RBE by applying measures and restrictions during treatment planning. To change RBE-
related clinical guidelines in the future more clinical data on RBE are explicitly demanded.