Technical Notes
TECHNICAL NOTES Steel Trimmer for Use with 60CO Tangential Fields 1 Irving A. Lerch, Ph.D., and Joseph Newall, M.D. A prototype steel trimmer provided by Atomic Energy of Canada Limited was modified for use with tangential wedged 60CO fields. The trimmer virtually eliminated the penumbra of the wedged fields, permitting a more uniform irradiation of the chest wall. There was no significant change in the wedge transmission factors for the trimmed and untrimmed fields.
( S)
INDEX TERM: Therapeutic radiology, apparatus and equipment Radiology 125:246-247, October 1977
90
The standard technique for the treatment of the chest wall at the New York University Medical Center is with opposed tangential wedged 60C0 fields. Compensation for body curvature with wedges (Fig. 1, B) as opposed to the use of the uncompensated tangential breast cone (Fig. 1, A) permits a more uniform irradiation of the target volume and a reduction of the hot spot at the surface of the skin. A surface bolus is used on alternate days of treatment to prevent excessive skin dose. However, the source size of the Theratron 780 60CO unit (2 cm) introduces a penumbra which reduces the uniformity of the field at the posterior edge of the treatment volume. We have elected to use a modified stainless steel trimmer plate, initially provided by Atomic Energy of Canada, Ltd. (AECL)for prototype testing, to trim the 60CO field (Fig. 1, C).
10
(C) 90
10
Fig. 1. Artronix PC-12studiesdemonstratethe dose distributions achievedin the irradiated volumeusing the uncompensated tangential breastcone, A, the untrimmed, opposed 60 0 wedged fields, B, andthe trimmed,opposed 60 0 wedged fields, C. All fields are 5 X 15 cm at an SSD of 80 cm.
MATERIALS AND METHODS
The AECLsteel trimmer, as originally provided, was unsuitable for our use owing to the lack of a provision for accommodating wedges. The trimmer system consisted of a steel plate designed to attach to a collimator leaf, a source-surface-distance (SSD) indicator scale with a magnet for affixing to the trimmer and a caliper for determining the edge of the exit field. The system was modified as follows (Figs. 2, A and B): the trimmer plate was split to provide a slot into which the edge of the wedge would fit, the trimmer mounting block was repositioned to permit mounting on an interior collimator leaf, the projection was cut off the trimmer plate to prevent partial clipping of the trimmed margin of the field (because of the finite source size), and the wedge mounting plates were cut out to accommodate the trimer plate (Fig. 2, B).
TABLE I: AECL WEDGE DESIGNATION G22-
151C 152C 153C 1518 1528 1538 151A 152A 153A
WEDGE TRANSMISSION FACTORS (WTF) FOR TRIMMED ANDUNTRIMMED FIELDS
FIELD SIZE· 10 10 10 8 8 8 6 6 6
X X X X X X X X X
The mounting of the trimmer plate on an interior collimator leaf resulted in the uniform masking of 7 mm from the margin of the geometric field at an SSD of 80 em. Dosimetric studies were made to determine the effects of trimming on the wedge transmission factor, dose distributions and build-up factors. Kodak RP/V2 X-Ornat processable therapy verification film was calibrated with 60CO reference fields and scanned with a microdensitometer employing a 10 #Lm X 1 mm light slit. Film studies were used to investigate the degree of trimming of the wedged fields at the surface and at build-up depth in a Lucite phantom. Build-up was estimated with film and ionometry. Ionization measurements were employed to determine the wedge transmission factors. A 0.6-cc Capintec ionization
10 10 10 10 10 10 10 10 10
WEDGE ANGLEt (DEGREES)
NOMINAL WTF
MEASURED WTF
TRIMMED WTF
30 45 60 30 45 60 30 45 60
0.71 0.58 0.42 0.74 0.62 0.47 0.79 0.68 0.55
0.714 0.581 0.410 0.739 0.608 0.450 0.786 0.654 0.524
0.713 0.580 0.414 0.611 0.457 0.793 0.656 0.530
• The first dimension is across the wedge. t The wedge angle is defined as the angle which the 50% isodose line makes with the normal to the central ray of the unwedged field. The wedge transmission factor is the ratio of the dose rate at build-up in the center of the wedged field to the dose rate at build-up in the center of the unwedged field.
246
Technical Notes
247
TECHNICAL NOTES
Vol. 125
16
60° WEDGE (A) I
14
1
,
I 12
U> ...... Z
:::J
I I
,
10
I 8
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TECHNICAL NOTES Steel Trimmer for Use with 60CO Tangential Fields 1 Irving A. Lerch, Ph.D., and Joseph Newall, M.D. A prototype steel trimmer provided by Atomic Energy of Canada Limited was modified for use with tangential wedged 60CO fields. The trimmer virtually eliminated the penumbra of the wedged fields, permitting a more uniform irradiation of the chest wall. There was no significant change in the wedge transmission factors for the trimmed and untrimmed fields.
( S)
INDEX TERM: Therapeutic radiology, apparatus and equipment Radiology 125:246-247, October 1977
90
The standard technique for the treatment of the chest wall at the New York University Medical Center is with opposed tangential wedged 60C0 fields. Compensation for body curvature with wedges (Fig. 1, B) as opposed to the use of the uncompensated tangential breast cone (Fig. 1, A) permits a more uniform irradiation of the target volume and a reduction of the hot spot at the surface of the skin. A surface bolus is used on alternate days of treatment to prevent excessive skin dose. However, the source size of the Theratron 780 60CO unit (2 cm) introduces a penumbra which reduces the uniformity of the field at the posterior edge of the treatment volume. We have elected to use a modified stainless steel trimmer plate, initially provided by Atomic Energy of Canada, Ltd. (AECL)for prototype testing, to trim the 60CO field (Fig. 1, C).
10
(C) 90
10
Fig. 1. Artronix PC-12studiesdemonstratethe dose distributions achievedin the irradiated volumeusing the uncompensated tangential breastcone, A, the untrimmed, opposed 60 0 wedged fields, B, andthe trimmed,opposed 60 0 wedged fields, C. All fields are 5 X 15 cm at an SSD of 80 cm.
MATERIALS AND METHODS
The AECLsteel trimmer, as originally provided, was unsuitable for our use owing to the lack of a provision for accommodating wedges. The trimmer system consisted of a steel plate designed to attach to a collimator leaf, a source-surface-distance (SSD) indicator scale with a magnet for affixing to the trimmer and a caliper for determining the edge of the exit field. The system was modified as follows (Figs. 2, A and B): the trimmer plate was split to provide a slot into which the edge of the wedge would fit, the trimmer mounting block was repositioned to permit mounting on an interior collimator leaf, the projection was cut off the trimmer plate to prevent partial clipping of the trimmed margin of the field (because of the finite source size), and the wedge mounting plates were cut out to accommodate the trimer plate (Fig. 2, B).
TABLE I: AECL WEDGE DESIGNATION G22-
151C 152C 153C 1518 1528 1538 151A 152A 153A
WEDGE TRANSMISSION FACTORS (WTF) FOR TRIMMED ANDUNTRIMMED FIELDS
FIELD SIZE· 10 10 10 8 8 8 6 6 6
X X X X X X X X X
The mounting of the trimmer plate on an interior collimator leaf resulted in the uniform masking of 7 mm from the margin of the geometric field at an SSD of 80 em. Dosimetric studies were made to determine the effects of trimming on the wedge transmission factor, dose distributions and build-up factors. Kodak RP/V2 X-Ornat processable therapy verification film was calibrated with 60CO reference fields and scanned with a microdensitometer employing a 10 #Lm X 1 mm light slit. Film studies were used to investigate the degree of trimming of the wedged fields at the surface and at build-up depth in a Lucite phantom. Build-up was estimated with film and ionometry. Ionization measurements were employed to determine the wedge transmission factors. A 0.6-cc Capintec ionization
10 10 10 10 10 10 10 10 10
WEDGE ANGLEt (DEGREES)
NOMINAL WTF
MEASURED WTF
TRIMMED WTF
30 45 60 30 45 60 30 45 60
0.71 0.58 0.42 0.74 0.62 0.47 0.79 0.68 0.55
0.714 0.581 0.410 0.739 0.608 0.450 0.786 0.654 0.524
0.713 0.580 0.414 0.611 0.457 0.793 0.656 0.530
• The first dimension is across the wedge. t The wedge angle is defined as the angle which the 50% isodose line makes with the normal to the central ray of the unwedged field. The wedge transmission factor is the ratio of the dose rate at build-up in the center of the wedged field to the dose rate at build-up in the center of the unwedged field.
246
Technical Notes
247
TECHNICAL NOTES
Vol. 125
16
60° WEDGE (A) I
14
1
,
I 12
U> ...... Z
:::J
I I
,
10
I 8
I
r
ll::
