High-resolution field shaping utilizing a masked multileaf collimator.

2.50
Hdl Handle:
http://hdl.handle.net/10541/85439
Title:
High-resolution field shaping utilizing a masked multileaf collimator.
Authors:
Williams, Peter C; Cooper, P
Abstract:
Multileaf collimators (MLCs) have become an important tool in the modern radiotherapy department. However, the current limit of resolution (1 cm at isocentre) can be too coarse for acceptable shielding of all fields. A number of mini- and micro-MLCs have been developed, with thinner leaves to achieve approved resolution. Currently however, such devices are limited to modest field sizes and stereotactic applications. This paper proposes a new method of high-resolution beam collimation by use of a tertiary grid collimator situated below the conventional MLC. The width of each slit in the grid is a submultiple of the MLC width. A composite shaped field is thus built up from a series of subfields, with the main MLC defining the length of each strip within each subfield. Presented here are initial findings using a prototype device. The beam uniformity achievable with such a device was examined by measuring transmission profiles through the grid using a diode. Profiles thus measured were then copied and superposed to generate composite beams, from which the uniformity achievable could be assessed. With the average dose across the profile normalized to 100%, hot spots up to 5.0% and troughs of 3% were identified for a composite beam of 2 x 5.0 mm grids, as measured at Dmax for a 6 MV beam. For a beam composed from 4 x 2.5 mm grids, the maximum across the profile was 3.0% above the average, and the minimum 2.5% below. Actual composite profiles were also formed using the integrating properties of film, with the subfield indexing performed using an engineering positioning stage. The beam uniformity for these fields compared well with that achieved in theory using the diode measurements. Finally sine wave patterns were generated to demonstrate the potential improvements in field shaping and conformity using this device as opposed to the conventional MLC alone. The scalloping effect on the field edge commonly seen on MLC fields was appreciably reduced by use of 2 x 5.0 mm grids, and still further by the use of 4 x 2.5 mm grids, as would be expected. This was also achieved with a small or negligible broadening of the beam penumbra as measured at Dmax.
Affiliation:
North Western Medical Physics, Christie Hospital NHS Trust, Manchester, UK.
Citation:
High-resolution field shaping utilizing a masked multileaf collimator. 2000, 45 (8):2313-29 Phys Med Biol
Journal:
Physics in Medicine and Biology
Issue Date:
Aug-2000
URI:
http://hdl.handle.net/10541/85439
PubMed ID:
10958196
Type:
Article
Language:
en
ISSN:
0031-9155
Appears in Collections:
All Christie Publications

Full metadata record

DC FieldValue Language
dc.contributor.authorWilliams, Peter Cen
dc.contributor.authorCooper, Pen
dc.date.accessioned2009-11-05T15:59:08Z-
dc.date.available2009-11-05T15:59:08Z-
dc.date.issued2000-08-
dc.identifier.citationHigh-resolution field shaping utilizing a masked multileaf collimator. 2000, 45 (8):2313-29 Phys Med Biolen
dc.identifier.issn0031-9155-
dc.identifier.pmid10958196-
dc.identifier.urihttp://hdl.handle.net/10541/85439-
dc.description.abstractMultileaf collimators (MLCs) have become an important tool in the modern radiotherapy department. However, the current limit of resolution (1 cm at isocentre) can be too coarse for acceptable shielding of all fields. A number of mini- and micro-MLCs have been developed, with thinner leaves to achieve approved resolution. Currently however, such devices are limited to modest field sizes and stereotactic applications. This paper proposes a new method of high-resolution beam collimation by use of a tertiary grid collimator situated below the conventional MLC. The width of each slit in the grid is a submultiple of the MLC width. A composite shaped field is thus built up from a series of subfields, with the main MLC defining the length of each strip within each subfield. Presented here are initial findings using a prototype device. The beam uniformity achievable with such a device was examined by measuring transmission profiles through the grid using a diode. Profiles thus measured were then copied and superposed to generate composite beams, from which the uniformity achievable could be assessed. With the average dose across the profile normalized to 100%, hot spots up to 5.0% and troughs of 3% were identified for a composite beam of 2 x 5.0 mm grids, as measured at Dmax for a 6 MV beam. For a beam composed from 4 x 2.5 mm grids, the maximum across the profile was 3.0% above the average, and the minimum 2.5% below. Actual composite profiles were also formed using the integrating properties of film, with the subfield indexing performed using an engineering positioning stage. The beam uniformity for these fields compared well with that achieved in theory using the diode measurements. Finally sine wave patterns were generated to demonstrate the potential improvements in field shaping and conformity using this device as opposed to the conventional MLC alone. The scalloping effect on the field edge commonly seen on MLC fields was appreciably reduced by use of 2 x 5.0 mm grids, and still further by the use of 4 x 2.5 mm grids, as would be expected. This was also achieved with a small or negligible broadening of the beam penumbra as measured at Dmax.en
dc.language.isoenen
dc.subject.meshBiophysical Phenomena-
dc.subject.meshBiophysics-
dc.subject.meshModels, Statistical-
dc.subject.meshParticle Accelerators-
dc.subject.meshRadiation Dosage-
dc.subject.meshRadiotherapy, Conformal-
dc.subject.meshReproducibility of Results-
dc.subject.meshTechnology, Radiologic-
dc.titleHigh-resolution field shaping utilizing a masked multileaf collimator.en
dc.typeArticleen
dc.contributor.departmentNorth Western Medical Physics, Christie Hospital NHS Trust, Manchester, UK.en
dc.identifier.journalPhysics in Medicine and Biologyen

Related articles on PubMed

All Items in Christie are protected by copyright, with all rights reserved, unless otherwise indicated.