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Electron beam characteristics at extended source-to-surface distances for irregular cut-outs.

Arunkumar T, Supe SS, Ravikumar M, Sathiyan S, Ganesh M - J Med Phys (2010)

Bottom Line: There is a +7 mm shift in the R(100) depth when compared with regular and irregular field sizes.The symmetry was found to be within limits for all the field sizes as the treatment distance extended as per International Electro technical Commision (IEC) protocol.This suggests that target coverage at extended SSD with irregular cut-outs may be inadequate unless relatively large fields are used.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Hosur Road, Bangalore, India.

ABSTRACT
Electron beam therapy is widely used in the management of cancers. The rapid dose fall-off and the short range of an electron beam enable the treatment of lesions close to the surface, while sparing the underlying tissues. In an extended source-to-surface (SSD) treatment with irregular field sizes defined by cerrobend cutouts, underdosage of the lateral tissue may occur due to reduced beam flatness and uniformity. To study the changes in the beam characteristics, the depth dose, beam profile, and isodose distributions were measured at different SSDs for regular 10 × 10 cm(2) and 15 × 15 cm(2) cone, and for irregular cutouts of field size 6.5 × 9 cm(2) and 11.5 × 15 cm(2) for beam energies ranging from 6 to 20 MeV. The PDD, beam flatness, symmetry and uniformity index were compared. For lower energy (6 MeV), there was no change in the depth of maximum dose (R100) as SSD increased, but for higher energy (20 MeV), the R(100) depth increased from 2 cm to 3 cm as SSD increased. This shows that as SSD increases there is an increase in the depth of the maximum dose for higher energy beams. There is a +7 mm shift in the R(100) depth when compared with regular and irregular field sizes. The symmetry was found to be within limits for all the field sizes as the treatment distance extended as per International Electro technical Commision (IEC) protocol. There was a loss of beam flatness for irregular fields and it was more pronounced for lower energies as compared with higher energies, so that the clinically useful isodose level (80% and 90%) width decreases with increase in SSD. This suggests that target coverage at extended SSD with irregular cut-outs may be inadequate unless relatively large fields are used.

No MeSH data available.


Related in: MedlinePlus

Depth dose curves for 20-MeV electron beam at 100 cm FSD, 108 cm FSD and 115 cm FSD for 11.5 × 15 cm2 field size
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Figure 0004: Depth dose curves for 20-MeV electron beam at 100 cm FSD, 108 cm FSD and 115 cm FSD for 11.5 × 15 cm2 field size

Mentions: Figure 1a–d shows a series of PDD curves obtained using the 6.5 × 9.0 cm2 and 11.5 × 15 cm2 cut-outs for electron beam energies of 6 and 20 MeV. The shapes of the PDD curves are characteristic of clinical electron beams. Each PDD displays a high surface dose, a buildup region, a broad dose maximum, a sharp dose fall-off, and a bremsstrahlung tail. These results are illustrated in the Tables 1a and Table 1b. Based on these datasets, the following conclusions can be made: The depth of dose maximum R 100 for 6 MeV which is 1.4 cm remains constant for regular (10 × 10 cm2 and 15 × 15cm2) as well as for irregular field sizes (6.5 × 9 cm2 and 11.5 × 15 cm2) as the treatment distance increases. For 20 MeV the R 100 depth increased from 2 cm to 3.5 cm as the distance increases. There was a +7 mm shift in the R 100 depth when compared with regular and irregular field sizes as the treatment distance increased. The change in depth dose curve for higher energies was because of large angular scattering of the electron beams. The relative surface dose increases with increase in energy and decreases with increase in SSD, irrespective of field size, by 76.4%–73.9% for 6 MeV and by 92%–88% for 20 MeV. It was noticed that the change in the depth dose curve was minimum and the bremsstrahlung dose component D x = 0.3 for 6 MeV and remains unaltered as the treatment distance increases. But for 20 MeV, the D x increased from 4.6% to 5.4% as the treatment distance increased. These values are in agreement with TG-25.[6] The increase in Dx at larger SSD for 20 MeV electron beam may be because of the lesser absorption of low-energy scattered electrons produced from the cerrobend cutout that contributes to the point of measurement.[7]


Electron beam characteristics at extended source-to-surface distances for irregular cut-outs.

Arunkumar T, Supe SS, Ravikumar M, Sathiyan S, Ganesh M - J Med Phys (2010)

Depth dose curves for 20-MeV electron beam at 100 cm FSD, 108 cm FSD and 115 cm FSD for 11.5 × 15 cm2 field size
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2990115&req=5

Figure 0004: Depth dose curves for 20-MeV electron beam at 100 cm FSD, 108 cm FSD and 115 cm FSD for 11.5 × 15 cm2 field size
Mentions: Figure 1a–d shows a series of PDD curves obtained using the 6.5 × 9.0 cm2 and 11.5 × 15 cm2 cut-outs for electron beam energies of 6 and 20 MeV. The shapes of the PDD curves are characteristic of clinical electron beams. Each PDD displays a high surface dose, a buildup region, a broad dose maximum, a sharp dose fall-off, and a bremsstrahlung tail. These results are illustrated in the Tables 1a and Table 1b. Based on these datasets, the following conclusions can be made: The depth of dose maximum R 100 for 6 MeV which is 1.4 cm remains constant for regular (10 × 10 cm2 and 15 × 15cm2) as well as for irregular field sizes (6.5 × 9 cm2 and 11.5 × 15 cm2) as the treatment distance increases. For 20 MeV the R 100 depth increased from 2 cm to 3.5 cm as the distance increases. There was a +7 mm shift in the R 100 depth when compared with regular and irregular field sizes as the treatment distance increased. The change in depth dose curve for higher energies was because of large angular scattering of the electron beams. The relative surface dose increases with increase in energy and decreases with increase in SSD, irrespective of field size, by 76.4%–73.9% for 6 MeV and by 92%–88% for 20 MeV. It was noticed that the change in the depth dose curve was minimum and the bremsstrahlung dose component D x = 0.3 for 6 MeV and remains unaltered as the treatment distance increases. But for 20 MeV, the D x increased from 4.6% to 5.4% as the treatment distance increased. These values are in agreement with TG-25.[6] The increase in Dx at larger SSD for 20 MeV electron beam may be because of the lesser absorption of low-energy scattered electrons produced from the cerrobend cutout that contributes to the point of measurement.[7]

Bottom Line: There is a +7 mm shift in the R(100) depth when compared with regular and irregular field sizes.The symmetry was found to be within limits for all the field sizes as the treatment distance extended as per International Electro technical Commision (IEC) protocol.This suggests that target coverage at extended SSD with irregular cut-outs may be inadequate unless relatively large fields are used.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Hosur Road, Bangalore, India.

ABSTRACT
Electron beam therapy is widely used in the management of cancers. The rapid dose fall-off and the short range of an electron beam enable the treatment of lesions close to the surface, while sparing the underlying tissues. In an extended source-to-surface (SSD) treatment with irregular field sizes defined by cerrobend cutouts, underdosage of the lateral tissue may occur due to reduced beam flatness and uniformity. To study the changes in the beam characteristics, the depth dose, beam profile, and isodose distributions were measured at different SSDs for regular 10 × 10 cm(2) and 15 × 15 cm(2) cone, and for irregular cutouts of field size 6.5 × 9 cm(2) and 11.5 × 15 cm(2) for beam energies ranging from 6 to 20 MeV. The PDD, beam flatness, symmetry and uniformity index were compared. For lower energy (6 MeV), there was no change in the depth of maximum dose (R100) as SSD increased, but for higher energy (20 MeV), the R(100) depth increased from 2 cm to 3 cm as SSD increased. This shows that as SSD increases there is an increase in the depth of the maximum dose for higher energy beams. There is a +7 mm shift in the R(100) depth when compared with regular and irregular field sizes. The symmetry was found to be within limits for all the field sizes as the treatment distance extended as per International Electro technical Commision (IEC) protocol. There was a loss of beam flatness for irregular fields and it was more pronounced for lower energies as compared with higher energies, so that the clinically useful isodose level (80% and 90%) width decreases with increase in SSD. This suggests that target coverage at extended SSD with irregular cut-outs may be inadequate unless relatively large fields are used.

No MeSH data available.


Related in: MedlinePlus