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2248 Comparison of dosimetric properties of three commercial multileaf collimator systems:
398
I. J. Radiation
Oncology
0 Biology
l Physics
Volume
36, Number
1, Supplement,
1996
2248 COMPARISON
OF DOSIMETRIC
Hover, K.H. I, Hesse, B.M.‘,
Haring,
PROPERTIES OF THREE COMMERCIAL
MULTILEAF
COLLIMATOR
SYSTEMS:
P.‘, Rhein, B.‘, Bannach B.*, Doll, T.‘, Domer K.J.’
‘German Cancer Reserch Center, Department of Medical Physics, D-69120 Heidelberg, Germany ‘University of Dusseldorf, Department of Radiation Therapy, D-40225 Dusseldorf Germany ‘Wellhofer Dosimitrie, D-90592 Schwarzenbtuck, Germany Purpose: The dosimetric properties of different designs of multileaf collimators used for the generation of irregular fields will be measured and compared with each other. Using multileaf collimators is a practical method of achieving conformal therapy. The use for complex conformal treatment fields to be given in either in static or dynamic mode depends much on the leaf end penumbra and the leaf side penumbra as well as the transmission through the leafs. Penumbra and leakage caused by the leaves therefore are of special interest in this intercomparison. Material and Methods: To investigate the dosimetric taken at two different facilities. Until now, comparative
properties of three multileaf collimators of different technical measurements have been performed for the following devices
The new Siemens double focusing MLC with 29 opposite Heidelberg. The energy used was 15 MV and 6 MV.
leaf pairs, installed at the Mevatron
Experimental
design, measurements
in the German
have been
Cancer Research Center,
The Philips quasi-double focusing MIX with 40 opposite leaf pairs, installed at the SL2S in the University Dusseldorf rather than on a circular arc and have rounded ends to reduce penumbra. The energy used was 25 MV and 6 MV.
The leaves move in a plane
The Leibinger non-focusing micro-MLC with 40 opposite leaf pairs. This MLC was specially comparative study is to be continued and extended to involve additional devices in the future.
irradiation
designed for stereotactic
of the brain. The
Both, the film densitometry and a newly designed ten-bit Beam Imaging System BIS-710 developed by Wellhofer company were used. The BIS-710 was developed especially for quantitative dose measuring, whereas most of the existing Portal Imaging Systems are used for image display only. The BIS-710 contains a camera for IO-bit digital data output. The size of each of the 512 x 5 12 detector elements is 0.6 mm x 0.6 mm Results: Measurements taken with the BIS-710 and with film densitometty respectively agree very well. within +/- 2% This is true for the maximum measurable field size of 30.1 x 30.7 cm’. We consider the beam image system BIS-710 to be well suited for real-time verification and quality assurance of all MLC-generated irradiation fields. In the penumbra measurements only small differences between the different collimators have been found. The values ranged from 4.5 mm to 7 mm depending on leaf position and field size. The differences in the transmission measurements were between less than 1.5% and 5%. Conclusion: The data from this comparative study prove that between the three MLC compared up to now there is only a small difference average leaf-end and leaf-side penumbras, Significant differences do, however, exist in the measured transmission of the collimator leaves.
in the
2249 THEORETICAL
ANALYSIS
Shidong Li, Arthur
Boyer, David Findley,
Physics Division,
Department
OF RADIATION
of Radiation
FIELD
PENUMBRA
FROM
A MULTILEAF
COLLIMATOR
and Ed Mok Oncology,
Stanford University
Purpose/Objective: Analysis and measurement of the difference (MLC) leaves that are constructed with curved ends.
School of Medicine,
Stanford,
between the light field and the radiation
CA 94305 field of the multileaf
collimator
Material & Methods: A Varian MLC with curved leaf ends was installed on a Clinac 2300 C/D. The leaves were 6.13 cm deep (dimension in beam direction) and were located 53.9 cm from the x-ray target. The leaf ends had an 8 cm radius of curvature. A relation was derived using three dimensional geometry predicting the location of the light field edge relative to the geometric projection of the tip of the curved leafend. This is a nonlinear relationship because the shadow of the leaf is generated by different points along the leaf end surface as the leaf moves across the field. The theoretical edge of the radiation tluence for a point source was taken to be located along the projection of a chord whose length was 1 Half-Value Thickness (HVT). The chords having projection points across the light field edge were computed using an analytical solution. The radiation transmission through the leaf end was then estimated. The HVT used for tungsten alloy, the leaf material, was 0.87 cm and 0.94 cm for the 6 MV and 15 MV photon beams, respectively. The location of the projection of the 1 HVT chord at a distance of 100 cm from x-ray target was also a nonlinear function of the projection of the leaf tip. Results: The displacement of the light field edge relative to the projection of the leaf tip varies from 0 mm when the leaf tip projects to the central axis, to approximately 3.2 mm for a 20 cm half-field width. The ljght field edge was always displaced into the unblocked area. The displacement of the projection of the I HVT chord relative to the projectton of the. leaf tip varies from 0.3 mm on the central axis to 3.0 mm for a 20 cm half-field width. The projection of 1 HVT chord was deviated from the light tield edge by only 0.3 mm which would be slightly increased to 0.4 mm on decreasing of the leaf end radius to 5 cm. Therefore, the light field edge and the point-source radiation tluence edge are displaced from each other by less than 0.5 mm for all leaf positions. However, calculations for a different MLC geometry, an upper-jaw-replacement MLC without secondary collimation, predict a tripling of the diiplacements. Conclusion: The observed penumbra of 2 mm-4 mm for the MLC leaf end is not due primarily to the shape of the leaf end, but rather is due to other factors such as the extrafocal scatter. This explains the observed similarity of the MLC penumbra to focused collimator jaw penumbra. Coincidence of the light field with the radiation field is theoretically predicted (and experimentally observed) throughout the range of motion of the leaves to be withii 0.5 mm. The calibration of the leaf position must account for the nonlinear relation between the physical location of the leaf (as determined by an encoder) and the true position of the light and radiation field edges. This investigation
was supported
in part by PHS grant number CA43840
awarded by the National
Cancer Institute.
I. J. Radiation
Oncology
0 Biology
l Physics
Volume
36, Number
1, Supplement,
1996
2248 COMPARISON
OF DOSIMETRIC
Hover, K.H. I, Hesse, B.M.‘,
Haring,
PROPERTIES OF THREE COMMERCIAL
MULTILEAF
COLLIMATOR
SYSTEMS:
P.‘, Rhein, B.‘, Bannach B.*, Doll, T.‘, Domer K.J.’
‘German Cancer Reserch Center, Department of Medical Physics, D-69120 Heidelberg, Germany ‘University of Dusseldorf, Department of Radiation Therapy, D-40225 Dusseldorf Germany ‘Wellhofer Dosimitrie, D-90592 Schwarzenbtuck, Germany Purpose: The dosimetric properties of different designs of multileaf collimators used for the generation of irregular fields will be measured and compared with each other. Using multileaf collimators is a practical method of achieving conformal therapy. The use for complex conformal treatment fields to be given in either in static or dynamic mode depends much on the leaf end penumbra and the leaf side penumbra as well as the transmission through the leafs. Penumbra and leakage caused by the leaves therefore are of special interest in this intercomparison. Material and Methods: To investigate the dosimetric taken at two different facilities. Until now, comparative
properties of three multileaf collimators of different technical measurements have been performed for the following devices
The new Siemens double focusing MLC with 29 opposite Heidelberg. The energy used was 15 MV and 6 MV.
leaf pairs, installed at the Mevatron
Experimental
design, measurements
in the German
have been
Cancer Research Center,
The Philips quasi-double focusing MIX with 40 opposite leaf pairs, installed at the SL2S in the University Dusseldorf rather than on a circular arc and have rounded ends to reduce penumbra. The energy used was 25 MV and 6 MV.
The leaves move in a plane
The Leibinger non-focusing micro-MLC with 40 opposite leaf pairs. This MLC was specially comparative study is to be continued and extended to involve additional devices in the future.
irradiation
designed for stereotactic
of the brain. The
Both, the film densitometry and a newly designed ten-bit Beam Imaging System BIS-710 developed by Wellhofer company were used. The BIS-710 was developed especially for quantitative dose measuring, whereas most of the existing Portal Imaging Systems are used for image display only. The BIS-710 contains a camera for IO-bit digital data output. The size of each of the 512 x 5 12 detector elements is 0.6 mm x 0.6 mm Results: Measurements taken with the BIS-710 and with film densitometty respectively agree very well. within +/- 2% This is true for the maximum measurable field size of 30.1 x 30.7 cm’. We consider the beam image system BIS-710 to be well suited for real-time verification and quality assurance of all MLC-generated irradiation fields. In the penumbra measurements only small differences between the different collimators have been found. The values ranged from 4.5 mm to 7 mm depending on leaf position and field size. The differences in the transmission measurements were between less than 1.5% and 5%. Conclusion: The data from this comparative study prove that between the three MLC compared up to now there is only a small difference average leaf-end and leaf-side penumbras, Significant differences do, however, exist in the measured transmission of the collimator leaves.
in the
2249 THEORETICAL
ANALYSIS
Shidong Li, Arthur
Boyer, David Findley,
Physics Division,
Department
OF RADIATION
of Radiation
FIELD
PENUMBRA
FROM
A MULTILEAF
COLLIMATOR
and Ed Mok Oncology,
Stanford University
Purpose/Objective: Analysis and measurement of the difference (MLC) leaves that are constructed with curved ends.
School of Medicine,
Stanford,
between the light field and the radiation
CA 94305 field of the multileaf
collimator
Material & Methods: A Varian MLC with curved leaf ends was installed on a Clinac 2300 C/D. The leaves were 6.13 cm deep (dimension in beam direction) and were located 53.9 cm from the x-ray target. The leaf ends had an 8 cm radius of curvature. A relation was derived using three dimensional geometry predicting the location of the light field edge relative to the geometric projection of the tip of the curved leafend. This is a nonlinear relationship because the shadow of the leaf is generated by different points along the leaf end surface as the leaf moves across the field. The theoretical edge of the radiation tluence for a point source was taken to be located along the projection of a chord whose length was 1 Half-Value Thickness (HVT). The chords having projection points across the light field edge were computed using an analytical solution. The radiation transmission through the leaf end was then estimated. The HVT used for tungsten alloy, the leaf material, was 0.87 cm and 0.94 cm for the 6 MV and 15 MV photon beams, respectively. The location of the projection of the 1 HVT chord at a distance of 100 cm from x-ray target was also a nonlinear function of the projection of the leaf tip. Results: The displacement of the light field edge relative to the projection of the leaf tip varies from 0 mm when the leaf tip projects to the central axis, to approximately 3.2 mm for a 20 cm half-field width. The ljght field edge was always displaced into the unblocked area. The displacement of the projection of the I HVT chord relative to the projectton of the. leaf tip varies from 0.3 mm on the central axis to 3.0 mm for a 20 cm half-field width. The projection of 1 HVT chord was deviated from the light tield edge by only 0.3 mm which would be slightly increased to 0.4 mm on decreasing of the leaf end radius to 5 cm. Therefore, the light field edge and the point-source radiation tluence edge are displaced from each other by less than 0.5 mm for all leaf positions. However, calculations for a different MLC geometry, an upper-jaw-replacement MLC without secondary collimation, predict a tripling of the diiplacements. Conclusion: The observed penumbra of 2 mm-4 mm for the MLC leaf end is not due primarily to the shape of the leaf end, but rather is due to other factors such as the extrafocal scatter. This explains the observed similarity of the MLC penumbra to focused collimator jaw penumbra. Coincidence of the light field with the radiation field is theoretically predicted (and experimentally observed) throughout the range of motion of the leaves to be withii 0.5 mm. The calibration of the leaf position must account for the nonlinear relation between the physical location of the leaf (as determined by an encoder) and the true position of the light and radiation field edges. This investigation
was supported
in part by PHS grant number CA43840
awarded by the National
Cancer Institute.