Photoneutron Dosimetry in Medical Linear Accelerators

Photoneutron contamination arises when high‐energy therapeutic X‐rays interacting with high‐Z components in a linear accelerator head induce (γ,n) reactions, producing stray neutrons that contribute to out-of-field dose. Accurate dosimetry of these photoneutrons is essential for patient safety, staff protection and shielding design. Traditional methods have relied on Bonner sphere systems, activation foils and thermoluminescent detectors to characterise neutron fluence and ambient dose equivalent in treatment rooms. In recent years, advances in computational modelling and detector technology have driven a shift towards combined measurement–simulation strategies. Monte Carlo codes now allow detailed prediction of neutron spectra and dose distributions, while novel miniature and directional detectors offer improved spatial resolution, energy discrimination and real-time monitoring. These developments underpin more precise second-cancer risk estimation, enhanced radiation protection protocols and the optimisation of bunker design worldwide.

Research from Nature Portfolio

A passive multi-directional neutron spectrometry system has been devised using polycarbonate/¹⁰B detectors mounted on the faces of polyethylene cubes of varying diameters. This arrangement produces well-resolved, directional photoneutron spectra in thermal and fast energy regions at multiple field sizes and bunker locations. Its simplicity, cost-effectiveness and insensitivity to low-LET radiation make it a practical tool for routine clinical assessment and cross-facility comparisons.

A complementary approach employs a miniature tissue-equivalent neutron dosimeter incorporating polycarbonate, boron and cadmium inserts, applied within whole-body phantoms under high-energy X-ray beams. Each device yields seven energy-specific ambient dose equivalent responses per measurement point, enabling the construction of detailed dose matrices across organ surfaces and depths. This high-resolution spectrometric dosimeter supports tissue-specific second-primary-cancer risk evaluation and has proven adaptable to wider health-physics and spaceflight applications.

Photoneutron Dosimetry in Medical Linear Accelerators publication trend

The graph below shows the total number of articles in photoneutron dosimetry in medical linear accelerators across all publications each year (not limited to Nature Index journals).

Technical terms

Photoneutron: Neutron emitted following a photonuclear reaction when high-energy photons interact with a nucleus.

Ambient dose equivalent (H*(10)): Dose quantity representing the dose to a 10 mm deep tissue layer from external radiation fields.

Monte Carlo simulation: Computational method using random sampling to model the transport and interactions of particles in complex geometries.

Neutron spectrometry: Technique for measuring and unfolding the energy distribution of neutrons across thermal, epithermal and fast ranges.

Dosimeter: Device that measures and records cumulative radiation dose, often energy- and particle-specific.

References

1. Photoneutron spectrometry by novel multi-directional spherical neutron spectrometry system. Scientific Reports (2021).
2. Breakthrough whole body energy-specific and tissue-specific photoneutron dosimetry by novel miniature neutron dosimeter/spectrometer. Scientific Reports (2021).
3. Photoneutrons and Gamma Capture Dose Rates at the Maze Entrance of Varian TrueBeam and Elekta Versa HD Medical Linear Accelerators. Toxics (2023).
4. Neutron and capture gamma along the mazes of linear accelerator vaults. Journal of Applied Clinical Medical Physics (2003).
5. Measuring neutron spectra in radiotherapy using the nested neutron spectrometer. Medical Physics (2015).
6. Measurement of Neutron Dose Equivalent within and Outside of a LINAC Treatment Vault Using a Neutron Survey Meter. Quantum Beam Science (2021).