Lateral electron equilibrium and electron contamination in measurements of head‐scatter factors using miniphantoms and brass caps

Li, X. Allen; Soubra, M.; Szántó, J.; Gerig, L

doi:10.1118/1.597508

Cited by 84 publications

(80 citation statements)

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“…10,[61][62][63][64] The thickness of material perpendicular to the beam direction should provide enough lateral scatter 65 Mini-phantom so that the accuracy of the measured S c is maintained. This task group recommends a 4-cm diameter cylindrical miniphantom 56 coaxial with the central axis of the beam with the detector at 10-cm depth for the measurement of S c independent of the normalization depth.…”

Section: B1c Tissue Phantom Ratiosmentioning

confidence: 99%

“…Waterequivalent materials are recommended for the construction of the mini-phantom, given reports of some variation in results using high Z build-up materials. 65 However, for field sizes between 1 and 5 cm, a high Z mini-phantom can be used as long as the S c is renormalized so that the S c measured at 5 cm field size matches that measured with a water equivalent miniphantom, as described in the TG-74 report. 10 S c should be measured with the detector at the isocenter unless the field size for the measurement does not encompass the whole phantom.…”

Section: B1c Tissue Phantom Ratiosmentioning

confidence: 99%

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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: B1c Tissue Phantom Ratiosmentioning

confidence: 99%

Section: B1c Tissue Phantom 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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“…In contrast the output-factor decreases in the same way when the ion chamber distance is 100 cm and the field size falls below 1.2 cm x 1.2 cm (the diameter of the lead build-up cap is 1.2 cm): the conditions for lateral electron equilibrium LEE are not achieved for smaller field sizes when using 15 MeV X-rays. To establish a complete LEE (cLEE) 18 , about 80 % of the range of secondary electrons is sufficient, i.e. for 15 MeV X-rays: r LEE = 2.6 cm Ł d max ; F min = 5.2 cm x 5.2 cm or cLEE is already reached at field size 4.2 cm x 4.2 cm (this field size holds for measurements in water or water equivalent material).…”

Section: Resultsmentioning

confidence: 99%

Determination of field size limitations in stereotactic and intensity modulated radiotherapy

Jensen

Ihnen

Eilf

et al. 2003

Australas. Phys. Eng. Sci. Med.

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For dose measurement in small photon fields, different detectors are currently in use: TLD, semiconductor, diamond-detector, film, etc. But for absolute dosimetry, ionization chambers show the most advantages. To meet the basic dosimetrical requirements for lateral electron equilibrium the field size F must not remain under specified values: i.e. 5.2 x 5.2 cm2 for 15 MeV X-bremsstrahlung. As well as increasing the focus-chamber-distance, changing the physical density of the build-up material in the close vicinity of the chamber will be helpful to determine the output factor OF for smaller fields. By means of a correction factor, k(mat), this is taken into account. For a 6 MeV X and a 15 MeV X-bremsstrahlung of linear accelerators the lower limit of the field size F is determined: F > or = 0.8 cm. This value is mainly dependent on the diameter of the focal-spot (phi = 3 mm) of the treatment unit including design characteristics of the treatment head. Beside the dosimetrical aspects, some geometrical parameters have to be considered, when accuracy of dose application should remain on the same level as in medium and large field treatment (4 cm < or = F < or = F(max)). To keep dose-volume errors as low as +/- 10 % (diameter of PTV: 20 mm), the mean total error delta of CT-scanning (delta(p)), planning (delta(pl)), patient positioning (delta(x)), and treatment unit instabilities (delta(m)) should not exceed +/- 0.8 mm.

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“…[15–18] Mini phantom material have no significant effect on Head scatter (S h ) measurement. [19–21] All the above reports reveal that an effective atomic number close to that of water equivalent material is the best one for fabricating a mini phantom.…”

Section: Introductionmentioning

confidence: 98%

“…[9] Measurements for output ratios, wedge factors and beam quality using polystyrene mini-phantom (relative electron density 1.02 g. cm -3 ) provides the quasi lateral equilibrium for high energy photon beam with a cross section of 4 × 4 cm 2 mini phantom. [1011]…”

Section: Introductionmentioning

confidence: 99%

Design of mini phantom and measurement of cobalt-60 beam data parameters

Senthilkumar¹,

Ramakrishnan²

2008

J Med Phys

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Low cost mini phantoms were fabricated indigenously with different water equivalent material such as polymethyl methacrylate and Bee's wax of different shapes (with dome top surface and flat top surface). The beam parameters of the Co-60 machine, such as head scatter correction factor (Sh), phantom scatter correction factor (SP), total scatter correction factor (SC,P), collimator exchange effect were measured. Output ratio measurements were taken for both mini phantom and water phantom for different square and rectangular field sizes. Normalized output ratios were compared with ESTRO published values and (Storchi and Van Gasteren) S and G data. The percentage of variation between the measured and the literature values is about 0.7%. Collimator exchange effect were measured for water and mini phantom for different field size, were compared with ESTRO value. This was found to be 0.5% and 1.0% respectively. Phantom scatter correction factors were calculated for square and rectangular filed sizes; this was compared with ESTRO values, found to be 0.7% for square and 1.0% for rectangular filed size. It was also noted that there were no appreciable variation observed in ion chamber readings of different materials of mini phantoms for dome and flat surfaces. Mini phantom measurements were done for all types of phantoms and the measured values were compared with the existing data and they were in good agreement with the published values.

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Lateral electron equilibrium and electron contamination in measurements of head‐scatter factors using miniphantoms and brass caps

Cited by 84 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

Determination of field size limitations in stereotactic and intensity modulated radiotherapy

Design of mini phantom and measurement of cobalt-60 beam data parameters

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