ifa23 said:
Is there a threshold for neutron contamination in tissue used LINAC that is safe and has been regulated by an international organization?
I haven't heard of such a thing.
When it comes to radiation protection, limits are established in terms of
effective doses. These are tissue-weighted sums of
equivalent doses. Equivalent doses are physical absorbed doses (energy per unit mass) weighted for stochastics effects (e.g. cancer induction). The weighting for neutrons is energy dependent and ranges roughly from 2-20 with a peak for kinetic energies around 1 MeV, compared to a weighting factor of 1 for photons.
The International Commission on Radiation Protection (ICRP) Report 103 (2007) recommends limiting effective doses to members of the general public to less than 1 mSv per year. This is relative to average background exposures of about 2-3 mSv per year (though this can fluctuate with factors such as geography).
Generally, in radiation protection, rather than declaring a "safe" threshold, we try to keep exposures ALARA--as low as reasonably achievable, social and economic factors considered. So in terms of linac shielding, we design facilities and processes to keep exposures well below the ICRP recommendations.
Patients irradiated by a linac are a different story. They need to receive quite high doses of radiation, and so protection limits don't apply. Historically there has been a reluctance to heavily modulate photon beams with energies in their spectra above 10 MeV to avoid producing neutrons, but more recently higher energy modulated treatments (VMAT or IMRT) are becoming more popular, particularly 10 MV beams without flattening filters, due to the high dose rates one can achieve with them. We also try to avoid irradiating patients with implanted electronic devices such as pacemakers with the higher energy beams, because neutrons can catastrophically damage the electronics.
