The British Journal of Radiology, 79 (2006), 818–827
Normalized data for the estimation of fetal radiation dose from radiotherapy of the breast 1
B BRADLEY,
BSc,
2
A FLECK,
BSc, MSc
and
2,3
E K OSEI,
BSc, MSc, PhD
1
Department of Systems Design Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, 2Department of Medical Physics, Grand River Regional Cancer Center, 835 King Street West, Kitchener, Ontario and 3Department of Physics, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada ABSTRACT. There can be several reasons why a pregnant patient may receive a radiological examination. It could have been a planned exposure, or the exposure might have resulted from an emergency when a thorough evaluation of pregnancy was impractical. Sometimes the pregnancy was unsuspected at the time of the examination and, with younger women being diagnosed with breast cancer, the likelihood of this will increase in radiotherapy departments. Whatever the reason, when presented with a pregnant patient who has received a radiological examination involving ionizing radiation, the dose to the fetus should be assessed based on the patient’s treatment plan. However, a major source of uncertainty in the estimation of fetal absorbed dose is the influence of fetal size and position as these change with gestational age. Consequently, dose to the fetus is related to gestational age. Various studies of fetal dose during pregnancy have appeared in the literature. Whilst these papers contain many useful data for estimating fetal dose, they usually contain limited data regarding the depth and size of the fetus within the maternal uterus. We have investigated doses to the fetus from radiation therapy of the breast of a pregnant patient using an anthropomorphic phantom. Normalized data for estimating fetal doses that takes into account the fetal size (gestational age: 8–20 weeks post-conception) and depth within the maternal abdomen (4–16 cm) for different treatment techniques have been provided. The data indicate that fetal dose is dependent on both depth within the maternal abdomen and gestational age, and hence these factors should always be considered when estimating fetal dose. The data show that fetal dose can be underestimated up to about 10% or overestimated up to about 30% if the dose to the uterus is assumed instead of the actual fetal dose. It can also be underestimated up to about 23% or overestimated up to about 12% if a mean depth of 9 cm is assumed, instead of using the actual depth of the fetus within the maternal abdomen. Multisegments sMLC technique showed consistently lower fetal doses compared with all the wedged plans employed.
Peripheral dose (PD), which is an inevitable consequence of radiotherapy, is mainly due to the radiation that is scattered within the patient and also the scattered and leakage radiation from the head of the machine and the collimator assembly. The magnitude of the scattered radiation from within the patient will mainly depend on the beam energy, distance from the radiation field edge, field size and depth. For a pregnant patient, such dose is of greater concern due to the fact that the developing fetus is more susceptible to the adverse effects of ionizing radiation, especially when the absorbed dose to the fetus may be high. However, using additional shielding can minimize the radiation dose due to scatter and leakage radiation from the machine head and collimator assembly [1, 2]. Address correspondence to: Ernest K Osei, Department of Medical Physics, Grand River Regional Cancer Center, 835 King Street West, Kitchener, Ontario, Canada. E-mail:
[email protected].
818
Received 24 January 2006 Revised 13 April 2006 Accepted 25 April 2006 DOI: 10.1259/bjr/16416346 ’ 2006 The British Institute of Radiology
Although radiotherapy treatment during a known pregnancy (especially during the 8–15 weeks gestational age period) should be avoided [3], it is not always possible to postpone treatment for the full duration of the pregnancy. Under such circumstances, it is essential to be able to estimate the absorbed dose (and hence risk) to the fetus if the patient must undergo treatment. Modifications to the plan and shielding techniques can then be incorporated where necessary to achieve an optimum balance between risk to the fetus and benefit to the mother, although the fetus also receives an indirect benefit. There are also occasions when pregnancy may not be known before the onset of a therapeutic treatment, and sometime later the patient realises she is pregnant. Under such a situation too, the fetal absorbed dose (and hence risk) as a result of the radiation treatment received should be assessed by a medical physicist. With younger women being diagnosed with breast cancer, the likelihood increases that a patient may be pregnant when The British Journal of Radiology, October 2006
Estimation of fetal dose from radiotherapy of the breast
treatment is indicated or that inadvertent treatment may take place for women who subsequently learn that they are pregnant. Fetal dose estimation is a difficult and time-consuming procedure that requires extensive dosimetric measurements and sometimes the construction of a specific phantom to simulate the patient’s geometry [3, 4]. A review of the scientific literature regarding the measurement of peripheral dose for the estimation of dose to the fetus from radiotherapy procedures has revealed an extensive collection of data [1, 5–14]. Mazonakis developed a method similar to the one described here for estimating fetal dose during brain radiotherapy. Using an anthropomorphic phantom, fetal absorbed dose was determined at two gestational ages and depths [4]. Rincon [9] has also estimated the dose to the fetus using peripheral dose to the uterus for two standard breast radiotherapy treatments (a non-wedged plan and a 15 ˚ physical wedged plan). Point dose measurements were taken in a phantom from the field edge to the uterus, and the fetal depth was assumed to be 15 cm. However, there are very limited data in the literature that take into account the different fetal positions (depth within the maternal abdomen) and gestational age (size) when estimating dose, although it has been shown that these fetal parameters are significant factors when estimating dose [2, 15]. Therefore, a source of data, which accounts for the different fetal parameters (i.e. fetal depth and size), would be very useful when estimating the fetal absorbed dose and hence risk. This paper presents the experimental procedure and results of fetal dose estimation from radiotherapy treatment of the breast. Data are presented for two different photon beam energies (6 MV and 15 MV), four different physical wedge angles (15 ˚, 30 ˚, 45 ˚ and 60 ˚), and a segmented multi-leaf collimator (sMLC) plan. Fetal sizes (gestational age) considered range from 8 weeks to 20 weeks post-conception, and fetal depth within the maternal abdomen ranges from 4 cm to 16 cm. By treating the fetus as a three-dimensional volume at various gestational ages and depths from the anterior surface of the mother’s abdomen, data are generated for a more accurate estimation of the mean dose to the fetus from radiotherapy of the breast. Fetal dose is assumed to be a whole body exposure of the fetus within the maternal abdomen. There are three scenarios where the data presented in this paper can be of benefit to the medical and paramedical personnel involved with fetal dose estimation. For the case of a patient who has already completed radiotherapy treatment and discovers later that she was pregnant during the time of treatment, the data would be useful in estimating the dose to the fetus and hence risk using the patient’s treatment plan parameters. Such inadvertent fetal exposures would be very rare if modern patient safety legislation is implemented well in radiological departments. For the case of a patient who, during the course of treatment, discovers that she is pregnant and there is clinical indication for her to complete treatment, the data would be useful to estimate the fetal dose from the treatment already received. The patient’s treatment plan could then be modified and optimized in order to minimize the fetal dose. Appropriate abdominal shielding could also be The British Journal of Radiology, October 2006
employed for the rest of the treatment to further reduce the fetal dose and hence risk and the actual dose measured for the rest of the treatment. Lastly, for the case of a patient who is pregnant and wishes to have radiotherapy treatment and there is also clinical indication for her to undergo treatment, the data would be useful to estimate the fetal dose without taking into account any form of shielding (maximum dose). A detailed study of the actual treatment plan could then be conducted, and the plan might be modified and optimized to reduce the fetal dose. Furthermore, appropriate abdominal shielding could then be employed to further reduce the fetal dose and risk from the treatment. Actual fetal dose should then be measured using the optimized treatment plan for risk assessment.
Materials and methods Experimental set-up The experimental set-up used for the estimation of fetal dose from radiation therapy of the breast is shown in Figure 1. An Alderson adult Rando anthropomorphic phantom (Alderson Research Labs, Stamford, CT) was used to represent the patient. The phantom consists of a human skeleton encased in tissue-equivalent material, with 33 transverse sections. It has an AP thickness of 20 cm, a width (shoulder-to-shoulder) of about 34 cm and a height of 90 cm. Radiographic film (Kodak X-OMAT TL, Rochester, NY) was used for all dosimetry measurements. The films were placed between each of the slices spanning the fetal region. The films spanned from slice 27 to 33 depending on the length of the fetus at the gestational age being studied (Figures 1 and 2). The film calibration curve was performed in a homogeneous medium. A sensitometric curve was generated for each batch of film at the time of measurement and used to convert optical densities into relative doses. The phantom was irradiated using the plans generated (6 MV and 15 MV (all wedges), and segmented MLC plan). The irradiated films were developed, scanned and analysed using a Vidar VXR-16 Dosimetry Pro scanner (Vidar, Herndon, VA) and RIT113 (v4) Radiation Therapy Dosimetry Software. Metal-oxide-semiconductor field-effect transistors (MOSFETs) were also used to measure point doses in order to verify the film dosimetry. The pegs along a central axis throughout the fetal region of the Rando phantom were removed and five high-sensitivity MOSFET dosemeters were placed with the sensitive region covering the holes (Figure 2). They were at a depth of approximately 11 cm from the anterior surface of the phantom and ranged from a distance of approximately 22.5 cm to 32.5 cm from the field edge. The phantom was placed in a supine position on the treatment couch in accordance with department protocol (Figure 1) and was ‘‘treated’’ with the same set-up for each treatment plan described below.
Treatment planning The phantom underwent a treatment planning CT scan. At the time of scanning, the phantom was placed in 819
B Bradley, A Fleck and E K Osei
Figure 1. The experimental setup for a typical two-field tangential breast treatment. The Rando phantom is placed in a supine position on the treatment couch and setup as per departmental protocol, and was ‘‘treated’’ with the same set-up for each treatment plan.
Figure 2. The films and Metaloxide-semiconductor field-effect transistors (MOSFETs) placement within the phantom showing the positioning of the films and MOSFETs. MOSFETs were located along a central axis within the fetal region. When measurements are being taken with film, the MOSFETs are removed and likewise, when measurements are being taken with the MOSFETs for comparison, the films are removed.
the treatment position for tangential breast radiation therapy adopted at the centre (i.e. supine position). A helical AcQSim CT scanner (Philips Medical Systems Cleveland Inc., Ohio, USA) was used to acquire contiguous 3 mm CT axial images. The CT data set was then transferred to a 3D treatment planning workstation (Pinnacle3; ADAC Laboratories, Milpitas, USA) for treatment planning. Using our departmental protocol for beam arrangements, a typical two-field tangential breast plan was developed. This standard plan was then modified to incorporate different photon energies (6 MV and 15 MV), wedge angles (15 ˚, 30 ˚, 45 ˚ and 60 ˚), and a multi-segment static MLC technique. The MLC leaves were fully retracted in all wedge plans. In total, 10 different treatment plans were generated for the study and they spanned the different plans usually used in the radiotherapy centre for the treatment of breast cancer.
Fetal parameters and dose calculation A chart of fetal dimensions was created for all gestational ages ranging from 8 weeks to 20 weeks post-conception, which are based on the mean of a series of measurements taken from the literature [16–19]. They gave the fetal crown–rump-length (CRL), biparietal diameter (BPD), abdominal circumference (AC), head circumference (HC) and fetal mass (wt) at the various 820
gestational ages, with statistical variation as 2 standard deviations from the mean. These data were used to create a three-dimensional rectangular volume to represent the fetus at the different gestational ages (Table 1). The density (r) of the fetus was assumed to be 1.061023 kg m23. For gestational ages ranging from 8 weeks to 12 weeks post-conception, the CRL was used for the length (l), the width (w) was estimated from the mean of the abdominal circumference (i.e. AC/p) and the anteroposterior (AP) thickness was calculated from: AP~
wt l|w|r
For gestational ages ranging from 13 weeks to 20 weeks post-conception, the biparietal diameter was used for the width, AP thickness was estimated from the mean abdominal circumference and the length was estimated in a similar way as above. Using the RTT113 software, a rectangular region of interest (ROI) (Figure 3) was selected on each film using the width and AP thickness dimensions of the fetus at the depth of interest. The mean dose and standard deviation for this region are recorded. This process is repeated for each subsequent film spanning the length of the fetus at the same depth and gestational age dimensions. From the complete set of mean doses for each film, the mean dose for the volume (gestational age) is estimated. The whole process is then The British Journal of Radiology, October 2006
Estimation of fetal dose from radiotherapy of the breast Table 1. Fetal dimensions at different gestational ages Gestational age (weeks)
Length (cm)
Width (cm)
AP thickness (cm)
Mass (g)
8 9 10 11 12
1.6 2.3 3.3 4.1 5.4
0.7 0.8 1.0 1.4 1.9
0.9 1.1 1.2 1.2 1.4
1.0 2.0 4.0 2.0 14.0
13 14 15 16 17 18 19 20
5.9 6.5 8.0 9.2 10.7 11.8 12.5 13.5
2.1 2.5 2.9 3.2 3.5 3.9 4.3 4.6
2.3 2.6 3.0 3.4 3.7 4.1 4.5 4.8
28.0 43.0 70.0 100.0 140.0 190.0 240.0 300.0
Calculations
Length5CRL Width5AC/p AP5calculated
Width5BPD AP5AC/p Length5calculated
CRL, crown–rump length; AC, abdominal circumference; AP, anteroposterior; BPD, biparietal diameter.
repeated for each gestational age (8–20 weeks) and depth (4–16 cm) of the fetus, and for all the films exposed from each treatment plan delivered.
Results and discussion In this paper, fetal dose is assumed to be a whole body exposure of the fetus within the maternal abdomen. The data collected and presented here were taken without any modification to the treatment plan setup to minimize the dose to the fetus. They would be useful for estimating dose to the fetus when a patient realises she is or was pregnant during or after the course of treatment. If there is a clinical indication for a pregnant patient to complete treatment when pregnancy is known, the data could be used to estimate the fetal dose without shielding and the setup could then be modified by the use of external shielding over the abdominal region of the pregnant patient during treatment to significantly reduce the fetal dose. The dimensions of the rectangular shape representing the fetus at different gestational ages are shown in
Table 1. Measured fetal doses normalized to the prescribed dose at the isocentre of the plan as a function of fetal depth within the maternal abdomen for different breast treatment plans are given in Tables 2–6. Figure 4 shows the relationship between the normalized fetal dose as a function of depth for all gestational ages using the multi-segment static MLC treatment plan and 6 MV photon beam. A similar relationship comparing the various treatment plans (sMLC, 15 ˚, 30 ˚, 45 ˚ and 60 ˚) and photon beam energies (6 MV and 15 MV) is shown in Figure 5. One of the most important parameters in the measurement of fetal dose from radiotherapy procedures is the distance from the radiation field edge to the point of measurement. The fetal dose decreases approximately exponentially with distance from the field edge. Published data [2, 4] show that the change in the peripheral dose with depth is small, although other data [1, 8] show greater change for a Co-60 treatment unit. The results of fetal dose estimation from radiotherapy of the breast show that fetal dose is dependent on both fetal depth within the maternal abdomen and gestational age (Figure 4). In general for all the techniques used and for
Figure 3. Region of interest representing a fetus at a gestational age of 20-weeks post-conception and at a depth of 8 cm from the abdominal surface of the maternal abdomen.
The British Journal of Radiology, October 2006
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B Bradley, A Fleck and E K Osei Table 2. Normalized mean fetal dose, as a percentage of prescribed dose (PD), for a typical two-field tangential beam plan using multi-segment static MLC for both 6 MV and 15 MV photon energies. Data are presented for various gestational ages (GA) and fetal depth within the maternal abdomen Photon energy
6 MV
15 MV
GA (weeks)
Fetal depth (cm) 4
5
6
7
8
9
10
11
12
16
8 9 10 11 12 13 14 15 16 17 18 19 20
0.103 0.104 0.106 0.107 0.110 0.112 0.114 0.119 0.125 0.133 0.140 0.144 0.151
0.102 0.103 0.105 0.106 0.109 0.111 0.113 0.118 0.124 0.131 0.138 0.142 0.149
0.100 0.101 0.103 0.104 0.107 0.109 0.111 0.116 0.121 0.129 0.135 0.139 0.146
0.099 0.099 0.100 0.102 0.105 0.106 0.108 0.113 0.118 0.126 0.132 0.136 0.142
0.097 0.097 0.099 0.100 0.103 0.104 0.106 0.110 0.115 0.122 0.128 0.132 0.138
0.095 0.096 0.097 0.098 0.100 0.101 0.103 0.108 0.112 0.119 0.124 0.128 0.133
0.093 0.093 0.094 0.096 0.098 0.099 0.100 0.105 0.109 0.115 0.119 0.123 0.128
0.090 0.091 0.092 0.093 0.095 0.096 0.098 0.101 0.105 0.110 0.114 0.117 0.122
0.087 0.088 0.089 0.090 0.093 0.094 0.095 0.098 0.101 0.106 0.109 0.111 0.115
0.068 0.069 0.070 0.071 0.073 0.073 0.074 0.076 0.079 0.081 0.083 0.085 0.087
8 9 10 11 12 13 14 15 16 17 18 19 20
0.088 0.089 0.090 0.091 0.093 0.095 0.095 0.099 0.102 0.105 0.109 0.111 0.115
0.086 0.087 0.089 0.090 0.092 0.093 0.094 0.097 0.100 0.103 0.106 0.108 0.110
0.084 0.085 0.087 0.088 0.090 0.091 0.092 0.095 0.098 0.101 0.103 0.105 0.107
0.082 0.083 0.085 0.086 0.088 0.089 0.090 0.093 0.096 0.099 0.101 0.102 0.105
0.080 0.081 0.083 0.084 0.086 0.087 0.088 0.091 0.093 0.096 0.098 0.100 0.102
0.077 0.078 0.080 0.082 0.084 0.085 0.086 0.088 0.091 0.093 0.095 0.097 0.099
0.075 0.076 0.078 0.079 0.081 0.082 0.083 0.086 0.088 0.091 0.093 0.094 0.096
0.072 0.073 0.075 0.076 0.079 0.079 0.080 0.083 0.085 0.087 0.089 0.090 0.092
0.069 0.070 0.072 0.073 0.075 0.076 0.077 0.079 0.081 0.084 0.085 0.086 0.088
0.054 0.054 0.055 0.056 0.057 0.057 0.058 0.060 0.061 0.063 0.064 0.065 0.067
Table 3. Normalized mean fetal dose, as a percentage of prescribed dose (PD), for a typical two-field tangential beam 15 ˚ wedged plan for both 6 MV and 15 MV photon energies. Data are presented for various gestational ages (GA) and fetal depth within the maternal abdomen Photon energy
6 MV
15 MV
822
GA (weeks)
Fetal depth (cm) 4
5
6
7
8
9
10
11
12
16
8 9 10 11 12 13 14 15 16 17 18 19 20
0.325 0.331 0.342 0.350 0.365 0.370 0.378 0.396 0.412 0.433 0.450 0.460 0.476
0.323 0.330 0.341 0.349 0.364 0.369 0.377 0.395 0.411 0.432 0.448 0.459 0.474
0.321 0.328 0.338 0.346 0.361 0.367 0.374 0.393 0.409 0.430 0.446 0.456 0.471
0.319 0.326 0.336 0.344 0.358 0.364 0.371 0.389 0.405 0.425 0.441 0.451 0.466
0.315 0.322 0.332 0.340 0.354 0.359 0.366 0.384 0.399 0.419 0.434 0.444 0.459
0.308 0.315 0.325 0.333 0.347 0.353 0.360 0.377 0.392 0.411 0.426 0.435 0.449
0.301 0.307 0.317 0.325 0.339 0.344 0.351 0.368 0.382 0.401 0.415 0.424 0.438
0.292 0.298 0.308 0.316 0.329 0.334 0.340 0.357 0.371 0.389 0.402 0.411 0.423
0.280 0.287 0.296 0.304 0.317 0.322 0.328 0.344 0.357 0.374 0.387 0.395 0.406
0.218 0.221 0.227 0.232 0.241 0.245 0.250 0.263 0.274 0.289 0.300 0.308 0.319
8 9 10 11 12 13 14 15 16 17 18 19 20
0.205 0.209 0.216 0.221 0.230 0.234 0.239 0.251 0.261 0.275 0.286 0.293 0.304
0.203 0.207 0.214 0.219 0.228 0.232 0.236 0.248 0.258 0.271 0.281 0.288 0.297
0.198 0.203 0.209 0.215 0.224 0.228 0.232 0.244 0.254 0.266 0.276 0.283 0.292
0.193 0.198 0.204 0.209 0.218 0.222 0.227 0.238 0.248 0.261 0.270 0.277 0.286
0.188 0.192 0.198 0.204 0.213 0.216 0.221 0.232 0.242 0.254 0.264 0.270 0.279
0.181 0.186 0.192 0.198 0.207 0.210 0.215 0.226 0.235 0.247 0.256 0.262 0.271
0.175 0.180 0.186 0.191 0.200 0.204 0.208 0.219 0.228 0.239 0.248 0.254 0.262
0.167 0.172 0.179 0.184 0.193 0.196 0.200 0.211 0.219 0.230 0.238 0.243 0.251
0.157 0.162 0.169 0.174 0.183 0.186 0.190 0.200 0.208 0.219 0.227 0.231 0.238
0.120 0.122 0.125 0.128 0.134 0.137 0.140 0.147 0.153 0.162 0.169 0.173 0.180
The British Journal of Radiology, October 2006
Estimation of fetal dose from radiotherapy of the breast Table 4. Normalized mean fetal dose, as a percentage of prescribed dose (PD), for a typical two-field tangential beam 30 ˚ wedged plan for both 6 MV and 15 MV photon energies. Data are presented for various gestational ages (GA) and fetal depth within the maternal abdomen Photon energy
6 MV
15 MV
GA (weeks)
Fetal depth (cm) 4
5
6
7
8
9
10
11
12
16
8 9 10 11 12 13 14 15 16 17 18 19 20
0.430 0.438 0.451 0.462 0.481 0.488 0.497 0.522 0.544 0.573 0.595 0.610 0.632
0.431 0.440 0.453 0.464 0.482 0.490 0.499 0.524 0.546 0.575 0.597 0.611 0.633
0.430 0.439 0.452 0.463 0.482 0.489 0.499 0.524 0.545 0.574 0.596 0.610 0.632
0.428 0.437 0.449 0.460 0.479 0.486 0.496 0.521 0.542 0.571 0.592 0.607 0.628
0.424 0.432 0.445 0.455 0.474 0.481 0.491 0.515 0.536 0.564 0.586 0.600 0.621
0.417 0.425 0.438 0.448 0.467 0.474 0.483 0.507 0.528 0.555 0.576 0.590 0.611
0.407 0.416 0.428 0.441 0.457 0.464 0.473 0.497 0.517 0.544 0.564 0.575 0.597
0.395 0.404 0.416 0.427 0.445 0.452 0.460 0.484 0.503 0.529 0.548 0.560 0.579
0.381 0.390 0.402 0.413 0.430 0.437 0.445 0.467 0.485 0.509 0.527 0.539 0.556
0.302 0.307 0.314 0.320 0.332 0.337 0.344 0.361 0.376 0.396 0.412 0.422 0.437
8 9 10 11 12 13 14 15 16 17 18 19 20
0.295 0.301 0.309 0.316 0.329 0.334 0.340 0.356 0.370 0.388 0.403 0.412 0.426
0.292 0.298 0.307 0.314 0.327 0.331 0.337 0.353 0.366 0.384 0.397 0.406 0.419
0.286 0.293 0.302 0.309 0.322 0.327 0.333 0.349 0.362 0.379 0.391 0.400 0.412
0.281 0.287 0.297 0.304 0.317 0.322 0.328 0.343 0.356 0.372 0.384 0.392 0.403
0.275 0.281 0.290 0.298 0.311 0.315 0.321 0.336 0.348 0.364 0.375 0.383 0.394
0.267 0.273 0.283 0.290 0.302 0.307 0.313 0.327 0.339 0.354 0.366 0.373 0.383
0.259 0.265 0.274 0.281 0.293 0.298 0.305 0.318 0.329 0.344 0.354 0.361 0.371
0.249 0.255 0.265 0.272 0.284 0.288 0.293 0.307 0.318 0.331 0.342 0.348 0.357
0.238 0.244 0.253 0.261 0.272 0.276 0.281 0.294 0.304 0.317 0.326 0.332 0.341
0.184 0.188 0.194 0.199 0.207 0.211 0.215 0.225 0.233 0.244 0.252 0.257 0.265
Table 5. Normalized mean fetal dose, as a percentage of prescribed dose (PD), for a typical two-field tangential beam 45 ˚ wedged plan for both 6 MV and 15 MV photon energies. Data are presented for various gestational ages (GA) and fetal depth within the maternal abdomen Photon energy
6 MV
15 MV
GA (weeks)
Fetal depth (cm) 4
5
6
7
8
9
10
11
12
16
8 9 10 11 12 13 14 15 16 17 18 19 20
0.334 0.341 0.350 0.359 0.373 0.379 0.386 0.405 0.421 0.443 0.459 0.470 0.486
0.330 0.337 0.347 0.355 0.370 0.375 0.382 0.401 0.417 0.438 0.454 0.465 0.480
0.324 0.331 0.341 0.349 0.364 0.370 0.377 0.395 0.411 0.432 0.447 0.458 0.473
0.318 0.324 0.334 0.343 0.357 0.363 0.370 0.388 0.403 0.424 0.439 0.449 0.464
0.312 0.318 0.328 0.336 0.350 0.355 0.362 0.380 0.395 0.414 0.429 0.439 0.454
0.304 0.311 0.320 0.328 0.342 0.347 0.354 0.371 0.385 0.405 0.419 0.429 0.443
0.297 0.303 0.312 0.320 0.333 0.338 0.344 0.361 0.375 0.394 0.408 0.417 0.431
0.287 0.293 0.302 0.310 0.322 0.327 0.333 0.349 0.363 0.381 0.395 0.403 0.416
0.273 0.279 0.288 0.296 0.308 0.313 0.319 0.335 0.348 0.366 0.378 0.386 0.399
0.208 0.211 0.217 0.222 0.231 0.236 0.241 0.254 0.266 0.281 0.293 0.301 0.313
8 9 10 11 12 13 14 15 16 17 18 19 20
0.299 0.304 0.312 0.319 0.331 0.335 0.341 0.356 0.370 0.387 0.401 0.410 0.424
0.294 0.299 0.308 0.315 0.326 0.331 0.336 0.351 0.364 0.380 0.392 0.400 0.413
0.287 0.292 0.301 0.308 0.320 0.324 0.330 0.344 0.356 0.372 0.384 0.392 0.404
0.279 0.284 0.293 0.299 0.311 0.315 0.321 0.335 0.347 0.363 0.375 0.383 0.394
0.271 0.276 0.284 0.291 0.302 0.306 0.312 0.326 0.338 0.353 0.365 0.373 0.384
0.262 0.267 0.275 0.282 0.293 0.298 0.303 0.317 0.329 0.344 0.355 0.362 0.373
0.254 0.259 0.267 0.274 0.285 0.289 0.295 0.308 0.320 0.334 0.345 0.352 0.362
0.244 0.250 0.258 0.265 0.276 0.280 0.286 0.299 0.309 0.323 0.333 0.339 0.349
0.233 0.239 0.248 0.255 0.266 0.269 0.274 0.287 0.297 0.310 0.320 0.326 0.334
0.183 0.187 0.193 0.198 0.206 0.209 0.213 0.224 0.232 0.244 0.252 0.258 0.267
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B Bradley, A Fleck and E K Osei Table 6. Normalized mean fetal dose, as a percentage of prescribed dose (PD), for a typical two-field tangential beam 60 ˚ wedged plan for both 6 MV and 15 MV photon energies. Data are presented for various gestational ages (GA) and fetal depth within the maternal abdomen Photon energy
6 MV
15 MV
GA (weeks)
Fetal depth (cm) 4
5
6
7
8
9
10
11
12
16
8 9 10 11 12 13 14 15 16 17 18 19 20
0.381 0.389 0.400 0.409 0.425 0.431 0.439 0.460 0.478 0.502 0.520 0.532 0.550
0.377 0.384 0.395 0.404 0.421 0.427 0.435 0.456 0.474 0.497 0.516 0.527 0.545
0.370 0.378 0.389 0.398 0.415 0.421 0.429 0.450 0.468 0.491 0.510 0.522 0.539
0.362 0.370 0.381 0.390 0.407 0.413 0.421 0.443 0.460 0.484 0.502 0.514 0.532
0.355 0.363 0.374 0.383 0.399 0.405 0.413 0.434 0.452 0.475 0.493 0.505 0.523
0.348 0.355 0.366 0.375 0.391 0.397 0.405 0.425 0.443 0.466 0.484 0.495 0.513
0.341 0.348 0.358 0.366 0.382 0.388 0.395 0.415 0.433 0.455 0.473 0.484 0.501
0.331 0.338 0.348 0.356 0.371 0.377 0.384 0.404 0.420 0.442 0.459 0.470 0.486
0.317 0.324 0.334 0.343 0.358 0.363 0.371 0.389 0.405 0.426 0.442 0.452 0.467
0.245 0.251 0.259 0.266 0.278 0.283 0.289 0.305 0.319 0.336 0.351 0.360 0.373
8 9 10 11 12 13 14 15 16 17 18 19 20
0.339 0.345 0.353 0.361 0.373 0.378 0.384 0.401 0.416 0.435 0.450 0.460 0.475
0.333 0.339 0.348 0.355 0.368 0.372 0.378 0.400 0.409 0.427 0.436 0.451 0.465
0.326 0.331 0.340 0.347 0.360 0.365 0.371 0.388 0.401 0.420 0.434 0.443 0.457
0.318 0.324 0.332 0.339 0.352 0.357 0.363 0.379 0.393 0.411 0.425 0.434 0.448
0.311 0.316 0.325 0.332 0.344 0.349 0.355 0.371 0.384 0.402 0.416 0.425 0.438
0.303 0.308 0.317 0.324 0.336 0.341 0.349 0.363 0.376 0.393 0.406 0.415 0.427
0.294 0.300 0.309 0.316 0.329 0.333 0.339 0.354 0.367 0.383 0.396 0.404 0.415
0.285 0.291 0.300 0.308 0.320 0.324 0.329 0.344 0.356 0.372 0.383 0.391 0.402
0.271 0.277 0.287 0.294 0.307 0.311 0.317 0.331 0.343 0.358 0.369 0.376 0.386
0.213 0.218 0.225 0.231 0.241 0.245 0.250 0.263 0.273 0.286 0.296 0.303 0.313
all gestational ages, the fetal dose decreases with increase in depth. The data show that fetal dose estimations based on using a constant fetal depth of 9 cm would either overestimate or underestimate the dose, depending on the depth of interest. For example, dose could be underestimated up to about 10% at a depth of 4 cm and overestimated as high as about 30% at a depth of 16 cm (using 6 MV, sMLC plan) if a depth of 9 cm is used instead of the actual fetal depth. Fetal dose is also dependent on the gestational age of the fetus. For all fetal depths, as gestational age increases, fetal dose also increases. The data show that if fetal dose were to be estimated as the dose to the uterus, the fetal dose would again be either overestimated or underestimated at any given depth depending on the gestational age of interest. A uterine mass of 66.3 g and a length, width and AP thickness of 7.5 cm, 3.5 cm and 2.5 cm, respectively, correspond most closely to the dimensions of a fetus at 15 weeks post-conception. Therefore, for dose estimates below 15 weeks postconception (i.e. uterus dose), the fetal dose would be overestimated by up to about 12%, and for estimates above 15 weeks, the fetal dose would be underestimated by up to about 23% (using 6 MV with the sMLC plan). The degree to which the uterus dose differs from fetal dose is dependent upon the difference in gestational age, as well as other parameters such as depth and orientation of the fetus within the maternal abdomen. According to the literature [1, 6, 7], the presence of a wedge in the beam could increase the fetal dose by a 824
factor of about 2–4 and our data give a similar result ranging from a factor of about 2–5 for all the wedges. The data show that the multi-segment sMLC technique results in consistently lower fetal doses compared with all the wedged techniques and could significantly reduce fetal doses. This reduction in the fetal dose with the use of a multi-segment (sMLC) plan as against that of a wedged plan may be as a result of a combination of a decrease in scatter and improved collimation when MLC leaves are used in conjunction with the jaws to shape the treatment field. The MLC leaves were fully retracted in all plans employing a wedge. The data indicate that fetal dose is dependent on both depth within the maternal abdomen and gestational age, and hence these factors should always be considered when estimating fetal dose. The measured fetal doses are in good agreement with other data obtained from the literature [1, 2, 4, 5, 9]. Comparison was made between these data and others previously published, although most of the published data refer to fetal doses at one specific gestational age and depth. Rincon et al [9] reported a fetal dose (assuming dose to the uterus) of about 40 mGy at a depth of 15 cm using a 6 MV photon beam and a prescription of 50 Gy at the isocentre. Assuming a fetal size of 15 weeks post-conception to be that of the uterus (as used in the Rincon study), 6 MV photon beam and sMLC plan, we estimated a fetal dose of about 38 mGy at a depth of 16 cm. The difference in the estimated doses may be as a result of differences in fetal depth and irradiation geometry used. For a fetus at 2–6 weeks The British Journal of Radiology, October 2006
Estimation of fetal dose from radiotherapy of the breast
Figure 4. Normalized mean fetal dose, as a percentage of prescribed dose (PD), for a typical two-field tangential beam plan using multi-segment static MLC for 6 MV photon beam as a function of fetal depth within the maternal abdomen. Data are presented for various gestational ages (GA).
gestational age at a depth of 9 cm, Antypas et al [5] reported relative fetal doses in the range of 0.079% to 0.085% of the prescribed dose. Using the same fetal depth and 6 MV photon beam and sMLC plan, we estimated a fetal dose of about 0.095% of the prescribed dose for a gestational age of 8 weeks. The difference in the estimated doses may be due to differences in gestational age and irradiation geometry employed.
Conclusion Normalized data for converting the prescribed dose at the isocentre from medical exposure of a pregnant woman undergoing radiation treatment of the breast, to absorbed dose to the fetus, have been presented. These data may be useful for estimating absorbed dose to the fetus from breast cancer radiotherapy of the mother and take into account the dependence of fetal age on gestational age (fetal size), fetal depth, wedge dose and photon beam energy. The results show that fetal dose is dependent on both gestational age and depth within the maternal abdomen, and hence these factors should be The British Journal of Radiology, October 2006
taken into account when estimating dose. The data collected and presented here were taken without any modification to the treatment setup to minimize the dose to the fetus. They would be useful for estimating dose to the fetus when a patient realises she is or was pregnant during or after the course of treatment. Again, if there is a clinical indication for a pregnant patient to complete treatment when pregnancy is known, the data could be useful for estimating the fetal dose and the treatment setup can then be modified and optimized, and also external shielding over the abdominal region of the pregnant patient could be employed during treatment to significantly reduce the fetal dose. The sources of radiation dose to the fetus are mainly due to the radiation that is scattered within the patient and also the scattered and leakage radiation from the head of the machine and the collimator assembly. Whereas little can be done to reduce the internal scatter within the patient contributing dose to the fetus, the other sources of radiation exposure contributing dose to the fetus could be reduced significantly by the application of external shielding over the abdominal region of the pregnant patient. 825
B Bradley, A Fleck and E K Osei
Figure 5. Normalized mean fetal dose, as a percentage of prescribed dose (PD), for a typical two-field tangential beam plan as a function of fetal depth within the maternal abdomen. Data are presented for various treatment plans (sMLC, 15 ˚, 30 ˚, 45 ˚ and 60 ˚ wedged plans) and for 6 MV and 15 MV photon energies. The gestational age is 15 weeks post-conception.
References 1. Stovall M, Blackwell CR, Cundiff J, Novack DH, et al. Fetal dose from radiotherapy with photon beams: Report of AAPM Radiation Therapy Committee Task Group No. 36. Med Phys 1995;22:63–82. 2. Islam MK, Saeedi F, Al-Rajhi N. A simplified shielding approach for limiting fetal dose during radiation therapy of pregnant patients. Int J Radiat Oncol Biol Phys 2001;49:1469–73. 3. ICRP-60. International Commission on Radiological Protection (ICRP). 1990 Recommendations of the ICRP. ICRP publication 60, Annals of the ICRP 21, No 1-3, Pergamon Press, Oxford, 1991. 4. Mazonakis M, Damilakis J, Varveris H, Theoharopoulos N, Gourtsoyiannis N. A method of estimating fetal dose during brain radiation therapy. Int J Radiat Oncol Biol Phys 1999;44:455–9. 5. Antypas C, Sandilos P, Kouvaris J, Balafouta E, et al. Fetal dose evaluation during breast cancer radiotherapy. Int J Radiat Oncol Biol Phys 1998;40:995–9. 6. Fraass B, Van de Geijn J. Peripheral dose from megavolt beams. Med Phys 1983;10:809–18. 7. Diallo I, Lamon A, Shamsaldin A, Grimaud E, de Vathaire F, Chavaudra J. Estimation of the radiation dose delivered
826
8.
9.
10.
11.
12.
to any point outside the target volume per patient treated with external beam radiotherapy. Radiother Oncol 1996;38: 269–71. Van der Giessen P. A simple and generally applicable method to estimate the peripheral dose in radiation teletherapy with high energy x-rays or gamma radiation. Int J Radiat Oncol Biol Phys 1996;35:1059–68. Rincon CM, Jerez Sainz I, Farree IM, Espana Lopez ML, Franco PL, et al. Evaluation of the peripheral dose to uterus in breast carcinoma radiotherapy. Radiat Prot Dosim 2002;101:469–71. Nuyttens JJ, Prado KL, Jenrette JM, Williams TE. Fetal dose during radiotherapy: clinical implementation and review of the literature. Cancer (Radiothe´rapie) 2002;6: 352–7. Prado KL, Nelson SJ, Nuyttens JJ, Williams TE, Vanek KN. Clinical implementation of the AAPM Task Group 36 recommendations on fetal dose from radiotherapy with photon beams: a head and neck irradiation case report. J Appl Clin Med Phys 2000;1:1–7. Sneed PK, Albright NW, Wara WM, Prados MD, Wilson CB. Fetal dose estimates for radiotherapy of brain tumors during pregnancy. Int J Radiat Oncol Biol Phys 1995;32:823–30.
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Estimation of fetal dose from radiotherapy of the breast 13. Antolak JA, Strom EA. Fetal dose estimates for electronbeam treatment to the chest wall of a pregnant patient. Med Phys 1998;25:2388–91. 14. Ngu SL, Duval P, Collins C. Fetal radiation dose in radiotherapy for breast cancer. Aust Radiol 1992;36:321–2. 15. Osei EK, Faulkner K. Fetal position and size data for dose estimation. Br J Radiol 1999;72:363–70. 16. Doublet PM, Benson CD, Nadel AS. Improved birth weight table for neonates developed from gestations dated by early ultrasonography. J Ultrasound Med 1997;16:241.
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17. Hadlock FP, Shah YP, Kanon DJ, Lindsey JV. Fetal crownrump length: reevaluation of relation to menstrual age (5–18 weeks) with high-resolution real-time US. Radiology 1992;182:501–5. 18. Chitty LS, Altman DG, Henderson A, Campbell S. Charts of fetal size: 3. Abdominal measurements. Br J Obstet Gynaecol 1994;101:125–31. 19. Hadlock FP, Deter RL, et al. Estimating fetal age: computerassisted analysis of multiple fetal growth parameters. Radiology 1984;152:497–501.
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