3h'dtcal I.hmmetO'. Vol. 17. pp. 181-185
0739-0211/92 $6.00 + .00 Copyright @ 1993 American Association of Medical Dosimetrisls
Printed in the U.S.A. All rights reserved.
VARIOUS
WEDGE
ISODOSE
ANGLES
FOR TREATMENT
PLANNING
CHENG B. SAW, P H . D . , TODD PAWLICKI, M.S., a n d ANDREW WU, PH.D. Department of Radiation Oncology, University of Pittsburgh School of Medicine, Joint Radiation Oncology Center and Pittsburgh Cancer Institute, Pittsburgh, PA 15213, U.S.A. Abstract--Various wedge isodose angles or simply wedge angles smaller than the nominal wedge angle were created by combining the isodose distributions generated from a single physical wedge with the isodose distributions of the open field for the 8 - M V photon beam. The particular wedge angle generated depends on the weights imposed on these isodose distributions. The relationship between these weights and the wedge angle were examined and found to be nonlinear. The difference between the wedge angles defined at 10 cm depth and those defined using the 50% isodose curve is less 6 °. The present data was fitted using two proposed empirical equations. Key Words: External beam therapy, Dosimetry, Treatment Planning, Photon beam.
isodose distributions of an open or nonwedged field produce different wedge angles. The particular wedge angle generated depends on the weights imposed on these isodose distributions. This technique of generating different wedge angles can also be extended to those linear accelerators having a limited number of wedge filters. The interest in this paper is to evaluate the relationship between the wedge angle and the weights imposed on the isodose distributions of the wedged and nonwedged fields. In addition, two empirical equations that have been proposed to describe this relationship will be discussed.
INTRODUCTION Wedge filters are routinely used to modify the photon dose distributions. A wedge filter usually made of dense materials like steel, lead, or copper is designed to create differential attenuation and hence progressive change in the intensity across the photon beam width. The design of wedge filters have been described in standard reference books. L2The dose distributions consist of tilted isodose curves with respect to the normal of the photon beam central axis. The degree of isodose curves tilting toward the thin end of the wedge filter depend on the slope of the wedge. The characteristics of a physical wedge filter is specified by the wedge isodose angle. The wedge isodose angle or simple wedge angle has been defined as the angle through which an isodose curve is tilted on the beam axis at some reference depth) At present there is no general agreement to the choice of the reference depth. The International Commission on Radiation Units and Measurements (ICRU) recommended the depth at which the 50% isodose curve intersects the photon beam central axis as the reference depth/However for higher energy photon beam, this choice has been rendered impractical. 3 Recently, the ICRU recommended a single reference depth of 10 cm. 3 A set of wedge filters is usually provided with the linear accelerator. This set consists of wedges whose nominal wedge angles are typically 15° , 30 ° , 45 ° , and 60 ° . The advances in linear accelerator technology features has motivated one of the linear accelerator manufacturers to have varying wedge angles. This feature incorporates a single physical motorized wedge filter into the linear accelerator? The isodose distributions from this wedge filter in combination with the
where D represents the dose distributions. 6 The subscripts o and w indicate the nonwedged field and wedged field, respectively. The values of A and B represent the weights imposed on each isodose distributions. For simplicity, the weights should be normalized as
Reprint requests to: Cheng B. Saw, Ph.D., Department of Radiation Oncology, Presbyterian University Hospital, 230 Lothrop Street, Pittsburgh, PA 15213.
* Philips Medical System. North America Co., Shelton, CT. t Theratronics International Limited, Kanata, Ontario, Canada.
METHODS AND MATERIALS The isodose distributions of a nonwedged field and a wedged field from a Philips SL75-20" linear accelerator 8-MV photon beam are shown in Figs. 1 and 2, respectively. These isodose distributions were normalized to the depth of maximum dose along the photon beam central axis. Different wedge angles were produced by combining these isodose distributions using different weights. Such combination of isodose distributions were implemented using the treatment planning computer system TPI l/Theraplan.t The combination can be written mathematically as D = ADO+ BDw.
181
(1)
182
Medical Dosimetry
I
,
,
,
,
,
~
:
.
.
.
.
weight was varied from 0.0 to 1.0 in steps of 0.2. After the combination, the resultant isodose distributions were normalized to the depth of maximum dose along the photon beam central axis. For each weight selection, the isodose curve that crossed the photon beam central axis at 10 cm depth and the 50% isodose curve were plotted. A normal to the central axis was drawn at the depth where these isodose curves crossed the photon beam central axis. The wedge angle, which is the angle between the isodose curve and the normal, was measured. Figure 3 depicts the location of the wedge angles for the isodose distributions generated using equal weightings.
.
:100~=~ ~"~-90-
~-.
"-J
_80..J
~'70-
J
~ - - - . - . -.60 -
' so-
J J
RESULTS
_,oJ k'-30 ~
j
Fig. I. lsodose distributions of an open field 8-MV photon beam. The isodose distributions were obtained at 100 cm SSD using a 10 cm x 10 cm field size. A + B = I.
Volume17, Number 4, 1992
(2)
By using this constraint, A and B are related. As such, it is sufficient to mention either the value of A or B. The latter choice was adopted in this study. The
The variation of the wedge angle as a function of the weight imposed on the wedged isodose distributions is depicted in Fig. 4 and tabulated in Table 1. Both the wedge angles determined using the 50% isodose curves and the wedge angles determined at 10 cm deep are shown in the figure and the table. In the extreme case where there is no weight indicating an open field isodose distributions, the wedge angle is zero. In the other extreme case where the weight is unity, the wedge angle represents the nominal wedge angle of the wedge filter. The combination of isodose distributions can produce only those wedge angles smaller than the nominal wedge angle. As seen in the Fig. 4, the relationship between the weight and the
?,
.100
6~
72..j
50~ 3O
\ Fig. 2. Isodose distributions of a wedged field 8-MV photon beam. The isodose distributions were obtained at 100 cm SSD using a 10 cm x 10 cm field size.
Fig. 3. Resultant isodose distributions by combining an open field and a wedged field with equal weights. The wedge angle is the angle between the normal and the respective isodose curves.
Wedge isodose angles for treatment planning • C. B. SAW et al.
60 A
"1o
50
v
o.)
40
,
0739-0211/92 $6.00 + .00 Copyright @ 1993 American Association of Medical Dosimetrisls
Printed in the U.S.A. All rights reserved.
VARIOUS
WEDGE
ISODOSE
ANGLES
FOR TREATMENT
PLANNING
CHENG B. SAW, P H . D . , TODD PAWLICKI, M.S., a n d ANDREW WU, PH.D. Department of Radiation Oncology, University of Pittsburgh School of Medicine, Joint Radiation Oncology Center and Pittsburgh Cancer Institute, Pittsburgh, PA 15213, U.S.A. Abstract--Various wedge isodose angles or simply wedge angles smaller than the nominal wedge angle were created by combining the isodose distributions generated from a single physical wedge with the isodose distributions of the open field for the 8 - M V photon beam. The particular wedge angle generated depends on the weights imposed on these isodose distributions. The relationship between these weights and the wedge angle were examined and found to be nonlinear. The difference between the wedge angles defined at 10 cm depth and those defined using the 50% isodose curve is less 6 °. The present data was fitted using two proposed empirical equations. Key Words: External beam therapy, Dosimetry, Treatment Planning, Photon beam.
isodose distributions of an open or nonwedged field produce different wedge angles. The particular wedge angle generated depends on the weights imposed on these isodose distributions. This technique of generating different wedge angles can also be extended to those linear accelerators having a limited number of wedge filters. The interest in this paper is to evaluate the relationship between the wedge angle and the weights imposed on the isodose distributions of the wedged and nonwedged fields. In addition, two empirical equations that have been proposed to describe this relationship will be discussed.
INTRODUCTION Wedge filters are routinely used to modify the photon dose distributions. A wedge filter usually made of dense materials like steel, lead, or copper is designed to create differential attenuation and hence progressive change in the intensity across the photon beam width. The design of wedge filters have been described in standard reference books. L2The dose distributions consist of tilted isodose curves with respect to the normal of the photon beam central axis. The degree of isodose curves tilting toward the thin end of the wedge filter depend on the slope of the wedge. The characteristics of a physical wedge filter is specified by the wedge isodose angle. The wedge isodose angle or simple wedge angle has been defined as the angle through which an isodose curve is tilted on the beam axis at some reference depth) At present there is no general agreement to the choice of the reference depth. The International Commission on Radiation Units and Measurements (ICRU) recommended the depth at which the 50% isodose curve intersects the photon beam central axis as the reference depth/However for higher energy photon beam, this choice has been rendered impractical. 3 Recently, the ICRU recommended a single reference depth of 10 cm. 3 A set of wedge filters is usually provided with the linear accelerator. This set consists of wedges whose nominal wedge angles are typically 15° , 30 ° , 45 ° , and 60 ° . The advances in linear accelerator technology features has motivated one of the linear accelerator manufacturers to have varying wedge angles. This feature incorporates a single physical motorized wedge filter into the linear accelerator? The isodose distributions from this wedge filter in combination with the
where D represents the dose distributions. 6 The subscripts o and w indicate the nonwedged field and wedged field, respectively. The values of A and B represent the weights imposed on each isodose distributions. For simplicity, the weights should be normalized as
Reprint requests to: Cheng B. Saw, Ph.D., Department of Radiation Oncology, Presbyterian University Hospital, 230 Lothrop Street, Pittsburgh, PA 15213.
* Philips Medical System. North America Co., Shelton, CT. t Theratronics International Limited, Kanata, Ontario, Canada.
METHODS AND MATERIALS The isodose distributions of a nonwedged field and a wedged field from a Philips SL75-20" linear accelerator 8-MV photon beam are shown in Figs. 1 and 2, respectively. These isodose distributions were normalized to the depth of maximum dose along the photon beam central axis. Different wedge angles were produced by combining these isodose distributions using different weights. Such combination of isodose distributions were implemented using the treatment planning computer system TPI l/Theraplan.t The combination can be written mathematically as D = ADO+ BDw.
181
(1)
182
Medical Dosimetry
I
,
,
,
,
,
~
:
.
.
.
.
weight was varied from 0.0 to 1.0 in steps of 0.2. After the combination, the resultant isodose distributions were normalized to the depth of maximum dose along the photon beam central axis. For each weight selection, the isodose curve that crossed the photon beam central axis at 10 cm depth and the 50% isodose curve were plotted. A normal to the central axis was drawn at the depth where these isodose curves crossed the photon beam central axis. The wedge angle, which is the angle between the isodose curve and the normal, was measured. Figure 3 depicts the location of the wedge angles for the isodose distributions generated using equal weightings.
.
:100~=~ ~"~-90-
~-.
"-J
_80..J
~'70-
J
~ - - - . - . -.60 -
' so-
J J
RESULTS
_,oJ k'-30 ~
j
Fig. I. lsodose distributions of an open field 8-MV photon beam. The isodose distributions were obtained at 100 cm SSD using a 10 cm x 10 cm field size. A + B = I.
Volume17, Number 4, 1992
(2)
By using this constraint, A and B are related. As such, it is sufficient to mention either the value of A or B. The latter choice was adopted in this study. The
The variation of the wedge angle as a function of the weight imposed on the wedged isodose distributions is depicted in Fig. 4 and tabulated in Table 1. Both the wedge angles determined using the 50% isodose curves and the wedge angles determined at 10 cm deep are shown in the figure and the table. In the extreme case where there is no weight indicating an open field isodose distributions, the wedge angle is zero. In the other extreme case where the weight is unity, the wedge angle represents the nominal wedge angle of the wedge filter. The combination of isodose distributions can produce only those wedge angles smaller than the nominal wedge angle. As seen in the Fig. 4, the relationship between the weight and the
?,
.100
6~
72..j
50~ 3O
\ Fig. 2. Isodose distributions of a wedged field 8-MV photon beam. The isodose distributions were obtained at 100 cm SSD using a 10 cm x 10 cm field size.
Fig. 3. Resultant isodose distributions by combining an open field and a wedged field with equal weights. The wedge angle is the angle between the normal and the respective isodose curves.
Wedge isodose angles for treatment planning • C. B. SAW et al.
60 A
"1o
50
v
o.)
40
,
