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Photon and electron collimator effects on electron output and abutting segments in energy modulated electron therapy
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
2005 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 32, no 10, 3178-3184 p.Article in journal (Refereed) Published
Abstract [en]

In energy modulated electron therapy a large fraction of the segments will be arranged as abutting segments where inhomogeneities in segment matching regions must be kept as small as possible. Furthermore, the output variation between different segments should be minimized and must in all cases be well predicted. For electron therapy with add-on collimators, both the electron MLC (eMLC) and the photon MLC (xMLC) contribute to these effects when an xMLC tracking technique is utilized to reduce the x-ray induced leakage. Two add-on electron collimator geometries have been analyzed using Monte Carlo simulations: One isocentric eMLC geometry with an isocentric clearance of 35 cm and air or helium in the treatment head, and one conventional proximity geometry with a clearance of 5 cm and air in the treatment head. The electron fluence output for 22.5 MeV electrons is not significantly affected by the xMLC if the shielding margins are larger than 2-3 cm. For small field sizes and 9.6 MeV electrons, the isocentric design with helium in the treatment head or shielding margins larger than 3 cm is needed to avoid a reduced electron output. Dose inhomogeneity in the matching region of electron segments is, in general, small when collimator positions are adjusted to account for divergence in the field. The effect of xMLC tracking on the electron output can be made negligible while still obtaining a substantially reduced x-ray leakage contribution. Collimator scattering effects do not interfere significantly when abutting beam techniques are properly applied.

Place, publisher, year, edition, pages
2005. Vol. 32, no 10, 3178-3184 p.
URN: urn:nbn:se:umu:diva-14056DOI: 10.1118/1.2044431PubMedID: 16279071OAI: diva2:153727
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2010-08-13Bibliographically approved
In thesis
1. Energy and intensity modulated radiation therapy with electrons
Open this publication in new window or tab >>Energy and intensity modulated radiation therapy with electrons
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In recent years intensity modulated radiation therapy with photons (xIMRT) has gained attention due to its ability to reduce the dose in the tissues close to the tumour volume. However, this technique also results in a large low dose volume. Electron IMRT (eIMRT) has the potential to reduce the integral dose to the patient due to the dose fall off in the electron depth dose curves. This dose fall off makes it possible to modulate the dose distribution in the direction of the beam by selecting appropriate electron energies. The use of a computer based energy selection method was examined in combination with the IMRT technique to optimise the electron dose distribution. It is clearly illustrated that the energy optimisation procedure reduces the dose to lung and heart in a breast cancer treatment.

To shape the multiple electron subfields (beamlets) that are used in eIMRT, an electron multi leaf collimator (eMLC) is needed. However, photons produced in a conventional electron treatment head could penetrate such an added eMLC, thus producing an undesirable dose contribution. The leakage levels normally achieved are acceptable for standard single electron field treatments but could become unacceptably high in eIMRT treatments where a lot of small subfields are combined. To limit this photon contribution, the photon MLC (xMLC) was used to shield off large parts of the photon leakage.

The effect of this xMLC shielding on the reduction of photon leakage, the electron beam penumbras, and electron output (dose level), was studied using Monte Carlo methods for different electron treatment head designs. The use of helium as a mean to reduce the electron scatter in the treatment head, and thus the perturbating effect of the xMLC on electron beam penumbra and output, was also investigated.

This thesis shows that the effect of the xMLC shielding on the electron beam penumbra and output can be made negligible while still obtaining a significantly reduced x-ray leakage dose contribution. The result is a large gain in radiation protection of the patient and a better dynamic range for the eIMRT dose optimisation. For this optimisation a computer based electron energy selection method was developed and tested on two clinical cases.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2005. 48 p.
Umeå University medical dissertations, ISSN 0346-6612 ; 947
Radiation sciences, Radiation therapy, Conformal therapy, IMRT, Electrons, Electron treatment head, Electron MLC, Bremsstrahlung reduction, Integral dose, Penumbra, Output factor, Strålningsvetenskap
National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
urn:nbn:se:umu:diva-491 (URN)91-7305-839-4 (ISBN)
Public defence
2005-04-15, sal 244, by 7, Norrlands universitetssjukhus, Umeå, 13:00 (English)
Available from: 2005-04-01 Created: 2005-04-01 Last updated: 2012-04-03Bibliographically approved

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Olofsson, LennartKarlsson, Magnus GKarlsson, Mikael
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