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Energy and intensity modulated radiation therapy with electrons
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
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 [en]
Radiation sciences, Radiation therapy, Conformal therapy, IMRT, Electrons, Electron treatment head, Electron MLC, Bremsstrahlung reduction, Integral dose, Penumbra, Output factor
Keyword [sv]
Strålningsvetenskap
National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
radiofysik
Identifiers
URN: urn:nbn:se:umu:diva-491ISBN: 91-7305-839-4 (print)OAI: oai:DiVA.org:umu-491DiVA: diva2:143583
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
List of papers
1. Intensity modulated radiation therapy with electrons using algorithm based energy/range selection methods.
Open this publication in new window or tab >>Intensity modulated radiation therapy with electrons using algorithm based energy/range selection methods.
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2004 (English)In: Radiotherapy and Oncology, ISSN 0167-8140, Vol. 73, no 2, 223-231 p.Article in journal (Refereed) Published
Abstract [en]

BACKGROUND AND PURPOSE: In recent years photon intensity modulated radiation therapy (IMRT) has gained attention due to its ability to improve conformity of dose distributions. A potential advantage of electron-IMRT is that the dose fall off in the depth dose curve makes it possible to modulate the dose distribution in the direction of the beam by selecting different electron energies. This paper examines the use of a computer based energy selection in combination with the IMRT technique to optimise the electron dose distribution. MATERIALS AND METHODS: One centimetre square electron beamlets ranging from 2.5 to 50 MeV were pre-calculated in water using Monte Carlo methods. A modified IMRT optimisation tool was then used to find an optimum mix of electron energies and intensities. The main principles used are illustrated in some simple geometries and tested on two clinical cases of post-operated ca. mam. RESULTS: It is clearly illustrated that the energy optimisation procedure lowers the dose to lung and heart and makes the dose in the target more homogeneous. Increasing the energy at steep gradients compensates for lack of target coverage at beam edges and steep gradients. Comparison with a clinically acceptable four segment plan indicates the advantage of the used electron IMRT technique. CONCLUSIONS: Using an intensity optimised mix of computer selected electron energies has the potential to improve electron treatments for mastectomy patients with good target coverage and reduced dose to normal tissue such as lung and heart.

Keyword
Radiotherapy, conformal therapy, IMRT, electrons
Identifiers
urn:nbn:se:umu:diva-14818 (URN)10.1016/j.radonc.2004.08.020 (DOI)15542170 (PubMedID)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2009-11-25Bibliographically approved
2. Effects on electron beam penumbra using the photon MLC to reduce bremsstrahlung leakage for an add-on electron MLC
Open this publication in new window or tab >>Effects on electron beam penumbra using the photon MLC to reduce bremsstrahlung leakage for an add-on electron MLC
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.

Identifiers
urn:nbn:se:umu:diva-4462 (URN)10.1088/0031-9155/50/6/010 (DOI)15798316 (PubMedID)
Available from: 2005-04-01 Created: 2005-04-01 Last updated: 2010-08-13Bibliographically approved
3. Photon and electron collimator effects on electron output and abutting segments in energy modulated electron therapy
Open this publication in new window or tab >>Photon and electron collimator effects on electron output and abutting segments in energy modulated electron therapy
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.

Identifiers
urn:nbn:se:umu:diva-14056 (URN)10.1118/1.2044431 (DOI)16279071 (PubMedID)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2010-08-13Bibliographically approved
4. Reduction of the unmodulated dose in energy modulated radiation therapy with electrons
Open this publication in new window or tab >>Reduction of the unmodulated dose in energy modulated radiation therapy with electrons
Article in journal (Refereed) Submitted
Identifiers
urn:nbn:se:umu:diva-4464 (URN)
Available from: 2005-04-01 Created: 2005-04-01Bibliographically approved

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