Investigation of correction factors for non-reference conditions in ion chamber photon dosimetry with Monte-Carlo simulations

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.

Download Full-text

The determination of beam quality correction factors: Monte Carlo simulations and measurements

Physics in Medicine and Biology ◽

10.1088/0031-9155/54/15/006 ◽

2009 ◽

Vol 54 (15) ◽

pp. 4723-4741 ◽

Cited By ~ 19

Author(s):

D M González-Castaño ◽

G H Hartmann ◽

F Sánchez-Doblado ◽

F Gómez ◽

R-P Kapsch ◽

...

Keyword(s):

Monte Carlo ◽

Monte Carlo Simulations ◽

Beam Quality ◽

Correction Factors

Download Full-text

Beta-efficiency of a typical gas-flow ionization chamber using GEANT4 Monte Carlo simulations

Nuclear Technology and Radiation Protection ◽

10.2298/ntrp1103193h ◽

2011 ◽

Vol 26 (3) ◽

pp. 193-200 ◽

Cited By ~ 2

Author(s):

Abid Hussain ◽

Sikander Mirza ◽

Nasir Mirza ◽

Muhammad Siddique

Keyword(s):

Monte Carlo ◽

Monte Carlo Simulations ◽

Ionization Chamber ◽

Gas Flow ◽

Conversion Factor ◽

Wall Material ◽

Conversion Factors ◽

Beta Particles ◽

Ion Chamber ◽

Predicted Values

GEANT4 based Monte Carlo simulations have been carried out for the determination of efficiency and conversion factors of a gas-flow ionization chamber for beta particles emitted by 86 different radioisotopes covering the average-b energy range of 5.69 keV-2.061 MeV. Good agreements were found between the GEANT4 predicted values and corresponding experimental data, as well as with EGS4 based calculations. For the reported set of b-emitters, the values of the conversion factor have been established in the range of 0.5?1013-2.5?1013 Bqcm-3/A. The computed xenon-to-air conversion factor ratios have attained the minimum value of 0.2 in the range of 0.1-1 MeV. As the radius and/or volume of the ion chamber increases, conversion factors approach a flat energy response. These simulations show a small, but significant dependence of ionization efficiency on the type of wall material.

Download Full-text

Abstract ID: 13 Monte Carlo simulations for the beam quality factors of a parallel-plate ion-chamber in the presence of magnetic field

Physica Medica ◽

10.1016/j.ejmp.2017.11.024 ◽

2018 ◽

Vol 45 ◽

pp. S1

Author(s):

Jaegi Lee ◽

Sung-Joon Ye

Keyword(s):

Magnetic Field ◽

Monte Carlo ◽

Monte Carlo Simulations ◽

Parallel Plate ◽

Beam Quality ◽

Quality Factors ◽

Ion Chamber

Download Full-text

SU-E-T-242: Monte Carlo Simulations Used to Test the Perturbation of a Reference Ion Chamber Prototype Used for Small Fields

Medical Physics ◽

10.1118/1.4888573 ◽

2014 ◽

Vol 41 (6Part15) ◽

pp. 279-279

Author(s):

L Vazquez Quino ◽

O Calvo ◽

C Huerta ◽

M DeWeese

Keyword(s):

Monte Carlo ◽

Monte Carlo Simulations ◽

Ion Chamber ◽

Small Fields

Download Full-text

Evaluation of a Scatter Correction Technique for Single Photon Transmission Measurements in PET by Means of Monte Carlo Simulations

Nuklearmedizin ◽

10.1055/s-0038-1632247 ◽

2000 ◽

Vol 39 (03) ◽

pp. 67-71 ◽

Cited By ~ 1

Author(s):

K. Wegmann ◽

G. Brix

Keyword(s):

Monte Carlo ◽

Monte Carlo Simulations ◽

Single Photon ◽

Scatter Correction ◽

Whole Body ◽

Correction Factors ◽

Complex Objects ◽

Major Drawback ◽

Correction Technique ◽

2D And 3D

Summary Purpose: Single photon transmission (SPT) measurements offer a new approach for the determination of attenuation correction factors (ACF) in PET. A major drawback of this method is the high fraction of scattered photons in the transmission sinogram resulting in a marked underestimation of the ACFs. It was, therefore, the aim of the present work, to evaluate a scatter correction algorithm proposed by C. Watson by means of Monte Carlo simulations. Methods: SPT measurements with a Cs-137 point source were simulated for a whole-body PET scanner (ECAT EXACT HR+) in both the 2D and 3D mode. To examine the scatter fraction (SF) in the transmission data, the detected photons were classified as unscattered or scattered. The simulated data were used to determine (i) the spatial distribution of the SFs, (ii) an ACF sinogram from all detected events (ACFtot) and (iii) from the unscattered events only (ACFunscottered), and (iv) an ACFcor = (ACFtot)l+k sinogram corrected according to the Watson algorithm. In addition, density images were reconstructed in order to quantitatively evaluate linear attenuation coefficients. Results: A high correlation was found between the SF and the ACFtot sinograms. For the cylinder and the EEC phantom, similar correction factors K were estimated. The determined values resulted in an accurate scatter correction in both the 2D and 3D mode. Conclusions: The algorithm proposed by Watson allows an accurate correction of scattered radiation in SPT measurements. The correction factor k can by determined experimentally using simple phantoms and then applied to more complex objects. SPT measurements should be performed in the 3D mode, in order to increase the total numb of counts and/or to reduce the measurement time.

Download Full-text