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PURPOSE: The physical and dosimetric differences between three different wedge systems on a multileaf collimator (MLC) equiped linear accelerator are discussed in this report. In particular, the in-field and peripheral surface doses from these wedge systems are measured and their clinical differences discussed. METHODS AND MATERIALS: A parallel-plate chamber was used in a solid water phantom to measure the surface doses of the wedges. Published correction factors were used to convert relative ionization to relative surface dose. Measurements were performed for 6 and 18 MV photon beams for different field sizes, source-surface distances (SSD), and distances outside the field for peripheral dose measurements. Surface-dose profiles across a field in the wedge-gradient direction were measured for the dynamic and upper wedges. Dose profiles in the nonwedge gradient direction were measured for open beam as well as the three wedges using films at depths of maximum dose (d(max)). RESULTS: At 85 cm SSD, surface doses on the central axis under a dynamic wedge or upper wedges are similar to those of an open field, while those of a lower wedged field are as much as 100% higher. Differences in surface doses due to beam energy are relatively minor compared with differences due to SSD or wedge systems. Dynamic and upper wedges produce similar peripheral doses, much lower than those produced by the lower wedges. The surface dose profile across the field under the dynamic wedge has a higher slope than that under the upper wedge, when the difference in wedge angles is compensated for by normalization to the dose profile at d(max). In the nonwedge gradient direction, the dose profiles at d(max) of both the upper and the lower wedges demonstrate a marked effect of oblique filtration of the primary beam, resulting in an off-axis ratio at 80% of the field width of 0.95, in contrast to the off-axis ratio of 1.05 in the open and the dynamic wedged fields. CONCLUSIONS: The three wedge systems produce significantly different surface and peripheral doses that should be considered in properly choosing a wedge system for clinical use. Dynamic wedge and upper wedge systems deliver surface and peripheral doses similar to those of open fields and much lower than the lower wedge system. Both physical wedge systems degrade beam profiles in the nonwedged direction.
This study verifies the concept of the super-omni wedge by validating its equations for effective wedge orientation and wedge angle and by comparing the dose distributions it produces with those produced by other wedge techniques. To validate the equations, we calculated dose distributions for 20 combinations of wedge orientations and wedge angles: we then determined the differences between the wedge orientations and angles predicted by the equations and those calculated by a three-dimensional treatment-planning system. To compare the super-omni wedge concept with other techniques, we calculated the dose and position differences between the dose distributions produced by the super-omni wedge concept and those produced by other wedge techniques. The error of wedge orientations ranged from -0.5 degrees to 0.4 degrees, and that of wedge angles ranged from -0.6 degrees to 1.7 degrees. The dose distributions produced by the super-omni wedge were similar and therefore equivalent to those produced by other wedge techniques. Serving as an intermediate step in treatment-planning optimization. the super-omni wedge is a reliable method for producing wedged dose distributions with arbitrary wedge orientations and wedge angles.
The beam characteristics of a dual physical wedge system, upper and lower, for Varian accelerators are studied over the energy range 6-18 MV. Wedge factors for both systems are measured in a water phantom as a function of field size, depth and source-to-wedge (SWD) distance. Our results indicate that apart from their physical differences, dosimetrically, the two wedge systems have <2% difference in central axis percentage depth dose beyond the build-up region. The lower wedge central axis percentage depth dose is consistently lower than that of the corresponding upper wedge, with the effect more pronounced for large field sizes. The wedge profiles are identical within 2% for all field sizes, depths and energies. The wedge factors for both wedge systems are also within 2% for all field sizes and depths for both 6 and 15 MV photons and slightly higher for the 18 MV beam and 45 degrees-60 degrees wedge angle. The wedge factor variation with SWD reveals an interesting fact that thinner wedges (15 degrees and 30 degrees) result in a higher surface dose in the central axis region than thicker wedges. As the SWD increases beyond 80 cm, the reverse is true, i.e. thicker wedges produce higher surface dose than thinner wedges. It is also verified that the wedge factor at any depth and for any field size can be calculated from the wedged and open field central axis percentage depth dose, and the wedge factor at dmax, resulting in nearly 44% reduction in water phantom scanning and 80% reduction in point measurements during commissioning.
AIM: To investigate variation of wedge factors on field size and depth for physical and dynamic wedges of identical wedge angles for Clinac 2100C linear accelerator and its clinical implementation. MATERIAL AND METHODS: A computer controlled water phantom dosimetric system is used to generate profile data for physical wedges, whereas a 0.6 cm3 ion chamber is used for generation of profiles for dynamic wedge and wedge factors for both types of wedges. The method has been discussed to handle the dynamic wedge dosimetry in absence of linear array of detectors or film densitometer. RESULTS: A systematic dependence on wedge factor is observed for physical wedge, with respect to depth and wedge angle but not depending on field size. Whereas dynamic wedge shows strong dependence on field size and is not systematic because the dynamic wedge is controlled by segmented treatment tables depending on field size and energy and no significant variation is observed on depth for various wedge angles. The handling of beam data in a commercially available treatment planning system is discussed and a comparison has been made for iso-doses of both types of wedges. CONCLUSION: The dynamic wedge isodose curves shows rather straight lines than physical wedge but larger hot spots at thin edge which needs careful consideration during planning.
PURPOSE: Evaluating the validity of the universal wedge equation over the range of 60Cobalt (60CO) to 25 MV photon beam energies. METHODS AND MATERIALS: The universal wedge equation relates the beam weight of the wedged field to the effective wedge angle produced by combining the wedged field to the nonwedged field and is expressed as tan (theta) = B tan (theta w), where B is the normalized weight imposed on the wedged field, theta w is the maximum wedge angle of the wedge filter, and theta is the effective wedge angle. The isodose distributions from 60Co to 25 MV photon beam energies were used. For each photon energy, the isodose distributions of wedged and nonwedged fields were combined in different proportions. The effective wedge angle was determined from each resultant isodose distribution. RESULTS: The relationship between the weight of wedged field and the effective wedge angle was found to be nonlinear for wedge filters with maximum wedge angle greater than 30 degrees. The universal wedge equation predicts the effective wedge angle to within 2 degrees compared to the measured value.
The dependence of the wedge factor and central axis depth dose on field size was evaluated for 6-, 10-, and 24-MV wedged photon beams for field sizes up to 40 x 40 cm2. The wedge factor for 60 degrees, 45 degrees, 30 degrees, and 15 degrees wedges in a 24-MV beam was found to vary by as much as 25%, 12%, 9%, and 5%, respectively, over a field size range of 5 x 5 to 40 x 40 cm2. For 10 and 6 MV wedged beams, the wedge factors varied by up to 17% and 15%, respectively, over the same field size range. The depth dose curves for the wedged beams differed significantly from the open beam profiles. At 6 MV, the wedges caused beam hardening while at 24 MV, with the exception of the 15 degrees wedge, all wedged beams were softer than the open beams, for all field sizes. At 10 MV, wedged fields of size less than 20 x 20 cm2 were hardened relative to the open beam, whereas larger wedged fields had depth dose values within +/- 1% of the 10-MV open-beam depth dose data. Accurate treatment planning for large wedged fields and high-energy photon beams thus requires the use of wedged beam depth dose curves and field size specific wedge factors. It was established that an equivalent square field for a rectangular wedged field can be determined using the standard open beam formulation. The largest difference between the wedge factor for a rectangular beam and its equivalent square beam was 2.5% and occurred for 24-MV elongated fields.(ABSTRACT TRUNCATED AT 250 WORDS)
The concommitant use of a multileaf collimator (MLC) and a wedge can result in conflicts in the optimal collimator angle if both MLC and wedge are fixed relative to one another. This is particularly true of linacs in which a single wedge orientation is provided. In this paper, a solution is provided that makes use of two orthogonal universal wedges (omni wedge). Although this technique can be applied regardless of the means by which the wedged fields are implemented, the measurements reported in this paper were performed using a fixed, internal mechanical wedge coupled with a dynamic wedge, formed by the motion of one of the backup jaws. An implementation of a dynamic wedge for the Elekta SL series of linear accelerators is presented. Results of measurements of the dosimetric characteristics of both the particular implementation of the dynamic wedge and of the omni field are presented. For the dynamic wedge, measurements were made of the wedge factor and dose profile as a function of field size and depth. In addition, the effects of variables, such as dynamic delivery technique and direction of diaphragm motion, on the dynamic wedge profiles were studied and discussed. For the omni wedge, measurements were made of the degree to which the mathematical formalism for describing an omni wedge matches the measured isodose distributions. Comparisons between mechanical wedge dose distributions and the omni wedge were also made.
STUDY DESIGN: Biomechanical study. OBJECTIVES: This paper describes the biomechanical comparison of calf spines instrumented with a wedge alone versus a wedge-rod construct for the fusionless correction of scoliosis. SUMMARY OF BACKGROUND DATA: Current techniques for the correction of scoliosis require either anterior or posterior spinal fusion for correction. We propose a technique allowing correction via vertebral body osteotomies along with fixation using a wedge-rod construct without the requirement of intervertebral segment fusion. METHODS: Calf spines were used to test the biomechanical difference between the intact spine, transverse osteotomized spine with wedge-rod reconstruction, and transverse osteotomized spine with wedge alone reconstruction. Unconstrained segments (L1-L5) were first tested under five nondestructive static loading conditions to evaluate the intact stability of the operative motion segments in axial compression (-600 N), axial rotation (+/-5.0 Nm, 150 axial preload), flexion and extension (+/-5.0 Nm), and lateral bending (+/-5.0 Nm). Following the intact analysis, vertebral wedge osteotomies in the transverse plane were performed at the L2, L3, and L4 levels. The defects were reconstructed using the Sofamor Danek Wedge Spacer, and stainless steel TSRH one-quarter inch single rod with modified CD HORIZON 6.5-mm diameter vertebral body screws at each level (L2-L4). Standard CD HORIZON 6.5-mm bone screws and staples were used at the superior and inferior ends of the five-level construct. The wedge was on the left side and the rod and screw heads on the right side. After testing the reconstructed specimen, the TSRH rod was removed and the construct retested to evaluate the stability of the wedge alone reconstruction. RESULTS: Construct stiffness was calculated as the peak applied load (N or Nm) divided by the corresponding segmental displacement (mm or degrees) normalized to the intact specimen. Reconstruction static data are expressed as a percentage change from the intact condition. Statistical analysis included descriptives, a one-way analysis of variance, and the Student-Newman-Keuls test for multiple comparisons among the reconstruction groups. Axial compression: under axial compressive loads, the stiffness of the wedge-rod construct was approximately equal to that of the intact group. The stiffness of the wedge alone construct was 56% less than that of the intact group except for extension and left lateral bending. For the other modes of loading (right rotation, left rotation, flexion, extension, right lateral bending, and left lateral bending), the wedge-rod construct was stiffer than that of the intact group. The stiffness of the wedge alone construct was consistently less than that of the intact group. CONCLUSIONS: Based on the results of this biomechanical comparison, the calf spines instrumented with the wedge-rod system for fusionless correction were significantly stiffer as compared to the intact calf spine. The wedges alone were not as stiff as the intact spine. This suggests that the theory of performing transverse osteotomies of vertebral bodies with fixation with wedge-rod construct for 8 to 12 weeks, followed by removal of the rod, could provide adequate fixation and correction of a scoliotic deformity without requiring fusion of motion segments.
The beam characteristics of the wedged fields in the nonwedged planes (planes normal to the wedged planes) were studied for 6 MV and 15 MV x-ray beams. A method was proposed for determining the maximum field length of a wedged field that can be used in the nonwedged plane without introducing undesirable alterations in the dose distributions of these fields. The method requires very few measurements. The relative wedge factors of 6 MV and 15 MV X-rays were determined for wedge filters of nominal wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees as a function of depth and field size. For a 6 MV beam the relative wedge factors determined for a field size of 10 x 10 cm2 for 30 degrees, 45 degrees, and 60 degrees wedge filters can be used for various field sizes ranging from 4 cm2 to 20 cm2 (except for the 60 degrees wedge for which the maximum field size that can be used is 15 x 20 cm2) without introducing errors in the dosimetric calculations of more than 0.5% for depths up to 20 cm and 1% for depths up to 30 cm. For the 15 degrees wedge filter the relative wedge factor for a field size of 10 x 10 cm2 can be used over the same range of field sizes by introducing slightly higher error, 0.5% for depths up to 10 cm and 1% for depths up to 30 cm. For a 15 MV beam the maximum magnitude of the relative wedge factors for 45 degrees and 60 degrees lead wedges is of the order of 1%, and it is not important clinically to apply a correction of that magnitude. For a 15 MV beam the relative wedge factors determined for a field size of 6 x 6 cm2 for the 15 degrees and 30 degrees steel wedges can be used over a range of field sizes from 4 cm2 to 20 cm2 without causing dosimetric errors greater than 0.5% for depths up to 10 cm.
The purpose of this study is to evaluate the magnitude of the error in dose delivery caused by the use of open beam depth dose data in dosimetry calculations for wedged photon beams. Isodose plans were calculated for treatments given in a 3-field isocentric prostate or rectal setup using an open AP beam with two lateral wedged beams. The dose distributions were first calculated using open beam depth dose data for all three fields. Next, the open beam data was used only for the AP field and true wedged beam depth dose data was substituted for the two lateral wedged fields. The magnitude of the depth dose variations for wedged vs open beams depends on the nominal beam energy, the wedge angle, and the depth of measurement. Consequently, isodose distributions calculated for wedged fields were found to be different when true wedged beam depth dose data was used instead of open beam data as is commonly done. Monitor unit calculations using a field size specific wedge factor show that dose delivery errors up to 4% can result from the use of open beam depth dose data in wedged beam dose distribution calculations for a 6-MV photon beam. Accurate treatment planning for wedged fields requires the use of wedged beam depth dose data specific to each wedge. Simply using open beam depth dose data in dose calculations for wedged beams will result in dose delivery errors, the magnitude of which depends on the combination of wedge angle, field size, and nominal beam energy.
The layering of macroions confined to a wedge slit formed by two uncharged hard walls is studied using a canonical Monte Carlo method combined with a simulation cell that contains both wedge-shaped slit and bath regions. The macroion solution is modeled within a one-component fluid approach that in an effective way incorporates the double layer repulsion due to simple electrolyte ions as well as the discrete nature of an aqueous solvent. The layer formation under a wedge confinement is analyzed by carrying out separate simulation runs for a set of consecutive wedge segments designed to represent a single wedge slit. As the wedge thickness progressively increases, the sequence of regions along the wedge film with distinct features of macroion layering has been established. This sequence comprises (i) a wedge region of the thickness smaller than the macroion diameter that is free of macroions; (ii) a region with a one-dimensional macroion chain along the wedge corner at a wedge thickness of a one macroion diameter; (iii) a region comprising a low-ordered macroion monolayer that extends until the wedge thickness slightly above two macroion diameters; (iv) a region comprising a pair of well-defined two-dimensional configurations of macroions segregated on each of the wedge walls; and (v) a free-of-macroions wedge region between two surface monolayers that now originates from an electrostatic repulsion imposed by the surface macroions, which is followed by (vi) a well-defined macroion monolayer film between two surface monolayers, a less defined bilayer film, a three-layer film, and so on up to the bulk solution. Once formed, the macroion surface monolayers persist for all remaining wedge thicknesses up to the bulk, forming in such a way effective charged wedge boundaries. Such a formation of the macroion surface monolayers on the uncharged confining walls is related to the haloing mechanism for regulating the stability in colloidal suspensions [Tohver et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8951] and is discussed as well. Finally, the estimated boundary of the free of macroion region between surface monolayers correlates well with the location of the boundary of the so-called "vacuum" phase that has been observed experimentally in an aqueous suspension of charged polystyrene spheres bounded by electrostatically repulsive glass walls [Pieransky et al. Phys. Rev. Lett. 1983, 50, 900].
The optimal field shape achieved using a multileaf collimator (MLC) often requires collimator rotation to minimize the adverse effects of the scalloped dose distribution the leaf steps produce. However, treatment machines are designed to deliver wedged fields parallel or perpendicular to the direction of the leaves. An analysis of cases from our clinic showed that for 25% of the wedged fields used to treat brain and lung tumors, the wedge direction and optimal MLC orientation differed by 20 degrees or more. The recently published omni wedge technique provides the capability of producing a wedged field with orientation independent of the orientation of the collimator. This paper presents a comparison of the three-dimensional (3D) dose distributions of the omni wedged field with distributions of wedged fields produced using both the universal and dynamic wedge techniques. All measurements were performed using film dosimetry techniques. The omni wedge generated fields closely matched the conventional wedged fields. Throughout 95% of the irradiated volume (excluding the penubra), the dose distribution of the omni wedged field ranged from +5.5 to -3.5 +/- 1.5% of that of the conventionally wedged fields. Calculation of the omni wedged field is as accurate as conventional wedged field calculation when using a 3D treatment planning systems. For two-dimensional treatment planning systems, where one must assume that the omni wedged field is identical to a conventional field, the calculated field and the delivered field differs by a small amount.
The beam-hardening effects of the wedges on a 6-MV spoiled beam has been studied. The beam quality of all the wedged beams was found to be the same as the open beam. The dmax also stayed unchanged for all the wedges at all field sizes. The relative wedge factors were found to reflect the beam-hardening effect, which is a function of the wedge angles, depths, and field sizes. For the 15 degrees and 30 degrees wedges, the relative wedge factor at depths less than 15 cm were found to deviate less than +2% for all fields, while those for the 45 degrees and 60 degrees wedges for the same depth range from +3% to +4%. The surface doses were found to decrease with the wedged fields. For 15 degrees, 30 degrees, and 45 degrees wedges, the decreases were found to be from 0 to -2.0%. For 60 degrees wedges, the largest deviation was found to be -2.5% for a field size of 10 cm x 10 cm at a depth of 2 mm. The wedge factors at dmax were found to depend slightly on the field sizes. The use of an averaged wedge factor for each individual wedge was found to produce less than +/- 1.2% of error for all field sizes.
To do calculations for wedged photon beams with the NRC Monte Carlo simulation package BEAM, a new Component Module for wedges called WEDGE has been designed and built. After an initial series of benchmarks using monoenergetic photon beams as well as realistic 6 MV and 10 MV beams, it was found, that the new CM did work fine for the large wedge (maximum field size 30 x 40 cm2) of the Elekta SL-linac. The next step was to calculate dose distributions and output factors for a range of wedged fields with field size from 3 x 3 cm2 to 30 x 30 cm2. Results from these simulations have been compared to measurements. Calculated values for the reference wedge transmission factor and the relative wedge transmission factors were within 1.5% from the measured data. Dose distributions showed an identical behavior; both depth-dose curves as well as cross profiles were within 1.5% from measured data, usually even better. Despite the increased mean energy, there was no indication that, as a result, the phantom scatter output factors will change for a 10 MV photon beam. It was found that by adding a wedge the contributions for the different sources of head scatter changed considerably as compared to the open fields, apart from the additional scatter from the wedge. Another consequence of inserting a wedge was an increase in the mean energy of both primary and scattered radiation with 0.3 MV and 0.7 MV, respectively, for all wedged fields with respect to the corresponding open fields. Despite the statistical uncertainty in the calculated data, which is in the same order of magnitude as the effect to be determined, it was possible to derive reliable data for the beam hardening from the calculated dose distributions. Only for the smallest field (field size 3 x 3 cm2) a large difference between the measured and calculated beam hardening factor was found due to the relative large voxel size of 1 x 1 x 1 cm3 compared to the field size. For a description of the influence of a wedge on a photon beam, the results of this study strongly support the use of a reference wedge transmission factor (determined under reference conditions) in combination with a relative wedge transmission factor. The product of these variables should replace the collimator scatter output factor used in open fields. The influence on the dose distribution should be incorporated by using the (field size dependent) beam hardening. The ultimate solution will be to make this beam hardening depending on the actual position in the radiation field, as the photon energy varies over the field (holds also for open fields).