- Email: [email protected]
A Simple Method for Reducing Ovarian Dose During Megavoltage Irradiation of the Breast
0739-021 l/89 $3.00 t .OO Copyright 0 1989 American Assoctation of Medical Dosimetrists
Medmi Dovime!rv. Vol. 14. pp. 269-272 Pnnted in the U.S.A. 411nghts reserved.
A SIMPLE METHOD FOR REDUCING OVARIAN DOSE DURING MEGAVOLTAGE IRRADIATION OF THE BREAST EDWARD C. PENNINGTON,M.S.,JOHN STAPLES,M.B.,B.S.,D.M.R.T., and SHIRISHK.JANI,PH.D. Department of Radiology, Division of Radiation Oncology, The University of Iowa College of Medicine, Iowa City, IA 52242, U.S.A. Abstract-Breast cancer in its early stages can be effectively treated with conservative surgery and irradiation. In young women who wish to subsequently bear children, reduction of ovarian dose during irradiation could be of great emotional significance. We describe a simple, convenient, cost-effective method by which ovarian dose was reduced from 18 cGy to 8 cGyduring tangential irradiation of the intact breast with 6 MV photons. Key Words: Breast, radiotherapy, ovary, protection.
INTRODUCTION
ployed during treatment. In order to address her anxieties and consider accommodation of her wishes, we had to estimate her ovarian dose by
Breast self-examination and mammography will eventually result in the detection of breast cancer in its early stages in an increasing proportion of patients. One consequence of this trend is that larger numbers of young women will be treated with breast conserving surgery and radiation therapy.lm3 These patients are likely to take an active interest in their management and may well be concerned about scattered radiation to other vital structures such as the ovaries, during their course of radiation therapy. We present the case of a woman who refused to consider irradiation to her right breast following lumpectomy unless every effort was made to minimize the dose to her ovaries. A simple, effective, and convenient method by which the ovarian dose was approximately halved is described.
1. extrapolation from the existing published data, and 2. by performing in-phantom measurements replicating treatment conditions as closely as possible. MATERIAL
AND METHODS
The patient was treated in the supine position, her right arm abducted at 90” and supported by an arm board. Treatment was to the whole right breast only in view of her uninvolved axilla. 6 MV X-rays from a Varian Clinac 2500 linear accelerator were used throughout treatment, which employed opposed tangential beams. The medial edge of the treatment field was the midsternal line, the lateral edge was at the midaxillary line, the superior margin was located at the second costochondral junction, and the inferior margin was 2 cm below the inferior limits of palpable breast tissue. 200 cGy daily fractions were employed, treating 5 days a week (Monday to Friday). Twentyfive such fractions were delivered, resulting in a TD of 5000 cGy. Our first step was to evaluate the dose expected to be received by the ovaries for the required fields. The ovaries were about 32 cm inferior to the central axes of the tangential fields. For calculation and measurement purposes, the location of the ovaries was taken at the midline of the patient, this being 8 cm from the anterior and 12.5 cm from the lateral skin surface. A schematic representation of the treatment setup is shown in Figs. 1 and 2. The dose to the ovaries comes from four sources:
Case history A 30-year old divorced female with no children presented for radiation therapy following wide excision and axillary dissection of a Stage I (AJC) adenocarcinoma of the right breast. She had refused adjuvant chemotherapy on the National Surgical Adjuvant Breast Project B-13 protocol on the basis of concerns regarding ovarian function. She also stated emphatically that she would only accept postoperative radiation to her breast if every effort was made to reduce the dose to her ovaries. Many studies have been published describing the dose peripheral to the primary radiation beam.4-8 However, most published data had limited applicability to our patient for the particular geometry em-
Reprint requests to: Dr. John Staples, Division of Radiation Oncology, The University of Iowa College of Medicine, Iowa City, IA 52242.
1. leakage, 2. collimator scatter,
269
210
Medical Dosimetry
Volume 14, Number 4, 1989
Scatter from the wedge would contribute to the dose’, but was not taken into account in these estimates. Wedge attenuation was taken into account in the irregular field scatter calculation since the wedge attenuation results in a longer treatment time (higher monitor units delivered), leading to an increase in leakage radiation impinging on the patient. The treatment geometry pointed to a rather simple method to reduce the dose to the ovary by placing a block on the standard treatment tray outside the primary beam. This block potentially reduces the radiation from three of the four sources, namely the leakage, collimator scatter, and wedge scatter. To verify this, measurements were made in a phantom duplicating the treatment geometry. A 0.6 cc ionization chamber was placed at the midline of the phantom at the same off axis distance as the ovaries. The charge produced by the ionization chamber was collected on a precision electrometer. The breast volume was mimicked using bolus material. Measurements were made with and without an additional block on the tray.
TRAY
I-24
cm-
PHANTOM
\
RESULTS 25 cm
I
I
Q OVARY
I
I
I
Fig. 1. Schematic diagram of lateral tangential treatment field for phantom measurements. The 6 MV X-ray field was 16 X 7.5 cm at 100 SSD. For clarity in identification, the wedge has been drawn 90” from its actual orientation. Measurements were taken at midline (8 cm depth, 12.5 cm laterally). The scatter reduction block, supported on the accessory tray, intercepts photons from leakage (a), collimator scatter (b), wedge scatter (c). Photons scattered from within the patient (d) cannot be shielded. The scatter block lies outside the primary beam and therefore generates no additional scatter.
Table 1 lists the results of the phantom measurements. At 32 cm off the central ray, the ovarian dose was 0.15% of the central axis dmax dose for nonwedged fields with no additional block. Placing a block just outside the primary beam between the collimators and the ovaries blocked the scatter radiation from the collimators and reduced the ovarian dose by a factor of 2. Insertion of a 30” wedge created more scatter and resulted in a doubling of the ovarian dose compared to non-wedged fields. For a tumor dose of 5000 cGy, our patient received 8 cGy to her ovaries. Without the scatter block, the dose would have been 18 cGy. DISCUSSION
3. wedge scatter, and 4. scatter from within the patient. An estimate of the dose based on the geometry of Fig. 1 was obtained by extrapolation of the published data. This yielded a dose to the ovary in the range of 0.5 to 0.6 percent of the central axis dmax dose.4,6 For a tumor dose of 5000 cGy this resulted in an estimated ovarian dose of 30 to 40 cGy. Sharma8 has shown that using the Clarkson method of computing scatter is effective in estimating the dose outside of the primary beam. Such a calculation was performed on a commercial treatment planning system (Theraplan L, V4AB, Theratronics International, LTD) using the distances shown. Adding 0.1% to account for leakage from the accelerator, a value of 43 cGy was obtained for the ovarian dose during the total treatment course.
Our measured dose was considerably less than the predicted dose. The difference was most likely due to the fact that the predicted dose assumed fields under full scatter conditions, whereas the values measured included scatter from the irradiated breast tissue only (Fig. 2). In addition, the 0.1% leakage value used in the irregular field estimation was probably higher than the true leakage. We anticipate that the efforts of this particular patient to minimize the dose to vital organs will be seen in other patients as well, particularly since increased public awareness on the value of breast selfexamination and mammography, combined with increasing demand by women for breast preserving treatment, will result in increased use of conservative surgery and irradiation of early stage cases. Young patients such as ours, who have not yet completed
Reducing ovarian dose during megavoltage irradiation of the breast 0 E. C. PENNINGTONet al.
0
’
.
'
0
0
0 .
0 0 0
0 0 0 .
/-
/ 0
/
'
'
'
d'
'
'
/
.
'
'
'
'
/
0
' Measurement paint 04
0 '
0 ' 0
0
/ /
./
beams ,,,"
.
0
'zRH
Tangential
'
/ /
.
.
/
.
. /
271
'32 cm off axis
. '
PHANTOM
0
0 0 , .
0
Fig. 2. Cross sectional view of patient/phantom setup. Breast tissue was mimicked with bolus material. Note that scatter from the irradiated area originates from a small volume of tissue. Wedges and blocks are not shown in this view. The ovary is not in the same plane as the field; it is 32 cm inferior to the central axes of the beams.
their families, are likely to be concerned about the long term effects of irradiation. Two such long term effects include mutations and ovarian failure. The doubling dose for mutations in humans was estimated by the Biological Effects of the Ionizing Radiations (BEIR) III committee of the National Academy of Sciences, to be between 0.5 and 2.5 Sv (50-200 rems). In 1986, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) suggested a single figure of 100 cGy for the doubling dose, which was a calculated rather than measured quantity for humans, based on measured data in mice. The radiation dose to induce ovarian failure is age dependent, higher doses being needed for younger women. Permanent cessation of menstruation in women less than 40 years of age requires a dose in the order of 500 cG~.~ Despite our
Table 1. Results of ovarian dose measurement tangential (cGy) breast fields with 6 MV X-rays. Condition Non-wedged fields no scatter block Non-wedged fields with scatter wedge 30” wedged fields no scatter block 30” wedged fields with scatter block
%Iof Dmax dose
Total ovarian dose for 5000 cGy TD
0.15%
9
0.08%
5
0.30%
18
0.14%
8
reassurance that irradiation of her breast would result in an ovarian dose substantially lower than these estimates, our patient insisted that the dose should be as low as could be reasonably achieved if she was to consent to irradiation. The scatter block as described above definitely reduced the dose to her ovaries, satisfying her request without compromise of treatment quality, convenience, or cost. Obviously, the dose to the ovaries cannot be eliminated. The dose to the ovaries under an unmodified treatment scheme is certainly less than any dose observed to cause detrimental effects; consequently, we do not advocate routine use of this technique. However, in our patient we reduced her anxiety level considerably by making a very simple, yet effective, modification to the treatment setup. This technique could also be of value in reducing fetal dose to acceptable limits in patients requiring breast irradiation during early pregnancy. We would like to emphasize that the doses reported here are very specific to this particular patient. Other patients will vary greatly in anatomical dimensions, and could need different field sites and different field arrangements. In addition, these measurements are machine specific, particularly for leakage, collimator scatter, and wedge design, which will vary with accelerator design. Therefore, the effectiveness of a scatter reducing block as described here will vary depending on the particular patient being treated. Only phantom measurements under close duplica-
Medical Dosimetry
272
tion of the patient’s geometry will yield a precise ovarian dose. We offer this idea of using a scatter reducing block to other clinicians who may eventually be faced with a similar dilemma of treating a patient’s concerns as well as her cancer.
Acknowledgement-The authors wish to thank Michelle Behnke for her time, effort, and patience on this project.
REFERENCES 1. Chamberlain, J. An insurance policy to reduce the risk ofdying from breast cancer. Clin. Radiology 40: 1-3; 1989. 2. Hellman, S.; Harris, J.R. Breast cancer: Considerations in local and regional treatment. Radiology 164:593-598; 1986.
Volume 14, Number 4, 1989 3. Tobias, J.S. Radiotherapy and breast conservation. Br. J. Radiology59~653-666; 1986. 4. Fraass, B.A.; van de Geijn, J. Peripheral dose from megavolt beams. Med. Phys. 10(6):809-818; 1983. 5. Francois, P.; Beurtheret, C.; Dutriex, A. Calculation of the dose delivered to organs outside the radiation beams. Med. Phys. 15(6):879-883; 1988. 6. Keller, B.; Mathewson, C.; Rubin, P. Scattered radiation dosage as a function of X-my energy. Radiology 111:447-449; 1974. 7. Scrimger, J.; Kolitsi, Z. Scattered radiation from beam modifiers used with megavoltage therapy units. Radiology 130:233236; 1979. 8. Sharma, S.C.; Williamson, J.F.; Khan, F.M.; Lee, C.K.K. Measurement and calculation of ovary and fetus dose in extended field radiotherapy for 10 MV X-rays. ht. J. Radiation Oncolo., Biol., Phys. 7~843-846; 1981. 9. Peck, W.S.; McGreer, J.T.; Kretschmar, N.R.; Brown, W.E. Castration of the female by irradiation. Radiology 34: 176- 187; 1940.
Medmi Dovime!rv. Vol. 14. pp. 269-272 Pnnted in the U.S.A. 411nghts reserved.
A SIMPLE METHOD FOR REDUCING OVARIAN DOSE DURING MEGAVOLTAGE IRRADIATION OF THE BREAST EDWARD C. PENNINGTON,M.S.,JOHN STAPLES,M.B.,B.S.,D.M.R.T., and SHIRISHK.JANI,PH.D. Department of Radiology, Division of Radiation Oncology, The University of Iowa College of Medicine, Iowa City, IA 52242, U.S.A. Abstract-Breast cancer in its early stages can be effectively treated with conservative surgery and irradiation. In young women who wish to subsequently bear children, reduction of ovarian dose during irradiation could be of great emotional significance. We describe a simple, convenient, cost-effective method by which ovarian dose was reduced from 18 cGy to 8 cGyduring tangential irradiation of the intact breast with 6 MV photons. Key Words: Breast, radiotherapy, ovary, protection.
INTRODUCTION
ployed during treatment. In order to address her anxieties and consider accommodation of her wishes, we had to estimate her ovarian dose by
Breast self-examination and mammography will eventually result in the detection of breast cancer in its early stages in an increasing proportion of patients. One consequence of this trend is that larger numbers of young women will be treated with breast conserving surgery and radiation therapy.lm3 These patients are likely to take an active interest in their management and may well be concerned about scattered radiation to other vital structures such as the ovaries, during their course of radiation therapy. We present the case of a woman who refused to consider irradiation to her right breast following lumpectomy unless every effort was made to minimize the dose to her ovaries. A simple, effective, and convenient method by which the ovarian dose was approximately halved is described.
1. extrapolation from the existing published data, and 2. by performing in-phantom measurements replicating treatment conditions as closely as possible. MATERIAL
AND METHODS
The patient was treated in the supine position, her right arm abducted at 90” and supported by an arm board. Treatment was to the whole right breast only in view of her uninvolved axilla. 6 MV X-rays from a Varian Clinac 2500 linear accelerator were used throughout treatment, which employed opposed tangential beams. The medial edge of the treatment field was the midsternal line, the lateral edge was at the midaxillary line, the superior margin was located at the second costochondral junction, and the inferior margin was 2 cm below the inferior limits of palpable breast tissue. 200 cGy daily fractions were employed, treating 5 days a week (Monday to Friday). Twentyfive such fractions were delivered, resulting in a TD of 5000 cGy. Our first step was to evaluate the dose expected to be received by the ovaries for the required fields. The ovaries were about 32 cm inferior to the central axes of the tangential fields. For calculation and measurement purposes, the location of the ovaries was taken at the midline of the patient, this being 8 cm from the anterior and 12.5 cm from the lateral skin surface. A schematic representation of the treatment setup is shown in Figs. 1 and 2. The dose to the ovaries comes from four sources:
Case history A 30-year old divorced female with no children presented for radiation therapy following wide excision and axillary dissection of a Stage I (AJC) adenocarcinoma of the right breast. She had refused adjuvant chemotherapy on the National Surgical Adjuvant Breast Project B-13 protocol on the basis of concerns regarding ovarian function. She also stated emphatically that she would only accept postoperative radiation to her breast if every effort was made to reduce the dose to her ovaries. Many studies have been published describing the dose peripheral to the primary radiation beam.4-8 However, most published data had limited applicability to our patient for the particular geometry em-
Reprint requests to: Dr. John Staples, Division of Radiation Oncology, The University of Iowa College of Medicine, Iowa City, IA 52242.
1. leakage, 2. collimator scatter,
269
210
Medical Dosimetry
Volume 14, Number 4, 1989
Scatter from the wedge would contribute to the dose’, but was not taken into account in these estimates. Wedge attenuation was taken into account in the irregular field scatter calculation since the wedge attenuation results in a longer treatment time (higher monitor units delivered), leading to an increase in leakage radiation impinging on the patient. The treatment geometry pointed to a rather simple method to reduce the dose to the ovary by placing a block on the standard treatment tray outside the primary beam. This block potentially reduces the radiation from three of the four sources, namely the leakage, collimator scatter, and wedge scatter. To verify this, measurements were made in a phantom duplicating the treatment geometry. A 0.6 cc ionization chamber was placed at the midline of the phantom at the same off axis distance as the ovaries. The charge produced by the ionization chamber was collected on a precision electrometer. The breast volume was mimicked using bolus material. Measurements were made with and without an additional block on the tray.
TRAY
I-24
cm-
PHANTOM
\
RESULTS 25 cm
I
I
Q OVARY
I
I
I
Fig. 1. Schematic diagram of lateral tangential treatment field for phantom measurements. The 6 MV X-ray field was 16 X 7.5 cm at 100 SSD. For clarity in identification, the wedge has been drawn 90” from its actual orientation. Measurements were taken at midline (8 cm depth, 12.5 cm laterally). The scatter reduction block, supported on the accessory tray, intercepts photons from leakage (a), collimator scatter (b), wedge scatter (c). Photons scattered from within the patient (d) cannot be shielded. The scatter block lies outside the primary beam and therefore generates no additional scatter.
Table 1 lists the results of the phantom measurements. At 32 cm off the central ray, the ovarian dose was 0.15% of the central axis dmax dose for nonwedged fields with no additional block. Placing a block just outside the primary beam between the collimators and the ovaries blocked the scatter radiation from the collimators and reduced the ovarian dose by a factor of 2. Insertion of a 30” wedge created more scatter and resulted in a doubling of the ovarian dose compared to non-wedged fields. For a tumor dose of 5000 cGy, our patient received 8 cGy to her ovaries. Without the scatter block, the dose would have been 18 cGy. DISCUSSION
3. wedge scatter, and 4. scatter from within the patient. An estimate of the dose based on the geometry of Fig. 1 was obtained by extrapolation of the published data. This yielded a dose to the ovary in the range of 0.5 to 0.6 percent of the central axis dmax dose.4,6 For a tumor dose of 5000 cGy this resulted in an estimated ovarian dose of 30 to 40 cGy. Sharma8 has shown that using the Clarkson method of computing scatter is effective in estimating the dose outside of the primary beam. Such a calculation was performed on a commercial treatment planning system (Theraplan L, V4AB, Theratronics International, LTD) using the distances shown. Adding 0.1% to account for leakage from the accelerator, a value of 43 cGy was obtained for the ovarian dose during the total treatment course.
Our measured dose was considerably less than the predicted dose. The difference was most likely due to the fact that the predicted dose assumed fields under full scatter conditions, whereas the values measured included scatter from the irradiated breast tissue only (Fig. 2). In addition, the 0.1% leakage value used in the irregular field estimation was probably higher than the true leakage. We anticipate that the efforts of this particular patient to minimize the dose to vital organs will be seen in other patients as well, particularly since increased public awareness on the value of breast selfexamination and mammography, combined with increasing demand by women for breast preserving treatment, will result in increased use of conservative surgery and irradiation of early stage cases. Young patients such as ours, who have not yet completed
Reducing ovarian dose during megavoltage irradiation of the breast 0 E. C. PENNINGTONet al.
0
’
.
'
0
0
0 .
0 0 0
0 0 0 .
/-
/ 0
/
'
'
'
d'
'
'
/
.
'
'
'
'
/
0
' Measurement paint 04
0 '
0 ' 0
0
/ /
./
beams ,,,"
.
0
'zRH
Tangential
'
/ /
.
.
/
.
. /
271
'32 cm off axis
. '
PHANTOM
0
0 0 , .
0
Fig. 2. Cross sectional view of patient/phantom setup. Breast tissue was mimicked with bolus material. Note that scatter from the irradiated area originates from a small volume of tissue. Wedges and blocks are not shown in this view. The ovary is not in the same plane as the field; it is 32 cm inferior to the central axes of the beams.
their families, are likely to be concerned about the long term effects of irradiation. Two such long term effects include mutations and ovarian failure. The doubling dose for mutations in humans was estimated by the Biological Effects of the Ionizing Radiations (BEIR) III committee of the National Academy of Sciences, to be between 0.5 and 2.5 Sv (50-200 rems). In 1986, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) suggested a single figure of 100 cGy for the doubling dose, which was a calculated rather than measured quantity for humans, based on measured data in mice. The radiation dose to induce ovarian failure is age dependent, higher doses being needed for younger women. Permanent cessation of menstruation in women less than 40 years of age requires a dose in the order of 500 cG~.~ Despite our
Table 1. Results of ovarian dose measurement tangential (cGy) breast fields with 6 MV X-rays. Condition Non-wedged fields no scatter block Non-wedged fields with scatter wedge 30” wedged fields no scatter block 30” wedged fields with scatter block
%Iof Dmax dose
Total ovarian dose for 5000 cGy TD
0.15%
9
0.08%
5
0.30%
18
0.14%
8
reassurance that irradiation of her breast would result in an ovarian dose substantially lower than these estimates, our patient insisted that the dose should be as low as could be reasonably achieved if she was to consent to irradiation. The scatter block as described above definitely reduced the dose to her ovaries, satisfying her request without compromise of treatment quality, convenience, or cost. Obviously, the dose to the ovaries cannot be eliminated. The dose to the ovaries under an unmodified treatment scheme is certainly less than any dose observed to cause detrimental effects; consequently, we do not advocate routine use of this technique. However, in our patient we reduced her anxiety level considerably by making a very simple, yet effective, modification to the treatment setup. This technique could also be of value in reducing fetal dose to acceptable limits in patients requiring breast irradiation during early pregnancy. We would like to emphasize that the doses reported here are very specific to this particular patient. Other patients will vary greatly in anatomical dimensions, and could need different field sites and different field arrangements. In addition, these measurements are machine specific, particularly for leakage, collimator scatter, and wedge design, which will vary with accelerator design. Therefore, the effectiveness of a scatter reducing block as described here will vary depending on the particular patient being treated. Only phantom measurements under close duplica-
Medical Dosimetry
272
tion of the patient’s geometry will yield a precise ovarian dose. We offer this idea of using a scatter reducing block to other clinicians who may eventually be faced with a similar dilemma of treating a patient’s concerns as well as her cancer.
Acknowledgement-The authors wish to thank Michelle Behnke for her time, effort, and patience on this project.
REFERENCES 1. Chamberlain, J. An insurance policy to reduce the risk ofdying from breast cancer. Clin. Radiology 40: 1-3; 1989. 2. Hellman, S.; Harris, J.R. Breast cancer: Considerations in local and regional treatment. Radiology 164:593-598; 1986.
Volume 14, Number 4, 1989 3. Tobias, J.S. Radiotherapy and breast conservation. Br. J. Radiology59~653-666; 1986. 4. Fraass, B.A.; van de Geijn, J. Peripheral dose from megavolt beams. Med. Phys. 10(6):809-818; 1983. 5. Francois, P.; Beurtheret, C.; Dutriex, A. Calculation of the dose delivered to organs outside the radiation beams. Med. Phys. 15(6):879-883; 1988. 6. Keller, B.; Mathewson, C.; Rubin, P. Scattered radiation dosage as a function of X-my energy. Radiology 111:447-449; 1974. 7. Scrimger, J.; Kolitsi, Z. Scattered radiation from beam modifiers used with megavoltage therapy units. Radiology 130:233236; 1979. 8. Sharma, S.C.; Williamson, J.F.; Khan, F.M.; Lee, C.K.K. Measurement and calculation of ovary and fetus dose in extended field radiotherapy for 10 MV X-rays. ht. J. Radiation Oncolo., Biol., Phys. 7~843-846; 1981. 9. Peck, W.S.; McGreer, J.T.; Kretschmar, N.R.; Brown, W.E. Castration of the female by irradiation. Radiology 34: 176- 187; 1940.