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Effects on electron beam penumbra using the photon MLC to reduce bremsstrahlung leakage for an add-on electron MLC
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: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 50, no 6, 1191-1203 p.Article in journal (Refereed) Published
Abstract [en]

Electron IMRT treatments have the potential to reduce the integral dose due to the limited range of the electrons. However, bremsstrahlung produced in the scattering foils could penetrate an added electron MLC (eMLC), thus producing an unmodulated dose contribution that could become unacceptable in electron IMRT treatments. To limit this bremsstrahlung contribution, the photon MLC (xMLC) was used to track the eMLC, but with a margin to avoid penumbra widening through partial screening of the effective electron source. The purpose of this work was to study the effect of the photon-electron MLC tracking on the electron beam penumbra for different treatment head designs. Both isocentric designs and designs where the eMLC is used close to the patient (proximity geometry) have been analysed using Monte Carlo simulations. At 22.5 MeV energy, a tracking margin of 1 cm was enough to avoid penumbra degradation for a helium-filled isocentric geometry, while air-filled geometries (including proximity geometries) require a 2-3 cm margin. Illustrated by an example of a chest wall treatment by electron IMRT, the use of 1 cm tracking margin will reduce the collimator leakage contribution by a factor of 36 as compared to using a static setting of the photon collimator.

Place, publisher, year, edition, pages
2005. Vol. 50, no 6, 1191-1203 p.
Identifiers
URN: urn:nbn:se:umu:diva-4462DOI: 10.1088/0031-9155/50/6/010PubMedID: 15798316OAI: oai:DiVA.org:umu-4462DiVA: diva2:143580
Available from: 2005-04-01 Created: 2005-04-01 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.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 947
Keyword
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
radiofysik
Identifiers
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)
Opponent
Supervisors
Available from: 2005-04-01 Created: 2005-04-01 Last updated: 2012-04-03Bibliographically approved

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Olofsson, LennartKarlsson, MagnusKarlsson, Mikael

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