Photoneutron production mechanisms, their characteristics, and shielding strategies in high-energy linac environment: A review

Radiotherapy, a mainstay cancer treatment for patients worldwide, utilizes medical linear accelerators (linacs) with photon energies ranging from 4 MV to 25 MV. However, concerns arise at higher energies (>6 MV, particularly >10 MV). These high-energy photons interact with high-atomic number materials in the linac head and collimation system, generating unwanted neutrons through (γ,n) reactions. This neutron contamination, present in both photon and electron beams, is a significant issue. Neutrons, with their high Linear Energy Transfer (LET), are more effective at causing clustered DNA damage (single and double-strand breaks). These neutrons not only impact shielding requirements in treatment rooms but also increase out-of-field radiation doses for patients receiving high-energy photon therapy. Therefore, for radiotherapy treatments exceeding 6 MV, additional precautions become crucial, including enhanced door shielding and optimized treatment planning. This review discusses in detail the multifaceted aspects of neutron production and shielding requirements during radiotherapy.