Evidence for using Monte Carlo calculated wall attenuation and scatter correction factors for three styles of graphite-walled ion chamber

Conception and realization of a parallel-plate free-air ionization chamber for the absolute dosimetry of an ultrasoft X-ray beam

We report the design of a millimeter-sized parallel plate free-air ionization chamber (IC) aimed at determining the absolute air kerma rate of an ultra-soft X-ray beam (E = 1.5 keV). The size of the IC was determined so that the measurement volume satisfies the condition of charged-particle equilibrium. The correction factors necessary to properly measure the absolute kerma using the IC have been established. Particular attention was given to the determination of the effective mean energy for the 1.5 keV photons using the PENELOPE code. Other correction factors were determined by means of computer simulation (COMSOL™ and FLUKA). Measurements of air kerma rates under specific operating parameters of the lab-bench X-ray source have been performed at various distances from that source and compared to Monte Carlo calculations. We show that the developed ionization chamber makes it possible to determine accurate photon fluence rates in routine work and will constitute substantial time-savings for future radiobiological experiments based on the use of ultra-soft X-rays.

Comparison of air-kerma strength determinations for HDR (192)Ir sources

PURPOSE

To perform a comparison of the interim air-kerma strength standard for high dose rate (HDR) (192)Ir brachytherapy sources maintained by the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) with measurements of the various source models using modified techniques from the literature. The current interim standard was established by Goetsch et al. in 1991 and has remained unchanged to date.

METHODS

The improved, laser-aligned seven-distance apparatus of the University of Wisconsin Medical Radiation Research Center (UWMRRC) was used to perform air-kerma strength measurements of five different HDR (192)Ir source models. The results of these measurements were compared with those from well chambers traceable to the original standard. Alternative methodologies for interpolating the (192)Ir air-kerma calibration coefficient from the NIST air-kerma standards at (137)Cs and 250 kVp x rays (M250) were investigated and intercompared. As part of the interpolation method comparison, the Monte Carlo code EGSnrc was used to calculate updated values of A(wall) for the Exradin A3 chamber used for air-kerma strength measurements. The effects of air attenuation and scatter, room scatter, as well as the solution method were investigated in detail.

RESULTS

The average measurements when using the inverse N(K) interpolation method for the Classic Nucletron, Nucletron microSelectron, VariSource VS2000, GammaMed Plus, and Flexisource were found to be 0.47%, -0.10%, -1.13%, -0.20%, and 0.89% different than the existing standard, respectively. A further investigation of the differences observed between the sources was performed using MCNP5 Monte Carlo simulations of each source model inside a full model of an HDR 1000 Plus well chamber.

CONCLUSIONS

Although the differences between the source models were found to be statistically significant, the equally weighted average difference between the seven-distance measurements and the well chambers was 0.01%, confirming that it is not necessary to update the current standard maintained at the UWADCL.

Investigation of the air kerma response of spherical ionization chambers for unfolded pulse height distributions of (60)Co and (137)Cs using the EGS4 Monte Carlo code

Data required for the determination of the absolute air kerma rate for (60)Co and (137)Cs gamma-rays using spherical cavity chambers were calculated using the EGS4 Monte Carlo system. Mass energy-absorption coefficient ratio and the stopping power ratio were calculated for a 10 cm(3) primary standard graphite-walled ionization chamber from the unfolded energy pulse height distributions of (60)Co and (137)Cs sources. Wall correction factors and non-uniformity correction factors for two graphite and one air equivalent plastic walled ionization chambers were also calculated with EGS4 code. The wall correction factors were compared with those determined by an experimental extrapolation method. To check the accuracy of the calculations the results were compared with those obtained from other primary standard laboratories such as NIST and NRCC. For a 10 cm(3) graphite ionization chamber, the mass energy-absorption coefficient ratios were 0.99917 for (60)Co and 1.0004 for (137)Cs. The values differed by 0.02-0.05 % for (60)Co and 0.11 % for (137)Cs from those of two laboratories. The stopping power ratios were 0.99984 for (60)Co and 1.0087 for (137)Cs. Comparison with NIST values showed differences of 0.06 % for (60)Co and 0.04 % for (137)Cs. The wall correction factors were obtained and they were different by 0.6-1.1 % for (60)Co and (137)Cs compared to the experimental linear extrapolation method. These values were compared with Monte Carlo derived values from other laboratories. The non-uniformity correction factors were also calculated and they differed from unity, the traditional value used in most standard national metrology laboratories.

A virtual-accelerator-based verification of a Monte Carlo dose calculation algorithm for electron beam treatment planning in clinical situations

BACKGROUND AND PURPOSE

The introduction of Monte Carlo (MC) techniques for treatment planning and also for verification purposes will have considerable impact on the radiation therapy planning process. The aim of this work was to use a virtual accelerator to study the performance of a MC-based electron dose calculation algorithm, implemented in a commercial treatment planning system.

METHODS

The performance in phantoms containing air and bone as well as in patient-specific geometries (thorax wall, nose, parotid gland and spinal cord) has been studied.

RESULTS

The agreement between the virtual accelerator and the MC dose calculation algorithm is generally very good. A gamma-evaluation with criteria of 0.03 Gy/3 mm (per Gy at the depth of maximum dose) shows that, even for the worst cases, only a small volume of about 1.5% has gamma>1.0. In the worst case, with the 0.02 Gy/2 mm criteria, about 92% of the volume receiving more than 0.85 Gy per 100 monitor units (MU) has gamma-values <1.0. The corresponding value for the volume receiving more than 0.10 Gy/100 MU is about 98%. For the 18 MeV spinal-cord case, where a 6 x 20 cm2 insert is used, the TPS underestimates the dose outside the primary field due to inadequate modelling of the insert.

CONCLUSION

The possibility of dose calculations in typical patient cases makes the virtual accelerator a powerful tool for validation and evaluation of dose calculation algorithms present in treatment planning systems.

On the accuracy of techniques for obtaining the calibration coefficient N(K) of 192Ir HDR brachytherapy sources

The accuracy of interpolation or averaging procedures for obtaining the calibration coefficient N(K) for 192Ir high-dose-rate brachytherapy sources has been investigated using the EGSnrc Monte Carlo simulation system. It is shown that the widely used two-point averaging procedure of Goetsch et al. [Med. Phys. 18, 462 (1991)] has some conceptual problems. Most importantly, they recommended, as did the IAEA, averaging A(wall)N(K) values whereas one should average 1/N(K) values. In practice this and other issues are shown to have little effect except for Goetsch et al.'s methods for determining A(wall) values. Their method of generalizing the A(wall) values measured in one geometry to other geometries is incorrect by up to 2%. However, these errors in A(wall) values cause systematic errors of only 0.3% in 192Ir calibration coefficients. It is shown that A(wall) values need not be included in the averaging technique at all, thereby simplifying the technique considerably. It is demonstrated that as long as ion chambers with a flat response are used and/or very heavily filtered 250 kV (or higher) beams of x rays are used in the averaging, then almost all techniques can provide adequate accuracy.

Fifty years of Monte Carlo simulations for medical physics

Monte Carlo techniques have become ubiquitous in medical physics over the last 50 years with a doubling of papers on the subject every 5 years between the first PMB paper in 1967 and 2000 when the numbers levelled off. While recognizing the many other roles that Monte Carlo techniques have played in medical physics, this review emphasizes techniques for electron-photon transport simulations. The broad range of codes available is mentioned but there is special emphasis on the EGS4/EGSnrc code system which the author has helped develop for 25 years. The importance of the 1987 Erice Summer School on Monte Carlo techniques is highlighted. As an illustrative example of the role Monte Carlo techniques have played, the history of the correction for wall attenuation and scatter in an ion chamber is presented as it demonstrates the interplay between a specific problem and the development of tools to solve the problem which in turn leads to applications in other areas.

Accuracy of the photon and electron physics in GEANT4 for radiotherapy applications

This work involves a validation of the photon and electron transport of the GEANT4 particle simulation toolkit for radiotherapy physics applications. We examine the cross sections and sampling algorithms of the three electromagnetic physics models in version 4.6.1 of the toolkit: Standard, Low-energy, and Penelope. The depth dose distributions in water for incident monoenergetic and clinical beams are compared to the EGSNRC results. In photon beam simulations, all three models agree with EGSNRC to within 2%, except for the buildup region. Larger deviations are found for incident electron beams, and the differences are affected by user-imposed electron step limitations. Particle distributions through thin layers of clinical target materials, and perturbation effects near high-Z and low-Z interfaces are also investigated. The electron step size artifacts observed in our studies indicate potential problems with the condensed history algorithm. A careful selection of physics processes and transport parameters is needed for optimum efficiency and accuracy.