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Treatment of the Chest Wall
Medical Dosmetr~, Vol. 13, pp. 191-193 Printed in the U.S.A. All rights reserved.
Copyright
TREATMENT
OF THE CHEST
D. V. CORMACK, Tom Baker Cancer
Centre,
PH.D.,
M. LIM,
Calgary, Alberta,
0
1988 American
0739-021 l/88 Association of Medical
$3.00 + .oo Dosimetrists
WALL A.C.T.
Canada
T2N 4N2
Abstract-Irradiation of the chest wall is sometimes given following radical or modified radical mastectomy. The aim of treatment planning in such cases is to deliver a uniform dose to a superficial layer of tissue a few centimeters thick with an acceptably low dose to underlying tissues, particularly the lung. Both tangential photon beams and appositional electron beams have been used for this purpose, the choice between them being determined by the radiation modalities available, the extent and thickness of the designated target volume and the curvature of the patient’s contour in the region. In this paper we will consider a few examples of both types of treatment with emphasis on the use of multiple electron fields. Dose distributions for the following plans were calculated using the system developed at the Cross Cancer Institute in Edmonton. In this system electron dose calculations are based on the Fermi-Evues theorv of multiple Coulomb scattering using the programs developed by Hogstrom and his co-workers. __ Key Words:
Breast, Electron,Tangential,TLD, Chest
PHOTON
BEAMS
face is about 20% lower than the maximum dose which for this beam occurs at a depth of about 2.5 cm. In treating the chest wall it may be desirable to give the maximum dose at or near the surface. This may be achieved using a layer of bolus material. It is important to ensure that the bolus is free from both surface and internal irregularities as these may result in significant non-uniformity of dose. To a good approximation the addition of a layer of tissue-equivalent bolus shifts the isodose curves by the thickness of the bolus. Thus, the isodose curve in tissue with an overlying layer of bolus 1 cm thick is given by the unshaded portion of the curve in Fig. 4. The layer of tissue enclosed by the 90% isodose curve would extend from the surface to a depth of 2 cm and dose would have dropped to less than 10% of the maximum at a depth of 4 cm. Large lesions, particularly those with considerable curvature cannot be treated adequately with single electron fields, and the use of two or more adjacent fields is indicated. With such combinations, however, the problem of achieving an adequately
Figure 1 shows a dose distribution obtained by using opposed wedge fields from a 6oCo unit. Two-thirds of the dose was delivered using 30-degree wedges and the remainder with open fields. For a lesion of this extent (up to about 20 cm around the contour) and curvature (about 90 degrees) this type of plan gives a reasonably uniform dose over a layer of tissue about 3 cm in thickness although the depth of the high isodose contours in the center of the region may be undesirably large. For more extensive lesions some advantage is gained by using high-energy x-ray beams. Figure 2 shows the dose distribution obtained using opposed wedge pairs of IO-MV x-rays for the treatment of a lesion extending about 25 cm with a “wrap-around” angle of some 120 degrees. This distribution gives reasonably uniform coverage of the lesion although as in Fig. 1 the depth of penetration in the center (about 7 cm) is of concern. A plan for an even more extensive lesion is illustrated in Fig. 3 together with the estimated border of the lung. The best distribution achievable with lo-MV x-ray wedge pairs gives a high-dose region extending to nearly 10 cm below the surface and including a substantial amount of lung tissue. The use of even higher energy x-ray beams gives only minor improvement. ELECTRON
BEAMS
Unlike photon beams, electrons have a finite range in a medium which offers potential advantages in treatment of layers of tissue adjacent to the skin. Figure 4 shows the depth dose distribution of a lo-MeV electron beam incident perpendicularly on the surface of a water phantom. The dose at the sur-
/ Fig. 1. Tangential 6oCo. Fields 1 and 2 are open where as Fields 3 and 4 have 30 degree wedges. 191
Medical Dosimetry
192 FIELD
2
lo-MV
x RAYS
FIELD
Volume 13, Number 4, 1988
1
Fig. 2. Tangential lo-MV x-ray fields. Fields 1 and 2 have a weight of 0.35 and Field 3 and 4 have weight of 1.O. Bolus is 1 cm thick “Superflilb.”
uniform dose distribution across the region of the field junctions arises. Figure 5 shows a plan for treating a very large region of the chest wall with four electron fields. To smooth out the distribution in the junctions the anterior dose was divided between two fields, 8 X 18 and 12 X 18, each given half the weight of the oblique fields. As shown in the figure this results in a very uniform dose distribution throughout the target volume including the junctions. As an experimental test of the validity of the calculated dose distributions measurements were carried out during the actual treatment using thermoluminescent dosimetry. In many treatment situa-
tions it is impossible to place the dosimeters at the locations at which one would like to know the dose. However, with irradiation of the chest wall the thermoluminescent dosimeters can be placed at point of interest, the skin surface. Our technique is to mount the small (4 mm by 25 mm) plastic vials containing the LiF powder on a strip of plastic with a spacing of 5 cm. Two of these strips were placed on the transverse contour of the patient’s skin and located at distances of about 4.5 and 13.5 cm from the superior border of the fields. Polaroid photographs of the region to be treated showing the TLD’s were taken before placing the “Superflab” bolus in position. Each of the TLD’s was assigned a number which was written on the plastic strip and could later be used to identify the location lo-
rl
X RAYS
MV
I
I
5 DEPTH
IN
CM
Fig, 4. Depth dose in water for a 10 cm X 10 cm electron beam. The shaded area indicates the effect of a 1 cm layer of unit density bolus.
of the TLD with respect to anatomical features of the patient. Following the treatment the TLD’s were measured on a TLD reader and the dose determined using a calibration factor for lo-MeV electrons. Figure 6 shows the TLD measurements for the treatment given by the combination of fields shown in Fig. 5. It indicates that the uniformity of the dose distribution is very good except for some deficiency in the junction between the left oblique field and the two anterior fields. Examination of the Polaroid photographs revealed that at this level the contour was considerably more irregular than the average contour upon which the plan was based. This irregularity resulted in
n FIELD
1
II-WEDGE
lo-
/’
LUNG I 0
5
1 10 rm
\
Fig. 3. Proposed plan using IO-MV 15” wedge fields for an extensive area of the chest wall.
MeV
ELECTRONS
L 0
5
I 10cm
Fig. 5. Composite dose distribution from the addition of four electron fields. The dotted curve indicates the extent of the target volume to be enclosed within the 90% isodose contour. Daily doses at 1.5 cm below the skin surface were: Field 1 ( 12 X 18): 100 cGy; Field 2 (8 X 18): 100 cGy; Field 3(16X 18):200cGy;Field4(18X 18):2OOcGy.
Treatment of the chest wall 0 D. V. CORMACK
193
with parallel opposed photon beams. Satisfactory plans can be achieved using multiple electon beams using moving field edges to smooth out the dose disof the tribution over the junctions. The uniformity dose distributions can be conveniently checked using thermoluminescent dosimeters.
Fig. 6. Doses in cGy derived from thermoluminescent dosimeters in place during a daily treatment given using the plan shown in Fig. 5.
Acknowledgements-The successful carrying out of complicated treatments such as these require the cooperative efforts of radiation oncologists, physicists, dosimetrists and radiation therapy technologists. We take pleasure in acknowledging the particular contributions of K. E. Breitman, H. R. Boese, S. MacDonald and C. Nechwediuk.
REFERENCES the point in question receiving almost no contribution from the left oblique field. CONCLUSION Regions of the chest wall which extend over a large part of the contour cannot be adequately treated
1. Battista, J.J.; Field, C.; Santon, L.: Bamett, R.; Radiotherapy planning on a VAX-l l/780 computer. Proceedings of the Eighth International Conference of the Use of Computers in Radiation Therapy. 1984:489-492. 2. Hogstrom, K.R.; Mills, M.D.; Almond, P.R.; Electron beam dose calculations. Phys. Med. Biol. 26:445-459. 3. Levitt, S.H.; Perez, C.A.; Breast cancer. In Perez, C.A.; Brady, L.W. editors. Principles and practice of radiation oncology. Philadelphia, 1987:753-754.
Copyright
TREATMENT
OF THE CHEST
D. V. CORMACK, Tom Baker Cancer
Centre,
PH.D.,
M. LIM,
Calgary, Alberta,
0
1988 American
0739-021 l/88 Association of Medical
$3.00 + .oo Dosimetrists
WALL A.C.T.
Canada
T2N 4N2
Abstract-Irradiation of the chest wall is sometimes given following radical or modified radical mastectomy. The aim of treatment planning in such cases is to deliver a uniform dose to a superficial layer of tissue a few centimeters thick with an acceptably low dose to underlying tissues, particularly the lung. Both tangential photon beams and appositional electron beams have been used for this purpose, the choice between them being determined by the radiation modalities available, the extent and thickness of the designated target volume and the curvature of the patient’s contour in the region. In this paper we will consider a few examples of both types of treatment with emphasis on the use of multiple electron fields. Dose distributions for the following plans were calculated using the system developed at the Cross Cancer Institute in Edmonton. In this system electron dose calculations are based on the Fermi-Evues theorv of multiple Coulomb scattering using the programs developed by Hogstrom and his co-workers. __ Key Words:
Breast, Electron,Tangential,TLD, Chest
PHOTON
BEAMS
face is about 20% lower than the maximum dose which for this beam occurs at a depth of about 2.5 cm. In treating the chest wall it may be desirable to give the maximum dose at or near the surface. This may be achieved using a layer of bolus material. It is important to ensure that the bolus is free from both surface and internal irregularities as these may result in significant non-uniformity of dose. To a good approximation the addition of a layer of tissue-equivalent bolus shifts the isodose curves by the thickness of the bolus. Thus, the isodose curve in tissue with an overlying layer of bolus 1 cm thick is given by the unshaded portion of the curve in Fig. 4. The layer of tissue enclosed by the 90% isodose curve would extend from the surface to a depth of 2 cm and dose would have dropped to less than 10% of the maximum at a depth of 4 cm. Large lesions, particularly those with considerable curvature cannot be treated adequately with single electron fields, and the use of two or more adjacent fields is indicated. With such combinations, however, the problem of achieving an adequately
Figure 1 shows a dose distribution obtained by using opposed wedge fields from a 6oCo unit. Two-thirds of the dose was delivered using 30-degree wedges and the remainder with open fields. For a lesion of this extent (up to about 20 cm around the contour) and curvature (about 90 degrees) this type of plan gives a reasonably uniform dose over a layer of tissue about 3 cm in thickness although the depth of the high isodose contours in the center of the region may be undesirably large. For more extensive lesions some advantage is gained by using high-energy x-ray beams. Figure 2 shows the dose distribution obtained using opposed wedge pairs of IO-MV x-rays for the treatment of a lesion extending about 25 cm with a “wrap-around” angle of some 120 degrees. This distribution gives reasonably uniform coverage of the lesion although as in Fig. 1 the depth of penetration in the center (about 7 cm) is of concern. A plan for an even more extensive lesion is illustrated in Fig. 3 together with the estimated border of the lung. The best distribution achievable with lo-MV x-ray wedge pairs gives a high-dose region extending to nearly 10 cm below the surface and including a substantial amount of lung tissue. The use of even higher energy x-ray beams gives only minor improvement. ELECTRON
BEAMS
Unlike photon beams, electrons have a finite range in a medium which offers potential advantages in treatment of layers of tissue adjacent to the skin. Figure 4 shows the depth dose distribution of a lo-MeV electron beam incident perpendicularly on the surface of a water phantom. The dose at the sur-
/ Fig. 1. Tangential 6oCo. Fields 1 and 2 are open where as Fields 3 and 4 have 30 degree wedges. 191
Medical Dosimetry
192 FIELD
2
lo-MV
x RAYS
FIELD
Volume 13, Number 4, 1988
1
Fig. 2. Tangential lo-MV x-ray fields. Fields 1 and 2 have a weight of 0.35 and Field 3 and 4 have weight of 1.O. Bolus is 1 cm thick “Superflilb.”
uniform dose distribution across the region of the field junctions arises. Figure 5 shows a plan for treating a very large region of the chest wall with four electron fields. To smooth out the distribution in the junctions the anterior dose was divided between two fields, 8 X 18 and 12 X 18, each given half the weight of the oblique fields. As shown in the figure this results in a very uniform dose distribution throughout the target volume including the junctions. As an experimental test of the validity of the calculated dose distributions measurements were carried out during the actual treatment using thermoluminescent dosimetry. In many treatment situa-
tions it is impossible to place the dosimeters at the locations at which one would like to know the dose. However, with irradiation of the chest wall the thermoluminescent dosimeters can be placed at point of interest, the skin surface. Our technique is to mount the small (4 mm by 25 mm) plastic vials containing the LiF powder on a strip of plastic with a spacing of 5 cm. Two of these strips were placed on the transverse contour of the patient’s skin and located at distances of about 4.5 and 13.5 cm from the superior border of the fields. Polaroid photographs of the region to be treated showing the TLD’s were taken before placing the “Superflab” bolus in position. Each of the TLD’s was assigned a number which was written on the plastic strip and could later be used to identify the location lo-
rl
X RAYS
MV
I
I
5 DEPTH
IN
CM
Fig, 4. Depth dose in water for a 10 cm X 10 cm electron beam. The shaded area indicates the effect of a 1 cm layer of unit density bolus.
of the TLD with respect to anatomical features of the patient. Following the treatment the TLD’s were measured on a TLD reader and the dose determined using a calibration factor for lo-MeV electrons. Figure 6 shows the TLD measurements for the treatment given by the combination of fields shown in Fig. 5. It indicates that the uniformity of the dose distribution is very good except for some deficiency in the junction between the left oblique field and the two anterior fields. Examination of the Polaroid photographs revealed that at this level the contour was considerably more irregular than the average contour upon which the plan was based. This irregularity resulted in
n FIELD
1
II-WEDGE
lo-
/’
LUNG I 0
5
1 10 rm
\
Fig. 3. Proposed plan using IO-MV 15” wedge fields for an extensive area of the chest wall.
MeV
ELECTRONS
L 0
5
I 10cm
Fig. 5. Composite dose distribution from the addition of four electron fields. The dotted curve indicates the extent of the target volume to be enclosed within the 90% isodose contour. Daily doses at 1.5 cm below the skin surface were: Field 1 ( 12 X 18): 100 cGy; Field 2 (8 X 18): 100 cGy; Field 3(16X 18):200cGy;Field4(18X 18):2OOcGy.
Treatment of the chest wall 0 D. V. CORMACK
193
with parallel opposed photon beams. Satisfactory plans can be achieved using multiple electon beams using moving field edges to smooth out the dose disof the tribution over the junctions. The uniformity dose distributions can be conveniently checked using thermoluminescent dosimeters.
Fig. 6. Doses in cGy derived from thermoluminescent dosimeters in place during a daily treatment given using the plan shown in Fig. 5.
Acknowledgements-The successful carrying out of complicated treatments such as these require the cooperative efforts of radiation oncologists, physicists, dosimetrists and radiation therapy technologists. We take pleasure in acknowledging the particular contributions of K. E. Breitman, H. R. Boese, S. MacDonald and C. Nechwediuk.
REFERENCES the point in question receiving almost no contribution from the left oblique field. CONCLUSION Regions of the chest wall which extend over a large part of the contour cannot be adequately treated
1. Battista, J.J.; Field, C.; Santon, L.: Bamett, R.; Radiotherapy planning on a VAX-l l/780 computer. Proceedings of the Eighth International Conference of the Use of Computers in Radiation Therapy. 1984:489-492. 2. Hogstrom, K.R.; Mills, M.D.; Almond, P.R.; Electron beam dose calculations. Phys. Med. Biol. 26:445-459. 3. Levitt, S.H.; Perez, C.A.; Breast cancer. In Perez, C.A.; Brady, L.W. editors. Principles and practice of radiation oncology. Philadelphia, 1987:753-754.