Neutron spectrometry and determination of neutron ambient dose equivalents in different LINAC radiotherapy rooms

Simulating the head of a TrueBeam linear particle accelerator and calculating the photoneutron spectrum on the central axis of a 10-MV photon using particle and heavy-ion transport system code

Photon energy is higher than the (γ,n) threshold, allowing it to interact with the nuclei of materials with high z properties and liberate fast neutrons. This represents a potentially harmful source of radiation for humans and the environment. This study validated the Monte Carlo simulation, using the particle and heavy-ion transport code system (PHITS) on a TrueBeam 10-MV linear particle accelerator's head shielding model and then used this PHITS code to simulate a photo-neutron spectrum for the transport of the beam. The results showed that, when comparing the simulated to measured PDD and crosslines, 100% of the γ-indexes were <1 (γ3%/3mm) for both simulations, for both phase-space data source and a mono energy source. Neutron spectra were recorded in all parts of the TrueBeam's head, as well as photon neutron spectra at three points on the beamline.

DEVELOPMENT OF A NEUTRON DOSIMETRY SYSTEM BASED ON DOUBLE SELF-ACTIVATED CSI DETECTORS FOR MEDICAL LINAC ENVIRONMENTS

In the present study, by using double self-activated CsI detectors, the development of a neutron dosemeter system whose response indicates better agreement with the International Commission on Radiological Protection-74 rem-response was carried out to simply evaluate the neutron dose with high accuracy. The present double neutron dosemeter system, using a slow-neutron dosemeter (thermal to 10 keV) and a fast-neutron dosemeter (above 10 keV), consists of CsI scintillators wrapped with two types of neutron energy filtering materials: polyethylene and B4C silicon rubber. After optimization of each filter thickness, to confirm the validity of our method, the neutron ambient dose equivalents under several operating conditions of medical linear accelerators (Linacs) were evaluated using a Monte Carlo simulation and an experiment with the present dosemeter. From these results, the present dosimetry system has enabled a more accurate neutron dose evaluation than our conventional dosemeter, and the present dosemeter was suitable for the neutron dosimetry for 10 MV Linac environments.

The influence of shielding reinforcement in a vault with limited dimensions on the neutron dose equivalent in vicinity of medical electron linear accelerator

Background

High energy electron linear accelerators (LINACs) producing photon beams with energies higher than 10 MeV are widely used in radiation therapy. In these beams, fast neutrons are generated, which results in undesired contamination of the therapeutic beam. In this study, measurements and Monte Carlo (MC) simulations were used to obtain neutron spectra and dose equivalents in vicinity of linear accelerator.

Materials and methods

LINAC Siemens Oncor Expression in Osijek University Hospital is placed in vault that was previously used for ⁶⁰Co machine. Then, the shielding of the vault was enhanced using lead and steel plates. Measurements of neutron dose equivalent around LINAC and the vault were done using CR-39 solid state nuclear track detectors. To compensate energy dependence of detectors, neutron energy spectra was calculated in measuring positions using MC simulations.

Results

The vault is a source of photoneutrons, but a vast majority of neutrons originates from accelerator head. Neutron spectra obtained from MC simulations show significant changes between the measuring positions. Annual neutron dose equivalent per year was estimated to be less than 324 μSv in the measuring points outside of the vault.

Conclusions

Since detectors used in this paper are very dependent on neutron energy, it is extremely important to know the neutron spectra in measuring points. Though, patient dosimetry should include neutrons, estimated annual neutron doses outside the vault were far below exposure limit of ionizing radiation for workers.

CubBonner - A novel passive neutron area monitor

Responses of a novel passive neutron area monitor, named CubBonner, were calculated. The responses were estimated for sixty monoenergetic neutrons, from 10^-9 to 20 MeV, with the MCNP5 code. The CubBonner is a cubic polyethylene moderator and a gold foil as thermal neutron detector. The ambient dose equivalent response was calculated for three cubes (5″, 8″ and 10" side) with the gold foil at the cube's centers. The moderator cube having the best ambient dose equivalent response was used to estimate the neutron fluence and the ¹⁹⁷Au(n,γ) responses per history. The ambient dose response per unit mass of gold was compared with the response of the Berthold LB 6411 active neutron area monitor, and the response for the (n,γ) reaction in the gold foil was compared with the evaporation photo neutrons produced in linear accelerators for radiotherapy.

Fast, epithermal and thermal photoneutron dosimetry in air and in tissue equivalent phantom for a high-energy X-ray medical accelerator

Photoneutron (PN) dosimetry in fast, epithermal and thermal energy ranges originated from the beam and albedo neutrons in high-energy X-ray medical accelerators is highly important from scientific, technical, radiation protection and medical physics points of view. Detailed dose equivalents in the fast, epithermal and thermal PN energy ranges in air up to 2m as well as at 35 positions from the central axis of 12 cross sections of the phantom at different depths were determined in 18MV X-ray beams of a Siemens ONCOR accelerator. A novel dosimetry method based on polycarbonate track dosimeters (PCTD)/¹⁰B (with/without cadmium cover) was used to determine and separate different PN dose equivalents in air and in a multilayer polyethylene phantom. Dose equivalent distributions of PNs, as originated from the main beam and/or albedo PNs, on cross-plane, in-plane and diagonal axes in 10cm×10cm fields are reported. PN dose equivalent distributions on the 3 axes have their maxima at the isocenter. Epithermal and thermal PN depth dose equivalent distributions in the phantom for different positions studied peak at ∼3cm depth. The neutron dosimeters used for the first time in such studies are highly effective for separating dose equivalents of PNs in the studied energy ranges (beam and/or albedo). The PN dose equivalent data matrix made available in this paper is highly essential for detailed patient dosimetry in general and for estimating secondary cancer risks in particular.

Real-time detection of fast and thermal neutrons in radiotherapy with CMOS sensors

The peripheral dose distribution is a growing concern for the improvement of new external radiation modalities. Secondary particles, especially photo-neutrons produced by the accelerator, irradiate the patient more than tens of centimeters away from the tumor volume. However the out-of-field dose is still not estimated accurately by the treatment planning softwares. This study demonstrates the possibility of using a specially designed CMOS sensor for fast and thermal neutron monitoring in radiotherapy. The 14 microns-thick sensitive layer and the integrated electronic chain of the CMOS are particularly suitable for real-time measurements in γ/n mixed fields. An experimental field size dependency of the fast neutron production rate, supported by Monte Carlo simulations and CR-39 data, has been observed. This dependency points out the potential benefits of a real-time monitoring of fast and thermal neutron during beam intensity modulated radiation therapies.