KEY POINTS
- This consensus brings together radiation-protection experience from five proton therapy centres participating in the FLASHForward Consortium, focusing on three areas that change materially at ultra-high dose rates: shielding, activation and accident scenarios.
- Proton FLASH commonly requires beam currents and energies far above conventional clinical operation. On Varian systems, research delivery may use up to 800 nA cyclotron current at 250 MeV, with single spots exceeding the commonly used FLASH threshold of 40 Gy/s.
- Existing conventional radiation surveys are useful for locating potential shielding weak points but cannot simply be scaled and accepted as sufficient. Monte Carlo simulation can guide survey planning, but direct UHDR measurements remain necessary, particularly because FLASH delivery can create new neutron hot spots.
- Monte Carlo calculations were reasonably conservative in available validation data: simulations overestimated neutron dose or dose rate by approximately 18–24% across several evaluated proton facilities, although discrepancies were larger where measurements approached background levels.
- Detector behaviour itself becomes a safety issue. The commonly used Wendi-2 neutron detector showed saturation at high beam currents, with nonlinear response observed around 15 nA in testing, illustrating why detector linearity must be checked before extrapolating measurements to FLASH currents.
- Activation can remain clinically relevant after the beam stops. A water beam stop used in Manchester produced activity of approximately 3 MBq 50 seconds after irradiation, with activity returning toward background over roughly 40 minutes; activated hardware and samples therefore require measurement before handling.
- Repeated UHDR irradiation can accumulate activation, while the short delivery time increases the consequence of room-access errors. The authors emphasize interlocked beam stops, area monitoring, personal dosimetry, strict room-clearance procedures and, in the Manchester workflow, a two-person room search/no-lone-working policy before FLASH delivery.
CLINICAL TAKEAWAY
A proton facility commissioned for conventional therapy should not assume that its existing radiation-protection program automatically covers FLASH operation. The practical message is to re-evaluate shielding, detector response, activation, occupancy and accident controls for the actual FLASH geometry and beam parameters; these are consensus considerations rather than universal regulatory requirements.