Dosimetric characteristics of acrylic and stainless steel cones for electron beam therapy.

Dosimetric characteristics of acrylic and stainless steel cones for electron beam therapy were investigated. Acrylic and stainless steel cylindrical cones of 6, 7, and 8 cm in diameter and electron beams of energies 6, 9, 12, 15, 18, and 21 MeV were used for the measurements. Both acrylic and stainless steel cones showed high dose areas along the rim. The dose along the rim grew with increasing electron beam energy. The highest dose along the rim was 115% of the maximum dose on a central axis when a 6-cm-diameter acrylic cone and 21-MeV electrons were combined.     

Lead shielding thickness for dose reduction of 6-MeV electrons for different square fields

The relative percent dose reduction by lead (Pb) of 6-MeV electrons produced by Clinac 1800 for 6 X 6, 10 X 10, 15 X 15, 20 X 20, and 25 X 25 cm2 cones both with and without buildup is measured. The thickness of Pb required to attenuate the intensity of the primary electron beam to 95% and 98% depends upon the cone size and upon the depth in phantom at which transmission measurements are made.     

Backscatter towards the monitor ion chamber in high-energy photon and electron beams: charge integration versus Monte Carlo simulation

In some linear accelerators, the charge collected by the monitor ion chamber is partly caused by backscattered particles from accelerator components downstream from the chamber. This influences the output of the accelerator and also has to be taken into account when output factors are derived from Monte Carlo simulations. In this work, the contribution of backscattered particles to the monitor ion chamber response of a Varian 2100C linac was determined for photon beams (6, 10 MV) and for electron beams (6, 12, 20 MeV). The experimental procedure consisted of charge integration from the target in a photon beam or from the monitor ion chamber in electron beams. The Monte Carlo code EGS4/BEAM was used to study the contribution of backscattered particles to the dose deposited in the monitor ion chamber. Both measurements and simulations showed a linear increase in backscatter fraction with decreasing field size for photon and electron beams. For 6 MV and 10 MV photon beams, a 2-3% increase in backscatter was obtained for a 0.5 x 0.5 cm2 field compared to a 40 x 40 cm2 field. The results for the 6 MV beam were slightly higher than for the 10 MV beam. For electron beams (6, 12, 20 MeV), an increase of similar magnitude was obtained from measurements and simulations for 6 MeV electrons. For higher energy electron beams a smaller increase in backscatter fraction was found. The problem is of less importance for electron beams since large variations of field size for a single electron energy usually do not occur.     

Comprehensive evaluation of a commercial macro Monte Carlo electron dose calculation implementation using a standard verification data set

A commercial electron dose calculation software implementation based on the macro Monte Carlo algorithm has recently been introduced. We have evaluated the performance of the system using a standard verification data set comprised of two-dimensional (2D) dose distributions in the transverse plane of a 15 X 15 cm2 field. The standard data set was comprised of measurements performed for combinations of 9-MeV and 20-MeV beam energies and five phantom geometries. The phantom geometries included bone and air heterogeneities, and irregular surface contours. The standard verification data included a subset of the data needed to commission the dose calculation. Additional required data were obtained from a dosimetrically equivalent machine. In addition, we performed 2D dose measurements in a water phantom for the standard field sizes, a 4 cm X 4 cm field, a 3 cm diameter circle, and a 5 cm X 13 cm triangle for the 6-, 9-, 12-, 15-, and 18-MeV energies of a Clinac 21EX. Output factors were also measured. Synthetic CT images and structure contours duplicating the measurement configurations were generated and transferred to the treatment planning system. Calculations for the standard verification data set were performed over the range of each of the algorithm parameters: statistical precision, grid-spacing, and smoothing. Dose difference and distance-to-agreement were computed for the calculation points. We found that the best results were obtained for the highest statistical precision, for the smallest grid spacing, and for smoothed dose distributions. Calculations for the 21EX data were performed using parameters that the evaluation of the standard verification data suggested would produce clinically acceptable results. The dose difference and distance-to-agreement were similar to that observed for the standard verification data set except for the portion of the triangle field narrower than 3 cm for the 6- and 9-MeV electron beams. The output agreed with measurements to within 2%, with the exception of the 3-cm diameter circle and the triangle for 6 MeV, which were within 5%. We conclude that clinically acceptable results may be obtained using a grid spacing that is no larger than approximately one-tenth of the distal falloff distance of the electron depth dose curve (depth from 80% to 20% of the maximum dose) and small relative to the size of heterogeneities. For judicious choices of parameters, dose calculations agree with measurements to better than 3% dose difference and 3-mm distance-to-agreement for fields with dimensions no less than about 3 cm.     

Technical note: on cerrobend shielding for 18-22 MeV electron beams

The purpose of this study is to investigate (1) the depth at which the measurement of the block transmission factor should be made, and (2) the level of the transmission of 18 and 22 MeV electron beams through conventional Cerrobend. We measured the block transmission in water phantom as ionization profiles across the beam and as ionization distributions along the central axis of the beam for 18 and 22 MeV electron beams, for cone sizes ranging from 6 x 10 cm2 to 25 x 25 cm2. In our analysis, we separated the bremsstrahlung component produced in the Cerrobend block from the component originating in the head in the transmitted dose under the standard Cerrobend block. The block transmission for both beam energies and cone sizes was maximum on the central axis of the beam at depths between 0.4 and 0.7 cm. For the 18 MeV beam, the maximum transmission was 6.2% for the 6 x 10 cm2 cone, and 7.4% for the 25 x 25 cm2 cone. For the 22 MeV beam, it was 9.5% for the 6 x 10 cm2 cone, and 11.3% for the 25 x 25 cm2 cone. For the 22 MeV beam and 15 x 15 cm2 cone, it takes 2.95 and 1.4 cm of Cerrobend to reduce the maximum block transmission to 5% and 10%, respectively. The maximum dose under a blocked electron beam occurs on the central axis closer to the surface than it does for the open beam, and the block transmission factor should be defined at this shallower depth. To decrease the block transmission factor to the level of 5% on the central axis, electron beams with energy 18 MeV and greater require additional shielding.     

Characterization of megavoltage electron beams delivered through a photon multi-leaf collimator (pMLC)

A study is presented that characterizes megavoltage electron beams delivered through an existing double-focused photon multi-leaf collimator (pMLC) using film measurements in a solid water phantom. Machine output stability and linearity were evaluated as well as the effect of source-to-surface distance (SSD) and field size on the penumbra for electron energies between 6 and 18 MeV over an SSD range of 60-100 cm. Penumbra variations as a function of field size, depth of measurement and the influence of the jaws were also studied. Field abutment, field flatness and target coverage for segmented beams were also addressed. The measured field size for electrons transported through the pMLC was the same as that for an x-ray beam up to SSDs of 70 cm. At larger SSD, the lower energy electron fields deviated from the projected field. Penumbra data indicated that 60 cm SSD was the most favourable treatment distance. Backprojection of P(20-80) penumbra data yielded a virtual source position located at 98.9 cm from the surface for 18 MeV electrons. For 6 MeV electrons, the virtual source position was at a distance of 82.6 cm. Penumbra values were smaller for small beam slits and reached a near-constant value for field widths larger than 5 cm. The influence of the jaws had a small effect on the penumbra. The R90 values ranged from 1.4 to 4.8 cm between 6 and 21 MeV as measured at 60 cm SSD for a 9 x 9 cm2 field. Uniformity and penumbra improvement could be demonstrated using weighted abutted fields especially useful for small segments. No detectable electron leakage through the pMLC was observed. Bremsstrahlung measurements taken at 60 cm SSD for a 9 x 9 cm2 field as shaped by the pMLC compared within 1% to bremsstrahlung measurements taken at 100 cm SSD for a 10 x 10 cm2 electron applicator field at 100 cm SSD.     

Evaluation of a photon and an electron beam of a 6-MV linear accelerator

The first Mitsubishi medical linear accelerator in the United States was commissioned in April 1985. This unit EXL-8 (marketed by Mitsubishi International Corporation) produces 8-MeV electron beams in addition to 6-MV x rays. It is a 100-cm source-axis distance isocentric machine. Acceptance testing and performance evaluation of this accelerator were completed. Our measurements included beam characteristics and dosimetry parameters for both modalities. Central axis % depth dose (% DD), tissue-maximum ratio, field size output factors, wedge factors, etc., for this Linac 6-MV beam, are reported. Characteristics of the 8-MeV electron beam, namely % DD data, isodose curves, and cone ratios for various electron applicators are presented.     

An empirical formula for calculating the output factors of electron beams from a therac 20 linear accelerator

A polynomial formula, deduced from the data published by Mills et al. [Med. Phys. 12, 473 (1985)], in predicting the relative electron beam output factors, is presented in this report. This formula contains four parameters. By choosing four measured output factors, from four field sizes normalized at the field size of (10,10), the values of these parameters can be determined. A comparison of the factors predicted with this formula and the values measured by Mills et al. shows that the differences between the field sizes of (4,4) to (30,30) are 0.5% or less in 31 out of 35 field sizes. All the 35 field sizes are within 1% for an electron beam of 20 MeV. With 6-MeV electron beams, the differences are 0.5% or less in 26 out of 35 field sizes, and 1% or less in 31 out of 35 field sizes. Those having differences greater than 1% have either a small field size (5 cm) or a large field size (20 cm). Considering that this formula requires only four accurately measured relative output factors, one can predict the factors of any field within an acceptable accuracy. The calculation is easy with a scientific hand calculator. This formula provides major improvement over the other methods which require many measurements to be taken in order to interpolate with acceptable accuracy.     

Calculation of electron beam dose distributions for arbitrarily shaped fields

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is presented. Isodose distributions and output factors of treatment fields can be predicted with good accuracy, without the need for any dose measurement in the actual field. A Gaussian pencil beam model is employed with two different pencil beams for each electron beam energy. The values of the parameters of the pencil beam dose distributions are determined from a set of measurements of broad beam distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The dose distribution of electrons scattered from high atomic number metal frames, which define the treatment field contour at the skin, is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for beams with 6, 10, 14 and 20 MeV electron energy. The distance between calculated and measured isodose lines with values between 10 and 90% is under 0.3 cm. The difference between calculated and measured output factors does not exceed 2%.     

Prediction of electron beam output factors

A method to predict square and rectangular field output factors from the measurement of selected fields of electron beams on the Therac 20 Saturne has been developed. A two parameter fit of the square field output factor data, based on the functional dependence as predicted by a pencil beam calculational model, has proven clinically acceptable. The pencil beam distributions are given by the Fermi-Eyges theory of multiple Coulomb scattering. For a rectangular field, the output factor can be calculated from the square root of the product of the two square field output factors wtih sides equal to those of the rectangular field. If however, there is a significant asymmetry between the X and Y collimator systems, then rectangular field output factors should be predicted from the product of the X and Y one-dimensional output factors. One-dimensional output factors are defined as output factors of rectangular fields where one side remains constant and equal to the side of the square reference field. Measured data indicate either of the two methods of determining rectangular field output factors to be clinically acceptable for the Therac 20, the use of one-dimensional output factors demonstrating greater accuracy. Data show agreement to within approximately 1.5% at electron energies of 6, 9, 13, and 17 MeV.     

Varian 2300 C／D直线加速器高能电子束铅挡野输出因子的测定与分析

目的：探讨Varian 2300 C/D直线加速器高能电子束射野输出因子变化规律。方法：用电离室法实测在各种能量下对四种限光筒的不同铅挡野的射野输出因子。结果：铅挡野输出因子随射野边长及限光筒大小变化没有明显的规律;铅挡野输出因子与能量有关。结论：射线能量、限光铜和铅挡野大小时输出因子的影响较大,临床应用时需要针对性地精确测量。     

Lead shielding thickness for dose reduction of 5 MeV electrons

The relative percent intensity reduction by lead (Pb) of 5 MeV electrons produced by Siemens Mevatron 77/74 for 5 cm diameter, 10 X 10, 15 X 15, and 20 X 20 cm2 cones both with and without buildup is measured. The thickness of lead (Pb) required to attenuate the intensity of the primary electron beam to 95% and 98% depends upon the cone size and upon the depth in phantom at which transmission measurements are made.     

Beam characteristics and stopping-power ratios of small radiosurgery photon beams

Small megavoltage (MV) photon fields of dimensions less than 3?×?3?cm(2)?are increasingly being used in modern radiation therapy. To our knowledge, small beam characteristics and dosimetric parameters, such as the energy spectra, particle fluence, and water-to-air stopping-power ratios (SPRs) directly affect the accuracy of small field dosimetry. This study presents the characteristics of small photon beams and investigates the variations of energy spectra of photons and electrons as a function of field size and their effects on the water-to-air SPRs for field sizes ranging from a small 4?mm diameter circular field to a 10?×?10?cm(2)?field. It sheds light on the differences between small fields collimated by the cone accessory and X- and Y-jaws and on beam characteristics outside the primary radiation fields. In addition, we also investigated the use of an 'intermediate machine-specific-reference field' (Alfonso et?al 2008 Med. Phys. 35 5179-86) to determine if the variations between a small and a reference field can be eased by introducing an intermediate 4?×?4?cm(2)?field instead of a standard 10?×?10?cm(2)?reference field. The Monte Carlo simulation codes BEAMnrc, DOSXYZnrc and SPRRZnrc were used in this study. The accelerator head and circular cone accessory were simulated in detail including two designs of flattening filters: one for a standard-dose rate (100-600 MU min(-1)) and the other for a high-dose rate (1000 MU min(-1)) 6 MV beam. The mean energy of photons at depths (1.5-30?cm) in water are 1.72-2.36 MeV, 1.55-1.97 MeV, and 1.44-1.74 MeV for field sizes of 4?mm diameter, 4?×?4?cm(2), and 10?×?10?cm(2), respectively. The mean energy also varies significantly for electrons at depths (1.5-30?cm): 0.99-1.25 MeV, 0.94-1.09?MeV, and 0.93-1.04 MeV for field sizes of 4?mm, 4?×?4?cm(2), and 10?×?10?cm(2), respectively. The calculated water-to-air SPRs at depths (1.5-30?cm) are 1.120-1.113, 1.121-1.117, and 1.122-1.119 for field sizes of 4?mm, 4?×?4?cm(2)?and 10?×?10?cm(2), respectively. Although the differences in mean energy are > 20% for photons and > 5% for electrons between 4?mm field and 10?×?10?cm(2), the effects on the water-to-air SPRs are small (<0.5%). For detectors with responses to energy changes that are not negligible, significant energy variations between small fields and the reference 10?×?10?cm(2)?field may have a significant impact on the dosimetry accuracy. However, the use of an intermediate field is capable of greatly reducing these energy variations. This study also found negligible differences in dosimetric parameters between beams with different flattening filters and different incident electron energies on the target when each has the same beam quality k(Q) values specified by %dd(10)(x).     

Monte Carlo investigation of electron beam output factors versus size of square cutout.

A major task in commissioning an electron accelerator is to measure relative output factors versus cutout size (i.e., cutout factors) for various electron beam energies and applicator sizes. We use the BEAM Monte Carlo code [Med Phys. 22, 503-524 (1995)] to stimulate clinical electron beams and to calculate the relative output factors for square cutouts. Calculations are performed for a Siemens MD2 linear accelerator with beam energies, 6, 9, 11, and 13 MeV. The calculated cutout factors for square cutouts in 10 X 10 cm2, 15 X 15 cm2, and 20 X 20 cm2 applicators at SSDs of 100 and 115 cm agree with the measurements made using a silicon diode within about 1% except for the smallest cutouts at SSD= 115 cm where they agree within 0.015. The details of each component of the dose, such as the dose from particles scattered off the jaws and the applicator, the dose from contaminant photons, the dose from direct electrons, etc., are also analyzed. The calculations show that inphantom side-scatter equilibrium is a major factor for the contribution from the direct component which usually dominates the output of a beam. It takes about 6 h of CPU time on a Pentium Pro 200 MHz computer to simulate an accelerator and additional 2 h to calculate the relative output factor for each cutout with a statistical uncertainty of 1%.     

Determination of effective electron source size using multislit and pinhole cameras

Two independent methods have been utilized for determination of effective source sizes for 6, 12, and 20 MeV electron beams generated by a Varian 2100C linear accelerator. First, a multislit camera has been constructed using parallel aluminum plates and plastic strip spacers, similar to the beam-spot camera for the photon source imaging. Second, pinhole imaging was performed using a lead plate with a small hole on the central axis of the beam. The plate thickness and the hole diameter varied with electron energy. The cameras were positioned directly at the opening of the movable photon collimator. The size of the source distribution from each camera was characterized by its full width at half-maximum (FWHM) value. The measured values of FWHM are different for each camera because of their different imaging principles. For the multislit camera, the measured FWHM values were (6.3 +/- 0.4) cm for the 6 MeV beam, (3.6 +/- 0.4) cm for 12 MeV, and (2.7 +/- 0.4) cm for 20 MeV. For the pinhole camera the measured values of FWHM were (7.9 +/- 0.6) cm for 6 MeV, (4.5 +/- 0.4) cm for 12 MeV, and (3.0 +/- 0.4) cm for the 20 MeV beam. Additionally, the effective source position was derived from output measurements at different values of the SSD, which were fitted to the inverse square law.     

Small-beam calibration by 0.6- and 0.2-cm3 ionization chambers

Small beams are often applied in radiotherapy, e.g., in shrinking field and high-dose techniques with curative intent. For a given beam, measured field size factors (FSF) may vary with responses of different detectors to nonuniform radiation fluence. Dose rates of small photon and electron beams with diverse profiles were measured in polystyrene with 0.6- and 0.2-cm3 Farmer ionization chambers. FSF of 60Co, 4-, 8-, and 16-MV photon beams, and of 6-, 12-, and 20-MeV electron beams, were determined as ratios of dose rates of beams with differing field dimensions to dose rates of 10 X 10 cm beams. 60Co and 4-MV photon beam FSF were also determined in air using acrylic buildup caps. FSF obtained via 0.6- and 0.2-cm3 chambers are compared as functions of beam flatness and quality.. It is shown that notable discrepancies can exist between FSF for the same field obtained with different detectors, even when chamber volumes are well within nominal beam dimensions. Possible dose delivery errors arising from use of the 0.6-cm3 chamber were found to be from 0% to 2% for 4 X 4 to 5 X 5 cm photon beams, and from - 1% to 5% for electron beams 4 cm in diameter. Possible errors greater than 5% were noted for most beams less than 3.5 X 3.5 cm. Consequently, it is recommended that detectors smaller than the Farmer 0.6-cm3 chamber be employed in determining FSF of beams less than or equal to 5 X 5 cm.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of an add-on multileaf collimator for electron beam therapy

An add-on multileaf collimator for electrons (eMLC) has been developed that provides computer-controlled beam collimation and isocentric dose delivery. The design parameters result from the design study by Gauer et al (2006 Phys. Med. Biol. 51 5987-6003) and were configured such that a compact and light-weight eMLC with motorized leaves can be industrially manufactured and stably mounted on a conventional linear accelerator. In the present study, the efficiency of an initial computer-controlled prototype was examined according to the design goals and the performance of energy- and intensity-modulated treatment techniques. This study concentrates on the attachment and gantry stability as well as the dosimetric characteristics of central-axis and off-axis dose, field size dependence, collimator scatter, field abutment, radiation leakage and the setting of the accelerator jaws. To provide isocentric irradiation, the eMLC can be placed either 16 or 28 cm above the isocentre through interchangeable holders. The mechanical implementation of this feature results in a maximum field displacement of less than 0.6 mm at 90 degrees and 270 degrees gantry angles. Compared to a 10 x 10 cm applicator at 6-14 MeV, the beam penumbra of the eMLC at a 16 cm collimator-to-isocentre distance is 0.8-0.4 cm greater and the depth-dose curves show a larger build-up effect. Due to the loss in energy dependence of the therapeutic range and the much lower dose output at small beam sizes, a minimum beam size of 3 x 3 cm is necessary to avoid suboptimal dose delivery. Dose output and beam symmetry are not affected by collimator scatter when the central axis is blocked. As a consequence of the broader beam penumbra, uniform dose distributions were measured in the junction region of adjacent beams at perpendicular and oblique beam incidence. However, adjacent beams with a high difference in a beam energy of 6 to 14 MeV generate cold and hot spots of approximately 15% in the abutting region. In order to improve uniformity, the energy of adjacent beams must be limited to 6 to 10 MeV and 10 to 14 MeV respectively. At the maximum available beam energy of 14 MeV, radiation leakage results mainly from the intraleaf leakage of approximately 2.5% relative dose which could be effectively eliminated at off-axis distances remote from the field edge by adjusting the jaw field size to the respective opening of the eMLC. Additionally, the interleaf and leaf-end leakage could be reduced by using a tongue-and-groove leaf shape and adjoining the leaf-ends off-axis respectively.     

Measurements of output factors with different detector types and Monte Carlo calculations of stopping-power ratios for degraded electron beams

The aim of the present study was to investigate three different detector types (a parallel-plate ionization chamber, a p-type silicon diode and a diamond detector) with regard to output factor measurements in degraded electron beams, such as those encountered in small-electron-field radiotherapy and intraoperative radiation therapy (IORT). The Monte Carlo method was used to calculate mass collision stopping-power ratios between water and the different detector materials for these complex electron beams (nominal energies of 6, 12 and 20 MeV). The diamond detector was shown to exhibit excellent properties for output factor measurements in degraded beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of output factors, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth (especially for lower electron energies). Application of ionization-chamber-based dosimetry, according to international dosimetry protocols, will introduce uncertainties smaller than 0.3% into the output factor determination for conventional IORT beams if the variation of the water-to-air stopping-power ratio is not taken into account. The IORT system at our department includes a 0.3 cm thin plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. This was verified by the measurements. In small-electron-beam dosimetry, the water-to-air stopping-power ratio variation with field size could mostly be ignored. For fields with flat lateral dose profiles (>3 x 3 cm2), output factors determined with the ionization chamber were found to be in close agreement with the results of the diamond detector. For smaller field sizes the lateral extension of the ionization chamber hampers its use. We therefore recommend that the readily available silicon diode detector should be used for output factor measurements in complex electron fields.     

Dosimetric study of total skin irradiation with a scanning beam electron accelerator

The Therac 20 6-MeV scanned electron beam may be used for partial or total skin therapy. The maximum field size at 1 m is 30 X 30 cm defined by a set of primary photon collimators in conjunction with secondary trimmers. We have studied electron beam profiles with and without trimmers at the nominal source-skin distance of 1 m versus extended distances of 3-5 m. We find that the trimmers limit the field size and add little to the beam uniformity at extended distances. Beam energy, dose distributions, and output factors at extended distances were measured for single and multiple field arrangements with and without trimmers. Beam parameters were measured after introducing a degrader that lowered the energy to 3.7 MeV.     

高能电子线放射治疗的剂量学参数测量与分析

目的：对高能电子线总输出因子、百分深度剂量、深度剂量分布的剂量学参数进行测量并分析讨论。方法：在Varian23EX直线加速器上,利用9606剂量测量仪和0．6cc指型电离室测量不同能量、不同限光筒及不同射野下的输出剂量并作归一,得到我们所要的剂量学参数,然后分析数据。结果：总输出因子在不同能量下与正方形射野边长的关系可满足等式：y=a·e^bx＋c·e^dx。水模体百分剂量分布中,6MeV电子线各限光筒的90％、85％等剂量深度基本不变,9MeV-15MeV下90％、85％等剂量深度随着限光筒尺寸增大而变深。对于水模体的深度剂量分布情况,6MeV和12MeV能量的10cmx10cm、15cmxl5cm限光筒均整区内对称点的最大相对剂量差分别都为0．04％、O．03％。结论：通过测量掌握实际照射中的剂量学特点．对于电子线剂量的准确计算以及临床计划制定具有很大的参考价值。