Dosimetry of asymmetric x‐ray collimators

Khan, Faiz M.; Gerbi, Bruce J.; Deibel, Firmin C.

doi:10.1118/1.595822

Cited by 66 publications

(42 citation statements)

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“…Khan et al [4] have suggested that the asymmetric collimators can be considered as diverging blocks in computerized treatment planning systems for dose calculations. However, the above assumption might lead to some error in dose delivery due to the presence of backscatter radiation into the beam monitor chambers.…”

Section: Discussionmentioning

confidence: 99%

Measurement of Backscattered Radiation from Secondary Collimator Jaws into the Beam Monitor Chamber from a Dual Energy Linear Accelerator

Ravikumar

Ravichandran

2001

Strahlenther Onkol

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From this study a maximum reduction of 4% and 4.3% in dose delivery was observed for 6 MV and 18 MV photon beams due to backscattered radiation originating mainly from upper collimator reaching the beam monitor chamber. For asymmetric fields it is felt that direct output measurement is more reliable in order to avoid errors in output factor due to radiation backscattered from the collimator jaws into beam monitor chamber.

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Section: Discussionmentioning

confidence: 99%

Measurement of Backscattered Radiation from Secondary Collimator Jaws into the Beam Monitor Chamber from a Dual Energy Linear Accelerator

Ravikumar

Ravichandran

2001

Strahlenther Onkol

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“…67,68 However, measured profile data inherently contain changes in the relative scatter contribution within the phantom which are not accounted for in the current formalism. 69 Although good agreement is obtained for points close to the central axis, 67,70 errors greater than 5% can be obtained using profile data close to the edge of the field (e.g., x > 10 cm). Although calculations to points this far off-axis are unlikely to be seen in the clinic with any frequency, caution should be used when using these data for OAR.…”

Section: B1f Off-axis Ratiosmentioning

confidence: 99%

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

et al. 2014

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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 d m , 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.

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“…This effect is largest for a short SSD (1 1). Since central-axis depth doses can be measured in conventional geometry at the TBI distance and collimator scatter is almost independent of the position of the jaws for fields of the same dimensions (15), off axis depth doses were measured in a large water phantom in fields defined by the asymmetric jaws (Fig. 2).…”

Section: Resultsmentioning

confidence: 99%