Determination of electron beam output factors for a 20‐MeV linear accelerator

Mills, Michael D.; Hogstrom, Kenneth R.; Fields, Robert S.

doi:10.1118/1.595674

Cited by 29 publications

(22 citation statements)

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“…SSD eff is smallest for small fields and low electron energy. For rectangular field sizes, it is recommended that the geometric mean of SSD eff for each side be used as the SSD eff , as this can be shown to be equivalent to the square root method of Mills et al 112 The field size dependence is caused by a lack of lateral scatter equilibrium for small apertures. 40,113 Potential dose-delivering electrons near the central axis are scattered out of the field and not fully replaced by electrons originating peripheral to the central axis.…”

Section: B2d Effectivementioning

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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Section: B2d Effectivementioning

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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“…For example, the incident energy for 11 MeV beam simulations is 11.95 MeV. However, at the surface of the phantom, the mean energy of the 11 MeV beam inside the 10ϫ10 cm 2 , field is about 10.5 MeV. For smaller beams the value of R 50 decreases substantially.…”

Section: Simulationsmentioning

confidence: 98%

“…This is not due to the change of the mean energy in the beam, but is entirely an in-phantom effect. For example, the average energy in the 2ϫ2 cm 2 11 MeV beam is 10.6 MeV, even slightly larger than that of the 10ϫ10 cm 2 field despite the fact that R 50 decreases from 4.5 cm for the 10ϫ10 cm 2 field to 3.6 cm for the 2ϫ2 cm 2 field. From the exit window, the particles travel in the geometry defined by the component modules.…”

Section: Simulationsmentioning

confidence: 99%

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

et al. 1999

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A major task in commissioning an electron accelerator is to measure relative output factors versus cutout size ͑i.e., cutout factors͒ for various electron beam energies and applicator sizes. We use the BEAM Monte Carlo code ͓Med Phys. 22, 503-524 ͑1995͔͒ 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 10ϫ10 cm 2 , 15ϫ15 cm 2 , and 20ϫ20 cm 2 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 SSDϭ115 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 200MHz computer to simulate an accelerator and additional 2 h to calculate the relative output factor for each cutout with a statistical uncertainty of 1%. ͓S0094-2405͑99͒01405-4͔

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“…In the literature, the methods for predicting cutout output factors for electron beams include the equivalent square,10, 11 square root,12 one‐dimensional,13 and sector‐integration methods 14. These algorithms predict the output factor of an irregular electron field using a database of parameters based on measurements.…”

Section: Introductionmentioning

confidence: 99%

Convolution‐based modified Clarkson integration (CMCI) for electron cutout factor calculation

Chang

Lin

et al. 2018

J Applied Clin Med Phys

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Electron therapy is widely used to treat shallow tumors because of its characteristic sharp dose fall‐off beyond a certain range. A customized cutout is typically applied to block radiation to normal tissues. Determining the final monitor unit (MU) for electron treatment requires an output factor for the cutout, which is usually generated by measurement, especially for highly irregular cutouts. However, manual measurement requires a lengthy quality assurance process with possible errors. This work presents an accurate and efficient cutout output factor prediction model, convolution‐based modified Clarkson integration (CMCI), to replace patient‐specific output factor measurement. Like the Clarkson method, we decompose the field into basic sectors. Unlike the Clarkson integration method, we use annular sectors for output factor estimation. This decomposition method allows calculation via convolution. A 2D distribution of fluence is generated, and the output factor at any given point can be obtained. We applied our method to 10 irregularly shaped cutouts for breast patients for 6E, 9E, and 15E beams and compared the results with measurements and the electron Monte Carlo (eMC) calculation using the Eclipse planning system. While both the CMCI and eMC methods showed good agreement with chamber measurements and film measurements in relative distributions at the nominal source to surface distance (SSD) of 100 cm, eMC generated larger errors than the CMCI method at extended SSDs, with up to −9.28% deviations from the measurement for 6E beam. At extended SSD, the mean absolute errors of our method relative to measurements were 0.92 and 1.14, while the errors of eMC were 1.42 and 1.79 for SSD 105 cm and 110 cm, respectively. These results indicate that our method is more accurate than eMC, especially for low‐energy beams, and can be used for MU calculation and as a QA tool for electron therapy.

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Determination of electron beam output factors for a 20‐MeV linear accelerator

Cited by 29 publications

References 0 publications

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

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

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

Convolution‐based modified Clarkson integration (CMCI) for electron cutout factor calculation

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