Entrance and exit dose regions for a clinac-2100c

Klein, Eric E.; Purdy, James A.

doi:10.1016/0360-3016(93)90256-u

Cited by 43 publications

(30 citation statements)

References 18 publications

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“…1 However, the amount of dose from contaminant electrons from the head depends strongly on clinical setup parameters such as field size, beam modifier ͑wedge͒, tray, block, and source-to-surface distance ͑SSD͒. [7][8][9][10][11] Therefore, skin dose can vary significantly from one setup to another even when in-phantom dosimetric characteristics of the beams are similar to each other. In this study, we measured skin doses for 8 MV and 18 MV photon beams from a Varian Clinac 2100C under various clinical setups including the multileaf collimator ͑MLC͒ and the dynamic wedge.…”

Section: Introductionmentioning

confidence: 99%

Photon beam skin dose analyses for different clinical setups

et al. 1998

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A comprehensive set of data on skin dose for 8 MV and 18 MV photon beams from a medical linear accelerator was measured using a parallel-plate chamber to document the effect of field size, source-to-surface distance (SSD), off-axis distance, acrylic block tray, wedge (external standard wedge), Lipowitz's metal block, multileaf collimator (MLC), and dynamic wedge. The skin dose increased as field size increased from 5 X 5 cm2 to 40 X 40 cm2 (6% to 38% for 8 MV and 5% to 44% for 18 MV beam). With the use of an acrylic block tray, the skin dose increased for all field sizes (7% to 59% for 8 MV and 5% to 62% for 18 MV beam), but the increase was minimal for small fields. The skin dose with a wedge showed a much more complex trend. It was generally lower than the dose for an open field, but higher in the case of large fields and higher degree wedges. When both wedge and block tray were used, the tray was a major contributor to the skin dose because some of the contaminant electrons from the wedge assembly were absorbed by the block tray. Field-shaping blocks increased the skin dose, but, interestingly, the block tray reduced the skin dose for small blocked fields treated with a high-energy photon beam. The effect of an MLC on skin dose was very similar to that of a Lipowitz's metal block, but its magnitude was less. The skin dose was higher for dynamic wedge fields than it was for standard wedge fields. As SSD decreased, the skin dose increased, and this effect was dominant in larger field sizes. The SSD effect was enhanced in the presence of an acrylic block tray. The skin dose off-axis was the same as at the central axis, or smaller. A similar pattern of behavior of the skin dose is expected for photon beams from other linear accelerators.

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Section: Introductionmentioning

confidence: 99%

Photon beam skin dose analyses for different clinical setups

et al. 1998

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“…The effect of various backscattering materials on exit dose was also studied. The entrance and exit dose regions are of current interest as our accelerator is equipped with a Kapton thin window monitor chamber, which is reported to vary the entrance and exit doses significantly [11].…”

mentioning

confidence: 99%

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

Ravikumar¹,

Ravichandran

2000

Strahlenther Onkol

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The increase in surface dose with field size for both photon energies is due to the electron scattering from the intervening materials. The use of wedge filters absorbs low-energy scattered electrons significantly and hence, the relative surface dose (RSD) is always less than unity. The increase in dose enhancement percentage with graphite compared to perspex supporting assembly indicates that the electron backscatter is proportional to the atomic number of the medium.

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“…A PTW c thin-window parallel-plate Markus ionization chamber was used for dose measurements along the central beam axis in front of air cavities (build-down region) and beyond air cavities (secondary build-up region) 17,18 .…”

Section: Markus Parallel Plate Ionisation Chambermentioning

confidence: 99%

Investigation of photon beam models in heterogeneous media of modern radiotherapy

Ding

Johnston

Wong

et al. 2004

Australas. Phys. Eng. Sci. Med.

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This study investigates the performance of photon beam models in dose calculations involving heterogeneous media in modern radiotherapy. Three dose calculation algorithms implemented in the CMS FOCUS treatment planning system have been assessed and validated using ionization chambers, thermoluminescent dosimeters (TLDs) and film. The algorithms include the multigrid superposition (MGS) algorithm, fast Fourier Transform Convolution (FFTC) algorithm and Clarkson algorithm. Heterogeneous phantoms used in the study consist of air cavities, lung analogue and an anthropomorphic phantom. Depth dose distributions along the central beam axis for 6 MV and 10 MV photon beams with field sizes of 5 cm x 5 cm and 10 cm x 10 cm were measured in the air cavity phantoms and lung analogue phantom. Point dose measurements were performed in the anthropomorphic phantom. Calculated results with three dose calculation algorithms were compared with measured results. In the air cavity phantoms, the maximum dose differences between the algorithms and the measurements were found at the distal surface of the air cavity with a 10 MV photon beam and a 5 cm x 5 cm field size. The differences were 3.8%. 24.9% and 27.7% for the MGS. FFTC and Clarkson algorithms. respectively. Experimental measurements of secondary electron build-up range beyond the air cavity showed an increase with decreasing field size, increasing energy and increasing air cavity thickness. The maximum dose differences in the lung analogue with 5 cm x 5 cm field size were found to be 0.3%. 4.9% and 6.9% for the MGS. FFTC and Clarkson algorithms with a 6 MV photon beam and 0.4%. 6.3% and 9.1% with a 10 MV photon beam, respectively. In the anthropomorphic phantom, the dose differences between calculations using the MGS algorithm and measurements with TLD rods were less than +/-4.5% for 6 MV and 10 MV photon beams with 10 cm x 10 cm field size and 6 MV photon beam with 5 cm x 5 cm field size, and within +/-7.5% for 10 MV with 5 cm x 5 cm field size, respectively. The FFTC and Clarkson algorithms overestimate doses at all dose points in the lung of the anthropomorphic phantom. In conclusion, the MGS is the most accurate dose calculation algorithm of investigated photon beam models. It is strongly recommended for implementation in modern radiotherapy with multiple small fields when heterogeneous media are in the treatment fields.

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Entrance and exit dose regions for a clinac-2100c

Cited by 43 publications

References 18 publications

Photon beam skin dose analyses for different clinical setups

Photon beam skin dose analyses for different clinical setups

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

Investigation of photon beam models in heterogeneous media of modern radiotherapy

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