Factors effecting the field size correction factor on a 6 MV linear accelerator

BackgroundOne 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 delivering 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.Materials 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 patients.

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Estimation of Dosimetric Parameters based on KNR and KNCSF Correction Factors for Small Field Radiation Therapy at 6 and 18 MV Linac Energies using Monte Carlo Simulation Methods

Journal of Biomedical Physics and Engineering ◽

10.31661/jbpe.v9i1feb.414 ◽

2019 ◽

Vol 9 (1Feb) ◽

Author(s):

S A Rahimi ◽

B Hashemi ◽

S R Mahdavi

Keyword(s):

Monte Carlo ◽

Correction Factor ◽

Reference Conditions ◽

Field Size ◽

Monte Carlo Code ◽

Small Field ◽

Correction Factors ◽

Photon Beams ◽

Small Field Dosimetry ◽

Circular Field

Background: Estimating dosimetric parameters for small fields under non-reference conditions leads to significant errors if done based on conventional protocols used for large fields in reference conditions. Hence, further correction factors have been introduced to take into account the influence of spectral quality changes when various detectors are used in non-reference conditions at different depths and field sizes.Objective: Determining correction factors (KNR and KNCSF) recommended recently for small field dosimetry formalism by American Association of Physicists in Medicine (AAPM) for different detectors at 6 and 18 MV photon beams.Methods: EGSnrc Monte Carlo code was used to calculate the doses measured with different detectors located in a slab phantom and the recommended KNR and KNCSF correction factors for various circular small field sizes ranging from 5-30 mm diameters. KNR and KNCSF correction factors were determined for different active detectors (a pinpoint chamber, EDP-20 and EDP-10 diodes) in a homogeneous phantom irradiated to 6 and 18 MV photon beams of a Varian linac (2100C/D).Results: KNR correction factor estimated for the highest small circular field size of 30 mm diameter for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.993, 1.020 and 1.054; and 0.992, 1.054 and 1.005 for the 6 and 18 MV beams, respectively. The KNCSF correction factor estimated for the lowest circular field size of 5 mm for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.994, 1.023, and 1.040; and 1.000, 1.014, and 1.022 for the 6 and 18 MV photon beams, respectively.Conclusion: Comparing the results obtained for the detectors used in this study reveals that the unshielded diodes (EDP-20 and EDP-10) can confidently be recommended for small field dosimetry as their correction factors (KNR and KNCSF) was close to 1.0 for all small field sizes investigated and are mainly independent from the electron beam spot size.

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Factor 10 Expedience of Monthly Linac Quality Assurance via an Ion Chamber Array and Automation Scripts

Technology in Cancer Research & Treatment ◽

10.1177/1533033819876897 ◽

2019 ◽

Vol 18 ◽

pp. 153303381987689

Author(s):

Lawrie B. Skinner ◽

Yong Yang ◽

Annie Hsu ◽

Lei Xing ◽

Amy S. Yu ◽

...

Keyword(s):

Quality Assurance ◽

Standard Deviation ◽

Linear Accelerator ◽

Data File ◽

Field Size ◽

Linear Accelerators ◽

Ion Chamber ◽

Medical Linear Accelerators ◽

Jaw Position ◽

Dosimetric Leaf Gap

Purpose: While critical for safe and accurate radiotherapy, monthly quality assurance of medical linear accelerators is time-consuming and takes physics resources away from other valuable tasks. The previous methods at our institution required 5 hours to perform the mechanical and dosimetric monthly linear accelerator quality assurance tests. An improved workflow was developed to perform these tests with higher accuracy, with fewer error pathways, in significantly less time. Methods: A commercial ion chamber array (IC profiler, Sun Nuclear, Melbourne, Florida) is combined with automation scripts to consolidate monthly linear accelerator QA. The array was used to measure output, flatness, symmetry, jaw positions, gated dose constancy, energy constancy, collimator walkout, crosshair centering, and dosimetric leaf gap constancy. Treatment plans were combined with automation scripts that interface with Sun Nuclear’s graphical user interface. This workflow was implemented on a standard Varian clinac, with no special adaptations, and can be easily applied to other C-arm linear accelerators. Results: These methods enable, in 30 minutes, measurement and analysis of 20 of the 26 dosimetric and mechanical monthly tests recommended by TG-142. This method also reduces uncertainties in the measured beam profile constancy, beam energy constancy, field size, and jaw position tests, compared to our previous methods. One drawback is the increased uncertainty associated with output constancy. Output differences between IC profiler and farmer chamber in plastic water measurements over a 6-month period, across 4 machines, were found to have a 0.3% standard deviation for photons and a 0.5% standard deviation for electrons, which is sufficient for verifying output accuracy according to TG-142 guidelines. To minimize error pathways, automation scripts which apply the required settings, as well as check the exported data file integrity were employed. Conclusions: The equipment, procedure, and scripts used here reduce the time burden of routine quality assurance tests and in most instances improve precision over our previous methods.

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Extended Source Size Correction Factor in Antenna Gain Measurements

Advanced Microwave and Millimeter Wave Technologies Semiconductor Devices Circuits and Systems ◽

10.5772/8771 ◽

2010 ◽

Author(s):

Aleksey Solovey ◽

Raj Mittr

Keyword(s):

Correction Factor ◽

Source Size ◽

Antenna Gain ◽

Extended Source ◽

Size Correction

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Extended Source Size Correction Factor in Antenna Gain Measurements

2008 38th European Microwave Conference ◽

10.1109/eumc.2008.4751620 ◽

2008 ◽

Author(s):

Aleksey Solovey ◽

Raj Mittra

Keyword(s):

Correction Factor ◽

Source Size ◽

Antenna Gain ◽

Extended Source ◽

Size Correction

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Grid Monte Carlo Simulation of a Medical Linear Accelerator

European Journal of Engineering Research and Science ◽

10.24018/ejers.2018.3.12.1001 ◽

2018 ◽

Vol 3 (12) ◽

pp. 40-43 ◽

Cited By ~ 1

Author(s):

Didi Samir ◽

Mustapha Zerfaoui ◽

Abdelilah Moussa ◽

Yassine Benkhouya ◽

Mehdi El Ouartiti

Keyword(s):

Monte Carlo ◽

Linear Accelerator ◽

Field Size ◽

Dose Profile ◽

Depth Dose ◽

Grid Simulation ◽

Medical Linear Accelerator ◽

Measured Dose ◽

Better Than ◽

Good Agreement

A full grid simulation of the head of an Elekta Synergy Platform medical linear accelerator is performed using the Geant4 Monte Carlo platform. The simulation includes all components of the accelerator head and a homogeneous water phantom. Results in terms of depth doses and lateral dose profiles are presented for 6 MV photon beam with the 10x10 cm2 reference field size at 100 cm distance from the source. Overall, a good agreement with the measured dose data is achieved with a precision better than 0.93% and 2.63% for the depth dose profile and lateral dose profiles respectively.

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Evaluation of Peripheral Dose for Varian Trilogy Linear Accelerator

10.21203/rs.3.rs-272397/v1 ◽

2021 ◽

Author(s):

Bo Yang ◽

Tingtian Pang ◽

Xiansong Sun ◽

Tingting Dong ◽

Rui Li ◽

...

Keyword(s):

Linear Accelerator ◽

Ionization Chamber ◽

Beam Energy ◽

Wedge Angle ◽

Field Size ◽

Measure Data ◽

Interest Points ◽

Peripheral Dose ◽

Radiation Beam ◽

Dosimetric System

Abstract Objective To measure and evaluate the peripheral dose(PD) for Trilogy linear accelerator in different setup condition and investigate the feasibility of the diode dosimetric system to measure the peripheral dose.Methods Peripheral dose were measured using a CC13 ionization chamber and the diode dosimetric system in a set of solid water phantom. Measurements were performed for different depths, field sizes, physical and virtual wedge, radiation beam energy and up at distance of 1cm to 31cm beyond the field edges. PD is separated into PDleakage and PDscatter by measure peripheral dose with or without scattering phantom. CRIS phantom was used for this research with the diode dosimetric system at the interest points of the breast, thyroid, and lens.Results All the measure data were normalized to isocenter. The measured PD decreases exponentially as a function of distance up to 31cm from the edge. PD shows no significant relevant to depth and it increases with the increased field size. As the physics wedge angle increase, PD increases about 1%, but enhanced dynamic wedge decreased 2-3% compared with open field. As the beam energy increase, PD decreased. All PD data difference less than 1% between CC13 ionization chamber and diode. The PD of CRIS phantom for Volume Modulated ARC Therapy (VMAT) is minimum and the mean dose for breast、thyroid and lens is 6.72 mGy、2.90 mGy and 2.37 mGy respectively.Conclusion The diode dosimetric system provides an sufficient assessment in peripheral regions of 6MV X-ray beam. PD changes because of field size、depth、beam energy etc and the assessment of PD would be helpful to evaluate the dose received by the relevant critical structures near the treatment field. Furthermore it is advantaged to use external shielding for critical organs.

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