Dosimetric Characteristics of Wedged Fields

Sidhu, Narinder; Breitman, K.

doi:10.1016/0958-3947(94)90031-0

Cited by 4 publications

(6 citation statements)

References 2 publications

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“…Figure 9͑b͒ also shows that the calculation predicts the dose profiles ''sagging'' laterally along the non-wedge gradient direction due to the beam hardening effect of the wedge. This phenomenon has also been noted by Chui et al 33 and Sidhu et al 34 The error of the calculated data particularly at the phantom surface along the non-wedge gradient direction could be caused by discrepancy in modeling the energy fluence distribution of the machine generated photons, which has been discussed elsewhere. 19 Also the attenuation by the wedge might be overestimated, which results in the sagging on the calculated dose profiles.…”

Section: Resultssupporting

confidence: 62%

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

1997

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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.

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Section: Resultssupporting

confidence: 62%

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

1997

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“…Two assumptions used for the wedge manufacture were unnecessary, but they were included for ease of wedge manufacture and to make the data collection easier. Sidhu and Breitman (1994) report noticeable sagging even for a 15 wedge. Their recommendation for limiting field size in the NW direction so that the dosimetric error is within an arbitrary percentage deviation is not necessary here, as the wedge modifications apply to the dose at all off-axis points.…”

Section: Discussionmentioning

confidence: 97%

“…Wedge filters are routinely used in radiotherapy centres worldwide to modify the dose distribution to obtain uniformity of dose in the target volume when summing fields or compensating for missing tissue. Typically the design of physical wedges supplied by linear accelerator manufacturers has uniform thickness in the non-wedged plane over the whole length of the wedge (Sidhu and Breitman 1994). However, dose profile curves in the non-wedged (NW) axis for wedged field irradiation show a decrease in dose (sagging) with off-axis distance, as shown in figure 1.…”

Section: Introductionmentioning

confidence: 99%

Improving wedged field dose distributions

et al. 1997

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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.

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“…Bidmead et al reported agreement of 2% between EWFs measured at d max and 10 cm. For comparison, at 6 MV the RWF (measured for a 10 × 10 cm 2 field with the 60 • fixed wedge) at 20 cm depth is about 1.07 (Sidhu and Breitman 1994).…”

Section: Effective Wedge Factorsmentioning

confidence: 99%

“…Some investigators (Ewald et al 1989) have proposed a design for fixed wedges that vary in thickness in the nonwedged direction, so that profiles match those of open fields. Others (Sidhu and Breitman 1994) have proposed a method for determining the maximum length of a fixed wedged field that can be used in the nonwedged direction without introducing undesirable alterations in the dose distribution. Approximating the fixed wedge fields, of limited size, to open fields is then valid.…”

Section: Introductionmentioning

confidence: 99%

Beam profiles in the nonwedged direction for dynamic wedges

Lydon¹,

Rykers²

1996

Phys. Med. Biol.

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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.

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Dosimetric Characteristics of Wedged Fields

Cited by 4 publications

References 2 publications

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

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

Improving wedged field dose distributions

Beam profiles in the nonwedged direction for dynamic wedges

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