摘要:

The Spencer–Attix water/air restricted mass collision stopping‐power ratio is calculated in realistic electron beams in the energy range from 5–50 MeV for a variety of clinical accelerators including the Varian Clinac 2100C, the Philips SL75‐20, the Siemens KD2, the AECL Therac 20, and the Scanditronix Medical Microtron 50. The realistic clinical beams are obtained from full Monte Carlo simulations of the clinical linear accelerators using the codebeam. The stopping‐power ratios calculated using clinical beams are compared with those determined according to the AAPM and the IAEA protocols which were calculated by using monoenergetic parallel beams. Using the energy–range relationship of Rogers and Bielajew [Med. Phys.13, 687–694 (1986)] leads to the most consistent picture in which the stopping‐power ratios atdmaxderived from mono‐energetic calculations underestimate the stopping‐power ratios calculated with the realistic beam by 0.3% at 5 MeV and up to 1.4% at 20 MeV. The stopping‐power ratios atdmaxdetermined according to the AAPM TG‐21 protocol (1983) are shown to overestimate the realistic stopping‐power ratios by up to 0.6% for a 5‐MeV beam and underestimate them by up to 1.2% for a 20‐MeV beam. Those determined according to the IAEA (1987) protocol overestimate the realistic stopping‐power ratios by up to 0.3% for a 5‐MeV beam and underestimate them by up to a 1.1% for a 20‐MeV beam at reference depth. The causes of the differences in the stopping‐power ratios between the realistic clinical mono‐energetic beams are analyzed quantitatively. The changes in the stopping‐power ratios atdmaxare mainly due to the energy spread of the electron beam and the contaminant photons in the clinical beams. The effect of the angular spread of electrons is rather small except at the surface. Data are presented which give the corrected stopping‐power ratios atdmaxor reference depth starting from those determined according to protocols for any energy of clinical electron beams with scattering foils. For scanned clinical electron beams the correction to stopping‐power ratios determined according to protocols is found to be less than 0.5% atdmaxor reference depth for all beam energies studied. We quantify the differences in the stopping‐power ratios determined using the depth of 50% ionization level and the depth of 50% dose level. The differences are very small except for very‐high‐energy beams (50 MeV) where they can be up to 0.8%.

ISSN:0269-3127    

DOI:10.1118/1.597581

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes BEAM, a general purpose Monte Carlo code to simulate the radiation beams from radiotherapy units including high‐energy electron and photon beams,60Co beams and ortho‐voltage units. The code handles a variety of elementary geometric entities which the user puts together as needed (jaws, applicators, stacked cones, mirrors, etc.), thus allowing simulation of a wide variety of accelerators. The code is not restricted to cylindrical symmetry. It incorporates a variety of powerful variance reduction techniques such as range rejection, bremsstrahlung splitting and forcing photon interactions. The code allows direct calculation of charge in the monitor ion chamber. It has the capability of keeping track of each particle's history and using this information to score separate dose components (e.g., to determine the dose from electrons scattering off the applicator). The paper presents a variety of calculated results to demonstrate the code's capabilities. The calculated dose distributions in a water phantom irradiated by electron beams from the NRC 35 MeV research accelerator, a Varian Clinac 2100C, a Philips SL75‐20, an AECL Therac 20 and a Scanditronix MM50 are all shown to be in good agreement with measurements at the 2 to 3% level. Eighteen electron spectra from four different commercial accelerators are presented and various aspects of the electron beams from a Clinac 2100C are discussed. Timing requirements and selection of parameters for the Monte Carlo calculations are discussed.Dedication:This paper is dedicated to the memory of our friend and colleague, Jiansu Wei, who made a significant contribution to this project before he passed away on March 15, 1993.

ISSN:0269-3127    

DOI:10.1118/1.597552

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Mean mass energy absorption coefficient ratios of acrylic, polystyrene, and water to air, were calculated using Monte Carlo generated energy spectra. The energy spectra were calculated for 4‐ to 50‐MV x‐ray beams, from machines using flattening filters and scanning beams. The validity of these spectra was verified by comparing the measured ionization ratios with the calculated values. The agreement was found to be within 1.9%. For beams of energy below 6 MV, our estimates of the mean mass energy absorption coefficient ratios agree well with those recommended by the TG‐21 protocol. For higher energy beams, the discrepancy increases to about 3%. It was found that the discrepancy is attributable to the different spectra used in these calculations.

ISSN:0269-3127    

DOI:10.1118/1.597553

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Values of electron mass scattering power,T/ρ, for various materials have been calculated by using the EGS4 Monte Carlo system and by integration of the Molière multiple‐scattering distribution. The energy range covered is 0.5–100 MeV. Monte Carlo calculations test the concept ofT/ρ “experimentally” and assess the contribution to electron mass scattering power from effects such as Moller scatter and energy‐loss straggling. The Monte Carlo results agree within 2% with the analytical results calculated from Molière multiple‐scattering theory at energies less than 20 MeV for high‐Zmaterials and for energies less than 50 MeV for low‐Zmaterials. At higher energies the Monte Carlo calculations include the effects of bremsstrahlung production which can significantly increase values ofT/ρ. For low‐Zmaterials and electron energies less than 60 MeV, the Monte Carlo calculatedT/ρ values are generally 22% higher than those given by ICRU Report 35, while those for high‐Zmaterials and energies less than 25 MeV are found to be consistent (within 1%) with ICRU Report 35. The effects of Moller scatter, which significantly affectT/ρ for low‐Zmaterials, as well as bremsstrahlung effects, are included in the present Monte Carlo calculations. If the tabulatedT/ρ data of ICRU Report 35 are modified to include the Moller scatter effect, then for energies less than 60 MeV they are generally 6% less than the present Monte Carlo data for low‐Zmaterials as well as for copper. It is shown thatT/ρ is a well‐defined constant over an appropriate range of slab thickness except when bremsstrahlung effects are significant. It is found thatT/ρ is proportional toE−n, wherenis in the range of 1.5–2.0 for the energies considered here. The Monte Carlo calculations are shown to agree well with various relevant experimental measurements. AccurateT/ρ data, which should include the effect of Moller scatter, are necessary in electron‐beam treatment planning, especially for a small field size. The choice of the depth step in the implementation of pencil‐beam codes should not violate the slab‐thickness limits forT/ρ data.

ISSN:0269-3127    

DOI:10.1118/1.597582

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dose profile data from small circular fields have been used in treatment dose planning for stereotactic radiosurgery. Generally, a two‐dimensional interpolation of the measured beam profiles from circular collimators is used to calculate the dose at any axial depth and radial distance from the central axis. Instead, the dose profile data can be transformed into a sigmoidal form. A new three parameter sigmoidal function was developed to fit the transformed (sigmoidal) dose profile data. The values of the three estimated parameters were found to follow either linearly or exponentially as a function of axial depth. Thus, instead of linear interpolation, these formulas can be used to calculate dose at any axial depth and radial distance from the central axis for circular collimators of various diameters. This new sigmoidal function provides another formula to describe dose profile data from circular collimator of small fields.

ISSN:0269-3127    

DOI:10.1118/1.597554

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The data from Meisbergeretal. [Radiology90, 953–957 (1968)] are often used as a basis for dose calculations in brachytherapy. In order to describe the absorbed dose in water around a brachytherapy point source, Meisberger provided a polynomial fit for different isotopes taking into account the effect of attenuation and scattering. The validity of the Meisberger coefficients is restricted to distances up to 10 cm from the source, which is regarded to be satisfactory for most brachytherapy applications. However, for more distant organs it may lead to errors in calculated absorbed dose. For this reason dose measurements have been performed in air and in water around a high activity60Co source used in high dose rate brachytherapy. Measurements were carried out to distances of 20 cm, using ionization chambers. These data show that at a distance of about 15 cm the amount of scattered radiation virtually equals the amount of primary radiation. This emphasizes the contribution of scattered radiation to the dose in healthy tissue far from the target volume, even with relatively high energy photon radiation of60Co. It is also shown that the Meisberger data as well as the approach of Van Kleffens and Star [Int. J. Radiat. Oncol. Phys.5, 557–563 (1979)] lead to significant errors in absorbed dose between distances of 10 and 20 cm from the source. In addition to these measurements, the Monte Carlo code has been used to calculate separately primary dose and scattered dose from a cobalt point source. The calculated results agree with the experimental data within 1% for a most distant dose scoring region.

ISSN:0269-3127    

DOI:10.1118/1.597583

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Thermoluminescence dosimetry is extensively used for quantitative dose measurements in various irradiation fields such as dosimetry of brachytherapy sources. In this application, small doses on the order of 0.5 cGy must be accurately measured, which requires careful control of instrumentation, energy‐dependence, and nonlinearity of detector response. Several investigators have observed the presence of some undesirable signals when the thermoluminescent dosimeters (TLDs) were read without any nitrogen gas flow in the TLD reader. Others have indicated that the “prereadout” annealing technique is the same as the “preirradiation” technique for doses above 10 cGy, but they have not extended their study to lower doses. The goal of this study is to investigate dependence of sensitivity and linearity of the TLD response to the flow of nitrogen gas in the TLD reader at low dose level, annealing technique, and TLD size. The effect of nitrogen flow sensitivity and linearity of two different sizes of lithium fluoride TLD‐100 chips has been studied. Our data indicate a large standard deviation of TLD sensitivity, up to a factor of 2, when TLDs were read without nitrogen gas flow in the TLD reader. In addition, a large deviation from linearity was observed for doses below 5 cGy. When the reading‐chamber was purged with nitrogen gas, dispersion of the responses of the TLDs that were exposed to the same dose fell to within 5%. At precision levels of 2% and 5%, the low dose limits are 1 cGy and 0.5 cGy, respectively, for large chips and 15 cGy and 1 cGy for small chips, if TLDs are read with nitrogen gas flow in the TLD reader. The full results of our investigation are presented.

ISSN:0269-3127    

DOI:10.1118/1.597555

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Modern computer controlled radiotherapy treatment equipment offers the possibility of delivering complex, multiple field treatments with minimal operator intervention, thus making multiple field conformal therapy practical. Conventional quality control programs are inadequate for this new technology, so new quality control procedures are needed. A reasonably fast, sensitive, and complete daily quality control program has been developed in our clinic that includes nearly automated mechanical as well as dosimetric tests. Automated delivery of these quality control fields is performed by the control system of the MM50 racetrack microtron, directed by the CCRS sequence processor [D. L. McShan and B. A. Fraass, Proceedings of the XIth International Conference on the use of computers in Radiation Therapy, 20–24 March 1994, Manchester, U.K. (North Western Medical Physics Department, Manchester, U.K., 1994), pp. 210–211], which controls the treatment process. The mechanical tests involve multiple irradiations of a single film to check the accuracy and reproducibility of the computer controlled setup of gantry and collimator angles, table orientation, collimator jaws, and multileaf collimator shape. The dosimetric tests, which involve multiple irradiations of an array of ionization chambers in a commercial dose detector (Keithly model 90100 TrackerTMSystem) rigidly attached to the head of the treatment gantry, check the output and symmetry of the treatment unit as a function of gantry and collimator angle and other parameters. For each of the dosimetric tests, readings from the five ionization chambers are automatically read out, stored, and analyzed by the computer, along with the geometric parameters of the treatment unit for that beam. The present work describes in detail these quality control tests that have been implemented in our clinic to help confirm the mechanical and dosimetric consistency of an MM50 racetrack microtron system [Mastersonetal., Int. J. Radiat. Oncol. Biol. Phys.28, 1219–1227 (1994)].

ISSN:0269-3127    

DOI:10.1118/1.597542

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dosimetric properties of a new diamond detector for the measurement of relative dose in photon beams have been investigated and compared to those of a siliconp‐type photon diode and a 0.14 cm3ionization chamber. The mass energy absorption ratio of carbon to water is nearly constant over a wide energy range making the diamond detector nearly tissue equivalent. The directional dependence of the radiation response of the diamond detector for cobalt 60, 6 MV and 18 MV photon beams was more uniform than that of the diode. As the incident photon beam moves from 0° (parallel to the detector stem) to a direction transverse to the detector stem (90°), the diamond detector sensitivity remains nearly uniform whereas the diode sensitivity diminishes by approximately 15%–22%. The spatial resolution of the diamond detector, as measured by penumbra width, is slightly less than that of the diode detector but clearly superior to that of the 0.14 cm3ionization chamber. The tissue maximum ratio measurements for small size photon fields (diameter ≤4 cm) with the diamond, diode, and a Markus parallel plate chamber are in excellent agreement. The diamond detector with high radiation sensitivity and spatial resolution is an excellent choice as a detector in photon fields with high dose gradients such as brachytherapy and radiosurgery.

ISSN:0269-3127    

DOI:10.1118/1.597543

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The primary goal of intraoperative radiation therapy is to irradiate the intraoperatively determined tumor target volume with a single fraction of tumoroidal dose while minimizing the dose to all adjacent healthy tissues. To reduce dose outside the treatment volume, lead sheets are often used to cover the external surface of the cone tip thus providing a shielding for the tissues outside the field. In this paper, the effect of the shielding on the depth dose distributions and dose profiles at different depths is studied based on experimental data. The results were also compared against an EGS4 Monte Carlo code for the same geometry as the measurements. The cones varied in size having diameters of 5 cm, 7 cm, and 9 cm, and the electron energies ranged from 6 MeV to 22 MeV. The depth dose curves and dose profiles (at two different depths in the phantom) were measured and computed with and without the lead shielding for the various combinations of cone sizes and electron energies using a water phantom to simulate the patient. It was found that the presence of lead increases on average across the treatment area the dose to the tumor from 2% up to 5%, while the dose outside the cone was reduced by as much as 75%. Both measurements and calculations were found to be in agreement.

ISSN:0269-3127    

DOI:10.1118/1.597584

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY