Beam profiles along the nonwedged direction for large wedged fields

Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71

A protocol is presented for the calculation of monitor units (MU) for photon and electron beams, delivered with and without beam modifiers, for constant source-surface distance (SSD) and source-axis distance (SAD) setups. This protocol was written by Task Group 71 of the Therapy Physics Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol defines the nomenclature for the dosimetric quantities used in these calculations, along with instructions for their determination and measurement. Calculations are made using the dose per MU under normalization conditions, D'0, that is determined for each user's photon and electron beams. For electron beams, the depth of normalization is taken to be the depth of maximum dose along the central axis for the same field incident on a water phantom at the same SSD, where D'0 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent D'0 ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of dm, with D'0 = 1 cGy/MU, although both systems are acceptable within the current protocol. For photon beams, the formalism includes the use of blocked fields, physical or dynamic wedges, and (static) multileaf collimation. No formalism is provided for intensity modulated radiation therapy calculations, although some general considerations and a review of current calculation techniques are included. For electron beams, the formalism provides for calculations at the standard and extended SSDs using either an effective SSD or an air-gap correction factor. Example tables and problems are included to illustrate the basic concepts within the presented formalism.

Radiation therapy of large intact breasts using a beam spoiler or photons with mixed energies

Radiation treatment of large intact breasts with separations of more than 24 cm is typically performed using x-rays with energies of 10 MV and higher, to eliminate high-dose regions in tissue. The disadvantage of the higher energy beams is the reduced dose to superficial tissue in the buildup region. We evaluated 2 methods of avoiding this underdosage: (1) a beam spoiler: 1.7-cm-thick Lucite plate positioned in the blocking tray 35 cm from the isocenter, with 15-MV x-rays; and (2) combining 6- and 15-MV x-rays through the same portal. For the beam with the spoiler, we measured the dose distribution for normal and oblique incidence using a film and ion chamber in polystyrene, as well as a scanning diode in a water tank. In the mixed-energy approach, we calculated the dose distributions in the buildup region for different proportions of 6- and 15-MV beams. The dose enhancement due to the beam spoiler exhibited significant dependence upon the source-to-skin distance (SSD), field size, and the angle of incidence. In the center of a 20 x 20-cm(2) field at 90-cm SSD, the beam spoiler raises the dose at 5-mm depth from 77% to 87% of the prescription, while maintaining the skin dose below 57%. Comparison of calculated dose with measurements suggested a practical way of treatment planning with the spoiler--usage of 2-mm "beam" bolus--a special option offered by in-house treatment planning system. A second method of increasing buildup doses is to mix 6- and 15-MV beams. For example, in the case of a parallel-opposed irradiation of a 27-cm-thick phantom, dose to D(max) for each energy, with respect to midplane, is 114% for pure 6-, 107% for 15-MV beam with the spoiler, and 108% for a 3:1 mixture of 15- and 6-MV beams. Both methods are practical for radiation therapy of large intact breasts.

Beam characteristics of upper and lower physical wedge systems of Varian accelerators

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.

Dose calculation along the nonwedged direction for externally wedged beams: improvement of dosimetric accuracy with comparatively moderate effort

Wedge filters ideally modify photon intensities only in one direction. However, in the other, the "nonwedged" direction, the intensity is affected too; it usually decreases with increasing off-axis distance. For external wedges on a particular treatment machine (Varian Clinac 2100C) and 6 MV photons, for example, this decrease is as big as 8%, depending on wedge angle and material, off-axis distance, and phantom depth. We present a way to account for this effect in prescriptions to points off-center in the nonwedged direction. The goal was to minimize the amount of additional data required for this purpose, without unduly compromising the final prescription accuracy. We measure the effective attenuation coefficients in narrow beam geometry for the wedge materials (lead and steel) as a function of thickness and off-axis angle, and the corresponding attenuation in water, again as a function of wedge material thickness and off-axis angle. The data allow us to extract a correction factor for off-axis distances in the nonwedged direction. Neglecting the contribution of scattered radiation and using primary beam data only, shortens data acquisition and simplifies calculations, but still yields surprisingly accurate results. Application of the derived correction reduces the off-axis distance related dose calculation error in wedged fields to < or =1%.

A Monte Carlo study on internal wedges using BEAM

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).

Verification of the super-omni wedge concept

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.

Comparison of dosimetric characteristics of Siemens virtual and physical wedges

Dosimetric properties of Virtual Wedge (VW) and physical wedge (PW) in 6 and 23 MV photon beams from a Siemens Primus linear accelerator, including wedge factors, depth doses, dose profiles, peripheral doses and surface doses, are compared. While there is a great difference in absolute values of wedge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have a stronger depth dependence than VWF due to beam hardening in PW fields. VW dose profiles in the wedge direction, in general, match very well with PW, except in the toe area of large wedge angles with large field sizes. Dose profiles in the nonwedge direction show a significant reduction in PW fields due to off-axis beam softening and oblique filtration. PW fields have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open fields while PW fields have lower surface doses. Surface doses for both VW and PW increase with field size and slightly with wedge angle. For VW fields with wedge angles 45 degrees and less, the initial gap up to 3 cm is dosimetrically acceptable when compared to dose profiles of PW. VW fields in general use less monitor units than PW fields.

Calculating dose and output factors for wedged photon radiotherapy fields using a convolution/superposition method

We have developed a convolution/superposition method to calculate dose distributions in photon treatment fields with beam modifiers such as physical wedges. The dose component due to wedge generated radiation was accounted for by using an extended phantom model, which integrated a wedge, an air gap, and a patient phantom as the calculation phantom. The inhomogeneities in the extended phantom and the effect of beam hardening by the wedge were both corrected for in the convolution dose calculation. The calculated dose was verified by Monte Carlo simulation of the same extended phantom. A new dual photon source model was also used in the convolution method to account for both primary photons from the target and extra-focal photons from the primary collimator and flattening filter. Thus, realistic photon energy fluence distributions in the extended phantom were used for the dose calculation. The calculated dose distributions and the wedge factors agreed with the measured data within 2% for a variety of treatment fields including asymmetric fields. Our results showed that the wedge-generated radiation could contribute a significant fraction of the total dose in patients. This dose component depends on a specific field configuration, thus wedge factor changes with photon energy, wedge angle, field size, depth, and patient phantom SSD. The variation of the wedge factor can be predicted accurately by our convolution approach with the extended phantom model, which allows for more accurate dose or monitor unit computation for photon fields with beam modifiers.

Improving wedged field dose distributions

Dose profiles produced by wedge filters in the non-wedged direction can exhibit a 7% or greater dose reduction at the outer ends of the field compared with open field profiles. However, many planning systems use open field profiles to model wedged dose distributions. In the present work, wedges have been modified to reproduce open field profile shapes. This modification involved removing varying thicknesses of the wedge using a simple milling machine. The wedge thickness was calculated using the assumption that dose is proportional to primary collision kerma. The discrepancies in dose between wedged field and open field profile shapes of up to 7% were reduced to less than 3% with the modifications, even for varying depths and off-axis distances. The necessary measurements are simple to perform, and hence this technique could be applied to improve wedged field dose distributions in other radiotherapy departments.

Surface and peripheral doses of dynamic and physical wedges

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.

Beam profiles in the nonwedged direction for dynamic wedges

One feature of the dynamic wedge is the improved flatness of the beam profile in the nonwedged direction when compared to fixed wedges. Profiles in the nonwedged direction for fixed wedges show a fall-off in dose away from the central axis when compared to the open field profile. This study will show that there is no significant difference between open field profiles and nonwedged direction profiles for dynamically wedged beams. The implications are that the dynamic wedge offers an improved dose distribution in the nonwedged direction that can be modelled by approximating the dynamically wedged field to an open field. This is possible as both the profiles and depth doses of the dynamically wedged fields match those of the open fields, if normalized to dmax of the same field size. For treatment planning purposes the effective wedge factor (EWF) provides a normalization factor for the open field depth dose data set. Data will be presented to demonstrate that the EWF shows relatively little variation with depth and can be treated as being independent of field size in the nonwedged direction.