  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 consideration 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 avoiding 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 resulting 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.
