Neutron spectrum and doses in a 18 MV LINAC

High-energy linacs produce secondary particles such as neutrons (photoneutron production). The neutrons have the important role during treatment with high energy photons in terms of protection and dose escalation. In this work, neutron dose equivalents of 18 MV Varian and Elekta accelerators are measured by thermoluminescent dosimeter (TLD) 600 and TLD700 detectors and compared with the Monte Carlo calculations. For neutron and photon dose discrimination, first TLDs were calibrated separately by gamma and neutron doses. Gamma calibration was carried out in two procedures; by standard 60Co source and by 18 MV linac photon beam. For neutron calibration by 241Am-Be source, irradiations were performed in several different time intervals. The Varian and Elekta linac heads and the phantom were simulated by the MCNPX code (v. 2.5). Neutron dose equivalent was calculated in the central axis, on the phantom surface and depths of 1, 2, 3.3, 4, 5, and 6 cm. The maximum photoneutron dose equivalents which calculated by the MCNPX code were 7.06 and 2.37 mSv.Gy-1 for Varian and Elekta accelerators, respectively, in comparison with 50 and 44 mSv.Gy-1 achieved by TLDs. All the results showed more photoneutron production in Varian accelerator compared to Elekta. According to the results, it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry inside the linac field due to high photon flux, while MCNPX code is an appropriate alternative for studying photoneutron production.

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“…This threshold depends on the atomic number of the target and is around 8 MeV for high atomic numbers (7.42 MeV for tungsten). [ 1 2 3 ]…”

Section: Introductionmentioning

confidence: 99%

Neutron dose measurements of Varian and Elekta linacs by TLD600 and TLD700 dosimeters and comparison with MCNP calculations

et al. 2014

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“…We simulated 2 × 10 9 electron histories for each run and the equivalent dose (both photon and neutron) was calculated in an ICRP soft tissue‐equivalent phantom (60 × 90 × 40 cm 3 ) at 100 cm SSD. Simulated neutrons were generated through the ( γ ,n) interaction only, ignoring the (e,n) interaction which has a cross‐section that is more than 100 times lower . We did not apply any energy cutoff for the neutrons.…”

Section: Methodsmentioning

confidence: 99%

“…In radiation therapy, high energy photons have several advantages when compared to low energy photons including deeper penetration for greater depth dose, reduced skin dose, and decreased peripheral dose due to smaller angle scatter making them desirable in many clinical situations . Conversely, the neutron contamination produced while creating these high energy photons is an undesirable side effect.…”

Section: Introductionmentioning

confidence: 99%

Neutron damage induced in cardiovascular implantable electronic devices from a clinical 18 MV photon beam: A Monte Carlo study

Ezzati

Studenski

2017

Medical Physics

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“…When these high energy photon beams collide with nuclei of high atomic number materials which are located along the path of the beam including linear accelerator (Linac) head structures (target, primary collimators, flattening filter and jaws), neutrons are produced by (γ,n) and (e,e’n) interactions and contaminate therapeutic beam. The cross-section for (γ, n) interaction is approximately 137 times higher than (e, e’n) interaction; as a result, (e, e’n) interaction generates negligibility neutrons versus (γ,n) interaction [5]. The average threshold energy for photoneutron interactions is 8 MV.…”

Section: Introductionmentioning

confidence: 99%

The Influence of Brass Compensator Thickness and Field Size on Neutron Contamination Spectrum in 18MV Elekta SL 75/25 Medical Linear Accelerator with and without Flattening Filter: A Monte Carlo Study

et al. 2018

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Background:One of the most significant Intensity Modulated Radiation Therapy treatment benefits is a high target to normal tissue dose ratio. To improve this advantage, an additional accessory such as a compensator is used to deliver doses. Compensator-based IMRT treatment is usually operated with an energy higher than 10 MV. Photoneutrons, which have high linear energy transfer and radiobiological effectiveness, are produced by colliding high-energy photon beams with linear accelerator structures, then they deliver the unwanted doses to patients and staff. Therefore, the neutron energy spectra should be determined in order to calculate and reduce the photoneutron risk. Objective:We have conducted a comprehensive and precise study on the influence of brass compensator thickness and field size on neutron contamination spectrum in an Elekta SL 75/25 medical linear accelerator with and without the flattening filter by Monte Carlo method. Material and Methods: MCNPX MC Code version 2.6.0 was utilized to simulate the detailed geometry of Elekta SL 75/25 head components based on Linac’s manual. This code includes an important feature to simulate the photo-neutron interactions. Photoneutrons spectrum was calculated after the Linac output benchmarking based on tuning the primary electron beam. Results and Conclusion: Based on the Friedman and Wilcoxon nonparametric tests results (P<0.05), photoneutron fluence directly depends on the field size and compensator thickness. Moreover, the unflattened beam provides lower photoneutron fluence than the flattened beam. Photoneutrons fluence is not negligible in compensator-based IMRT treatment. However, in order to optimize treatment plans, this additional and unwanted dose must be accounted for patientss.

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Neutron spectrum and doses in a 18 MV LINAC

Cited by 44 publications

References 15 publications

Neutron dose measurements of Varian and Elekta linacs by TLD600 and TLD700 dosimeters and comparison with MCNP calculations

Neutron dose measurements of Varian and Elekta linacs by TLD600 and TLD700 dosimeters and comparison with MCNP calculations

Neutron damage induced in cardiovascular implantable electronic devices from a clinical 18 MV photon beam: A Monte Carlo study

The Influence of Brass Compensator Thickness and Field Size on Neutron Contamination Spectrum in 18MV Elekta SL 75/25 Medical Linear Accelerator with and without Flattening Filter: A Monte Carlo Study

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