Neutron dosimetry in organs of an adult human phantom using linacs with multileaf collimator in radiotherapy treatments

MCNPX Estimation of Photoneutron Dose to Eye Voxel Anthropomorphic Phantom From 18 MV Linear Accelerator

The dose due to photoneutron contamination outside the field of irradiation can be significant when using high-energy linear accelerators. The eye is a radiation-sensitive organ, and this risk increases when high linear energy transfer neutron radiation is involved. This study aimed to provide a fast method to estimate photoneutron dose to the eye during radiotherapy. A typical high-energy linear accelerator operating at 18 MV was simulated using the Monte Carlo N-Particle Transport Code System extended version (MCNPX 2.5.0). The latest International Atomic Energy Agency photonuclear data library release was integrated into the code, accounting for the most known elements and isotopes used in typical linear accelerator construction. The photoneutron flux from a 5 × 5 cm² field size was scored at the treatment table plane and used as a new source for estimating the absorbed dose in a high-resolution eye voxel anthropomorphic phantom. In addition, common shielding media were tested to reduce the photoneutron dose to the eye using common shielding materials. Introducing a 2 cm thickness of common neutron shielding medium reduced the total dose received in the eye voxel anthropomorphic phantom by 54%. In conclusion, individualized treatment based on photoneutron dose assessment is essential to better estimate the secondary dose inside or outside the field of irradiation.

Linac natW target close-in geometry photoneutron study by nuclear track distribution function

During electron beam stopping on ^natW target in a linear accelerator, photonuclear (γ, xn) reactions occur; the tungsten converter provides a non-negligible neutron yield with an energy spectrum that significantly depends on surrounding mass nuclei. Reduction of the neutron radiation field is convenient to limit the side-effects that accompany the tumor or cancer radiotherapy. A close-in irradiation geometry is proposed to improve therapy effectiveness. The convenience of the proposed experimental arrangement is assessed using Monte Carlo simulation and experimental results based on nuclear track-etch methodology. Photoneutron yield for two energy groups (thermal and epithermal) are determined experimentally via boron (98%) converter and cadmium-filter employing a passive detector (poly allyl di-glicol carbonate polimer). Etched track diameter histograms are described by distribution functions to determine the ratio between thermal and higher energy neutrons. New insights are given into therapy beam quality and radiotherapy dose delivery based on bar histograms unfolding.

Feasibility of 18-MV grid therapy from radiation protection aspects: unwanted dose and fatal cancer risk caused by photoneutrons and scattered photons

PURPOSE

Photoneutron production is a common concern when using 18-MV photon beams in radiation therapy. In Spatially Fractionated Grid Radiation Therapy (SFGRT), the grid block in the collimation system modifies the neutron production, photon scattering, and electron contamination in and out of the radiation field. Such an effect was studied with grids made of different high-Z materials by Monte Carlo simulations. The results were also used to evaluate the lifetime risk of fatal cancers.

METHODS

MCNPX® code (2.7.0 extensions) was employed to simulate an 18-MV LINAC (Varian 2100 C/D). Three types of grid made of brass, cerrobend, and lead were used to study the neutron and electron fluence. Output factors for each grid with different field sizes were calculated. A revised female MIRD phantom with an 8-cm spherical tumor inside the liver was used to estimate the dose to the tumor and the critical organs. A 20-Gy SFGRT plan with Anterior Posterior (AP) - Posterior Anterior (PA) grid beams was compared with a Conventional Fractionated Radiation Therapy (CFRT) plan which delivered 40-Gy to the tumor by AP-PA open beams. Neutron equivalent dose, photon equivalent dose, as well as lifetime risks of fatal cancer were calculated in the organs at risk.

RESULTS

The grid blocks reduced the fluence of contaminant electrons inside the treatment field by more than 50%. The neutron fluences per electron-history in SFGRT plans with brass, cerrobend and lead were on average 55%, 31% and 31% less than that of the CFRT plan, respectively. However, when converting to fluences per delivered dose (Gy), the cerrobend and lead grid may incur higher neutron dose for 20 × 20 cm² field size and above. The changes in neutron mean energy, as well as the correlated radiation weighting factors, were insignificant. The total risk due to the photoneutrons in the SFGRT plans was 87% or lower than that in the CFRT plans. In both SFGRT and CFRT plans, the contribution of the primary and scattered photons to the fatal cancer risk was 2 times or more than the photoneutrons. The total risks from photons in SFGRT with brass, cerrobend, and lead blocks were 1.733, 1.374, and 1.260%, respectively, which were less than 30% of the total photon-risk in CFRT (5.827%).

CONCLUSION

In the brass, cerrobend, and lead grids, the attenuation of photoneutrons outweighs its photoneutron production in 18-MV SFGRT. The total cancer risks from photons and photoneutrons in the SFGRT plans were 30% or less of the risks in the CFRT plans (5.911%). Using 18 MV photon beams with brass, cerrobend, and lead grid blocks is still a feasible option for SFGRT.

Monte Carlo study of organ doses and related risk for cancer in Algeria from scattered neutrons in prostate treatment involving 3D-CRT

The present study aimed to evaluate organ doses and related risk for cancer from scattered neutrons involving 3D Conformational Radiotherapy (3D-CRT) for patients with prostate cancer in Algeria based on Monte Carlo technique and to estimate the secondary cancer risks. To this purpose, a detailed geometric Monte Carlo (MC) modeling of the LINAC Varian 2100C combined with a computational whole-body phantom was carried out. The neutron equivalent doses were calculated in-field and out-of field of patient's organs using the phase-space method. The obtained neutron equivalent doses were used to estimate the Lifetime Attributable Risks (LARs) for cancer incidence in out of field organs. LARs was evaluated assuming Biological Effects of Ionizing Radiation VII (BEIR VII) risk model for exposure age in the range 35-70 years, according to the interval's age of treated patients in Algeria. The baselines cancer risks and survival data were associated with the statistical data for the Algerian population. The results showed that the neutrons equivalent doses per prescribed dose (Photon Dose) mostly depend on the distance of organs from the treated volume. The highest and lowest equivalent doses of 1.18 mSv/Gy and 0.25 mSv/Gy were recorded in the bladder and heart, respectively. The highest estimated lifetime attributable risk per 100,000 population was found for 35 yrs' exposure age in colon 49.94, lung 16.63 and stomach 11.17. The lowest risks were found for 70 yrs' age, in spine 0.06 and thyroid 0.14. The results showed that LARs values decrease with the increase of the exposure age and cancer incidence risk is lower than the baseline cancer risk incidence for all organs. The present study may help in providing a database on the impact of radiotherapy-induced secondary cancer incidence during 3D-CRT for prostate cancer in Algeria.

New target with low photoneutron yield for LINAC radiotherapy applications

This paper presents three new materials composed of Ti_XV_0,035XCu_X-1,035 (X = 2, 4 and 6%), is proposed as Linear Accelerator target. Its response to electron beam based on photoneutron production, is assessed by MC simulation and nuclear track-etch methodology. The outcome is compared to a tungsten target irradiated by energetic 16 MeV electron beam. Photoneutron yield, of two energy groups (thermal and epithermal) were determined via converter matter ¹⁰B (98%) and Cd-filter by PADC-track density comparison. The multi-metal Ti₂V_0,07Cu_97.93 target related to therapy beam quality, resulted advantageous in comparison to that provided by W-target, commonly used in the LINAC.

Neutron track length estimator for GATE Monte Carlo dose calculation in radiotherapy

The out-of-field dose in radiation therapy is a growing concern in regards to the late side-effects and secondary cancer induction. In high-energy x-ray therapy, the secondary neutrons generated through photonuclear reactions in the accelerator are part of this secondary dose. The neutron dose is currently not estimated by the treatment planning system while it appears to be preponderant for distances greater than 50 cm from the isocenter. Monte Carlo simulation has become the gold standard for accurately calculating the neutron dose under specific treatment conditions but the method is also known for having a slow statistical convergence, which makes it difficult to be used on a clinical basis. The neutron track length estimator, a neutron variance reduction technique inspired by the track length estimator method has thus been developped for the first time in the Monte Carlo code GATE to allow a fast computation of the neutron dose in radiotherapy. The details of its implementation, as well as the comparison of its performances against the analog MC method, are presented here. A gain of time from 15 to 400 can be obtained by our method, with a mean difference in the dose calculation of about 1% in comparison with the analog MC method.

Dosimetric algorithm to reproduce isodose curves obtained from a LINAC

In this work isodose curves are obtained by the use of a new dosimetric algorithm using numerical data from percentage depth dose (PDD) and the maximum absorbed dose profile, calculated by Monte Carlo in a 18 MV LINAC. The software allows reproducing the absorbed dose percentage in the whole irradiated volume quickly and with a good approximation. To validate results an 18 MV LINAC with a whole geometry and a water phantom were constructed. On this construction, the distinct simulations were processed by the MCNPX code and then obtained the PDD and profiles for the whole depths of the radiation beam. The results data were used by the code to produce the dose percentages in any point of the irradiated volume. The absorbed dose for any voxel's size was also reproduced at any point of the irradiated volume, even when the voxels are considered to be of a pixel's size. The dosimetric algorithm is able to reproduce the absorbed dose induced by a radiation beam over a water phantom, considering PDD and profiles, whose maximum percent value is in the build-up region. Calculation time for the algorithm is only a few seconds, compared with the days taken when it is carried out by Monte Carlo.