Tissue Expander Radiation: Metallic Port Dosimetric Effects

Int. J. Radiation Oncology Biol. Phys., Vol. 66, No. 1, pp. 305–310, 2006
Copyright © 2006 Elsevier Inc.
Printed in the USA. All rights reserved
0360-3016/06/$–see front matter
doi:10.1016/j.ijrobp.2006.05.017
PHYSICS CONTRIBUTION
DO METALLIC PORTS IN TISSUE EXPANDERS AFFECT
POSTMASTECTOMY RADIATION DELIVERY?
SHARI DAMAST, M.D.,* KATHRYN BEAL, M.D.,* ÅSE BALLANGRUD, PH.D.,†
THOMAS J. LOSASSO, PH.D.,† PETER G. CORDEIRO, M.D.,‡ JOSEPH J. DISA, M.D.,‡
LINDA HONG, PH.D.,§ AND BERYL L. MCCORMICK, M.D.*
Departments of *Radiation Oncology, †Medical Physics, and ‡Surgery, Memorial Sloan-Kettering Cancer Center, New York, NY;
§
Department of Radiation Oncology, Montefiore Medical Center, Bronx, NY
Purpose: Postmastectomy radiation therapy (PMRT) is often delivered to patients with permanent breast
implants. On occasion, patients are irradiated with a tissue expander (TE) in place before their permanent
implant exchange. Because of concern of potential under-dosing in these patients, we examined the dosimetric
effects of the Magna-Site (Santa Barbara, CA) metallic port that is present in certain TEs.
Methods and Materials: We performed ex vivo film dosimetry with single 6-MV and 15-MV photon beams on a
water phantom containing a Magna-Site disc in two orientations. Additionally, using in vivo films, we measured
the exit dose from 1 patient’s TE-reconstructed breast during chest wall treatment with 15-MV tangent beams.
Finally, we placed thermoluminescent dosimeters (TLDs) on 6 patients with TEs who received PMRT delivered
with 15-MV tangent beams.
Results: Phantom film dosimetry revealed decreased transmission in the region of the Magna-Site, particularly
with the magnet in the parallel orientation (at 22 mm: 78% transmission with 6 MV, 84% transmission with 15
MV). The transmission measured by in vivo films during single beam treatment concurred with ex vivo results.
TLD data showed acceptable variation in median dose to the skin (86 –101% prescription dose).
Conclusion: Because of potential dosimetric effects of the Magna-Site, it is preferable to treat PMRT patients
with permanent implants. However, it is not unreasonable to treat with a TE because the volume of tissue affected
by attenuation from the Magna-Site is small. In this scenario, we recommend using 15 MV photons with
compensating bolus. © 2006 Elsevier Inc.
Breast cancer, Postmastectomy radiation therapy, Tissue expander, Film dosimetry, Thermoluminiscent dosimeters.
INTRODUCTION
A two-staged breast reconstruction is a common procedure
for breast cancer patients who are treated with mastectomy.
The first stage of the reconstruction often takes place at the
time of the initial mastectomy surgery when a tissue expander, a deflated balloon with an integrated injection port,
is placed in a subpectoral pocket that is created at the
mastectomy site. Postoperatively, the tissue expander is
inflated with weekly injections of saline solution through
the injection port. This expands and stretches the overlying
skin to accommodate a desired volume. In the second stage
of the reconstruction, the tissue expander is removed and
replaced with a permanent breast implant.
Several prospective randomized clinical trials have
shown a local control and survival benefit when adjuvant
radiation therapy (RT) is administered in the postmastectomy setting for high-risk breast cancer (1–3). RT is generally initiated within 4 – 8 weeks of the completion of
surgery, however, if adjuvant systemic chemotherapy is
necessary, a delay of up to 6 months to initiate RT does not
appear to compromise locoregional control (4). The optimal
time for irradiating patients who have opted for a twostaged reconstruction with a tissue expander and implant
remains controversial.
Based on complication rates and patient satisfaction, an
algorithm has been developed at Memorial Sloan-Kettering
Cancer Center regarding the optimum time to irradiate
patients who have the two-staged procedure for breast reconstruction (5): (1) reconstruction with tissue expander
placement at the time of mastectomy, (2) tissue expansion
during postoperative chemotherapy, (3) exchange of the
tissue expander for the permanent implant approximately 4
weeks after completion of chemotherapy, and (4) chest wall
irradiation beginning 4 weeks after the exchange. However,
because of scheduling difficulties or physician or patient
preference, there are a number of patients who may begin
RT with the temporary tissue expander still in place. Fur-
Reprint requests to: Kathryn Beal, M.D., Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, 1275
York Avenue, New York, NY 10021. Tel: (212) 639-5159; E-mail:
[email protected]
Received March 16, 2006, and in revised form April 28, 2006.
Accepted for publication May 28, 2006.
305
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thermore, with neoadjuvant chemotherapy followed by surgery and RT emerging as a common approach for treating
Stage III breast cancer (6 –7), there may be a growing
number of women who have radiation during the first stage
of the reconstruction, with the tissue expander in place.
Dosimetric concerns with delivering RT to an augmented
or reconstructed breast have been examined in several studies. More than a decade ago, Kuske et al. showed using a
phantom model of silicone and mammary breast implants
that there were no hot or cold spots because of the presence
of the prosthesis during radiation, and demonstrated that the
prosthesis itself did not alter photon beam distribution (8). A
more recent study by Shankar et al. reported on the dosimetric effect of temporary tissue expanders on chest wall
irradiation and showed no significant changes to the prescribed dose distribution from the temporary tissue expander (9). Only a few publications have documented the
specific effect of the metallic port that is present in certain
tissue expanders on the dosimetry of the radiation that is
prescribed. An ex vivo study by Moni et al. measured the
dosimetric changes around the metallic port in the McGhan
elastic silicone rubber tissue expander using both film dosimetry and thermoluminescent dosimeters (TLDs) (10).
The authors had hypothesized that there would be hot spots,
but they instead found a persistent decrease in measured
dose in the direct shadow of the metallic port from a single
beam. An ex vivo study by Thompson et al. used diodes and
a radiotherapy treatment planning system to calculate the
effect of the McGhan Style 133 Tissue Expander, a model
with a high-density magnet, on the dosimetry of radiation
delivered to a breast phantom. This study showed regions of
underdose of up to 30% in the area of phantom tissue
surrounding the magnet when radiated with a single 6-MV
beam (11).
The McGhan Style 133 Tissue Expander (Inamed Aesthetics, Santa Barbara, CA) is the same model of tissue
expander that is used at our institution. This tissue expander
is made of silicone and its textured surface contains an
integrated injection site, the Magna-Site (Santa Barbara,
CA), which is composed of a rare-earth magnet that is
20 mm in diameter and 2.7 mm thick encased in 0.4 mm
thick titanium with a diameter of 35 mm and width 6.6 mm.
This magnet allows the Magna-Site, an external locator
device, to accurately locate the injection site (12). As shown
in the Thompson et al. study, there is concern that because
of attenuation from the Magna-Site, there may be a decrease
in dose delivered to the tissue that lies in its shadow, and
this may translate clinically into a cold spot that would
underdose a portion of the targeted tissue.
The purpose of our investigation was to evaluate the
potential effects of the Magna-Site in the McGhan Style 133
Tissue Expander on radiation dose distribution in a clinical
setting. To do this, we performed ex vivo film dosimetry at
two photon energies, 6 and 15 MV, as well as in vivo film
dosimetry and in vivo TLD experiments in patients with
temporary tissue expanders in place.
Volume 66, Number 1, 2006
Fig. 1. Photograph of McGhan Style 133 Tissue Expander with
Magna-Site removed.
METHODS AND MATERIALS
Phantom (ex vivo) film dosimetry
The Magna-Site metallic port was removed from a McGhan
Style 133 tissue expander (Fig. 1, photograph). Film dosimetry
with single 6-MV and 15-MV photon beams (Varian Clinac
2100C; Varian Medical Systems Inc., Palo Alto, CA) was performed in a phantom setup shown diagrammatically in Fig. 2.
Kodak X-Omat V (XV) films (Eastman Kodak, Rochester, NY) in
Ready Pack form were placed 2.2 and 5.2 cm behind the magnet
by using solid water blocks. The solid water blocks with films were
immersed in water to a depth of 5 cm above the solid water. The
magnet was standing (parallel position relative to the beam) or
lying (perpendicular to the beam) on the solid water blocks (shown
in Fig. 2). Each measurement was performed with field size 15 ⫻
15 cm2 and irradiated to 50 MU. The same setup was used to
expose films when the magnet was not in the field (open field) to
correct for possible nonuniformities in the irradiated field. Calibration films were acquired by exposing films placed at depth 7.2
cm in a solid water phantom to 0 – 60 MU with 10 cm solid water
behind the film to provide full scatter. All films were processed
using an automatic film processor (Kodak RP X-Omat, model M6B)
and digitized with a Vidar VXR-16 Dosimetry Pro Scanner (Vidar
Systems, Herndon, VA) and optical density was converted to dose
based on known dose to water for a 15 ⫻ 15 cm2 field size at a depth
of 7.2 cm. The measured dose was sampled along a line on the film
crossing through the magnet and divided by the dose measured along
a line in the same position in the open field film to provide the relative
transmission through the magnet. The uncertainty associated with
individual film measurements using these methods is 1% (1␴) (13).
Radiation technique for in vivo studies
Radiation therapy to the chest wall was delivered using tangent
fields with 15-MV photons (Varian Clinac 2100C or 2100EX). A
6-MV photon field was delivered to the supraclavicular fossa for
all patients, and 2 patients had an additional 6-MV “boost” field to
the posterior axilla. All patients were treated to a total dose of 5000
cGy in 25 fractions. A daily skin bolus was placed on the chest
wall for the tangential fields of all patients to ensure adequate dose
to the skin. Bolus sizes ranged from 1.0 to 1.5 cm depending on
physician discretion.
Metallic ports in tissue expanders
● S. DAMAST et al.
307
Fig. 2. Diagram of water and solid water phantom setup showing metallic disc in two orientations.
In vivo film dosimetry
In vivo film dosimetry measurements were performed on 1
patient identified in December 2005 with a McGhan Style 133
tissue expander in place at the time of radiation. Two separate
radiographic films were cut to size from a Kodak X-Omat EDR2
film, Ready Pack form (Eastman Kodak), resealed, and placed on
the breast under the bolus to measure the exit dose from the medial
and lateral beams. Calibration films were exposed at a depth of
5 cm in a solid water phantom to 0 –140 MU with 10 cm solid
water behind the film to provide full scatter. All films were
processed and digitized as described previously. In-house software
was used to convert optical density to dose based on known dose
to water for a 15 ⫻ 15 cm2 field size at a depth of 5 cm.
Patient selection for TLD studies
The majority of patients at our center have their exchange for
permanent implants before initiation of RT; however, between
August 2002 and December 2003, we identified 6 patients with
temporary tissue expanders who were treated with postmastectomy
radiation. All of the patients underwent modified radical mastec-
tomy and patient characteristics are shown in Table 1. The median
patient age was 34.5 years old (range, 26 –50 years). Chemotherapy was administered to all patients; 3 patients received adjuvant
chemotherapy and 3 patients received neoadjuvant chemotherapy.
The expander was placed in a submuscular location on the chest
wall during the mastectomy operation at a median of 2 months
before the initiation of radiotherapy in the neoadjuvant group and
a median of 9 months before the initiation of radiotherapy in the
adjuvant group. In all cases the McGhan Style 133 model of tissue
expander was used.
TLD measurements
The TLD measurements were made on the first day of treatment for all patients except for 1 who had them performed in the
third week of treatment. TLD-100 chips (LiF chips 3.2 mm ⫻
3.2 mm ⫻ 0.9 mm, Harshaw Chemical Co., Solon, OH) were
placed in individual packets and two grids each of nine TLDs
(three-by-three matrix) were used for each patient. The spacing
between chips in the grid was in the range 7–10 mm. For each
patient, one grid of TLDs was placed medially and one laterally to
Table 1. Patient characteristics for thermoluminescent dosimeter studies
Patient
Age
Stage
Timing of
chemotherapy
Number of months between
surgery and radiation therapy
Bolus
(cm)
1
2
3
4
5
6
44
50
30
27
39
26
IIIa
IIb
IIb
IIIb
IIIa
IIIa
Adjuvant
Adjuvant
Adjuvant
Neoadjuvant
Neoadjuvant
Neoadjuvant
9
10
8
2
2
2.5
1.5
1.5
1.5
1.0
1.0
1.0
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Fig. 3. Film dosimetry results at depth 22 mm in solid water comparing the measured normalized dose profiles behind
the Magna-Site disc in 2 orientations, using two different beam energies. The measurements are normalized to 100%
at 22 mm depth without the metallic disc.
the Magna-Site metallic port under bolus (1 or 1.5 cm) and kept in
this location for the treatment of both tangent fields. TLD grid
placement over the Magna-Site was determined by reviewing the
simulation films and by palpation at the time of placement. Patient
TLDs and control TLDs, irradiated with a known dose, were read
out 24 h or more after irradiation in a Harshaw Model 2000
Thermoluminescence Analyzer (Harshaw Chemical Co.; Engelhard Corporation, Iselin, NJ). The TLD readings were converted to dose and corrected using their predetermined relative
sensitivities. Based on the individual relative sensitivities that were
obtained for the TLDs, the uncertainty is less than 2% (1␴) for the
dose range encountered in this study (⬃200 cGy).
RESULTS
Phantom film dosimetry
Figure 3 represents the dosimetry curves in the region of
the magnet that were created from the films placed at depth
2.2 cm in the phantom study. Similar results were seen at
depth 5.2 cm (results not shown). “Depth” refers to the
depth of the film in the solid water behind the magnet.
“Distance” refers to the cross-sectional distance on the film
with the center of the magnet located at 0 cm. The measured
dose is graphed as a percent of the dose in the open field at
the particular energy and depth of the film. The minimum
measured dose seen on films at the two different depths
studied is summarized in Table 2. The beam was attenuated
most significantly when the Magna-Site was oriented parallel to the incident beam. This greater attenuation in the
parallel position, compared with the perpendicular one, can
be explained by the larger thickness of high-density material
encountered by the beam in this orientation. The decrease in
Table 2. Summary of minimum measured dose (normalized) in
presence of MAGNA-SITE
Depth (mm) Perpendicular 6 vs. 15 MV Parallel 6 vs. 15 MV
22
52
94% vs. 97%
96% vs. 96%
78% vs. 84%
79% vs. 84%
dose from the magnet was less with 15-MV vs. 6-MV
beams in both the parallel and perpendicular positions.
Therefore, 15 MV was used for the in vivo studies.
In vivo film dosimetry
Both the medial and lateral in vivo films measuring exit
dose from a patient’s reconstructed breast with a tissue
expander in place showed a significant effect of the MagnaSite on radiation transmission (data not shown). The dose
behind the magnet is reduced to approximately 85% compared with the dose level in an area beside the magnet for
the lateral film (measuring exit dose from medial beam) and
approximately 90% for the medial film (measuring exit dose
from lateral beam). The slightly greater effect on transmission seen in the lateral film compared to the medial film may
be due to the location of the Magna-Site in this particular
patient, which was palpated several centimeters lateral of
midline during the experimental setup. The dose effect in
the in vivo films corresponds well with the ex vivo results for
a single 15 MV photon beam at a depth of 2–5 cm. Depending on the tangent beam angle relative to the MagnaSite, the transmitted radiation is in the range between the
two data sets reported above (perpendicular and parallel).
TLD measurements
The median dose measured by the TLDs for each patient
is outlined in Table 3. Eighteen TLD readings compose each
Table 3. Thermoluminescent dosimeter results
Patient
number
Median dose (cGy)
% prescription dose
1
2
3
4
5
6
178.7 (range, 148.4–197.3)
195.8 (range, 181.7–201.0)
186.8 (range, 178.6–195.8)
197.9 (range, 186.4–207.6)
171.2 (range, 163.2–179.3)
201.2 (range, 195.4–209.1)
89 (range, 74–99%)
98 (range, 91–100%)
93 (range, 89–98%)
99 (range, 93–104%)
86 (range, 82–90%)
101 (range, 98–104%)
Metallic ports in tissue expanders
entry. Overall, the results show acceptable variation in dose
to the skin. Notable, however, are the individual TLD results for Patient 1 in whom the TLDs comprising one row of
the three-by-three TLD matrix placed medially on her reconstructed breast measured values ranging from 74 –76%
of the prescription dose. In this case, it is likely that this row
of TLD chips were directly under the shadow of the MagnaSite. Although it is difficult to estimate how much of this
decrease in dose is due to the metallic port, it is not unreasonable to postulate that the effect seen here is mostly from
the presence of the metallic port. This is supported by the
fact that the adjacent six TLDs on this patient had readings
that were significantly higher (91–99%).
DISCUSSION
At our center, we typically deliver chest wall radiation to
postmastectomy patients with staged reconstruction after
the exchange for the permanent implant (5). However, we
occasionally encounter patients who require chest wall RT
who have not yet undergone exchange for the permanent
implant, and still have a temporary tissue expander in place.
Because of concern supported by recent physics literature
(10, 11) that the presence of such a tissue expander may lead
to underdosing of the target volume, we further investigated
the clinically relevant effects of the tissue expander’s Magna-Site on the prescription dose. To do this, we performed
film dosimetry both ex vivo on a phantom setup as well as
in vivo in 1 patient with a TE. We also performed TLD
experiments in vivo on 6 patients with TEs receiving RT
during a 17-month period. Our goal was to develop a
clinical strategy for managing the scenario of irradiating a
patient with a tissue expander.
Calculating dose in proximity to a high-Z metal, such as
the rare-earth metal of the Magna-Site, has been investigated in several studies (11, 14, 15). One difficulty encountered in using a treatment planning system to calculate dose
effects from the Magna-Site is assigning a computed tomography number for the rare-earth magnet, whose highdensity creates significant image artifact. This complicates
the localization of the magnet and the titanium housing.
Furthermore, calculated results depend on the specific calculation algorithm used. Therefore, whereas a comparison
between our measured results and calculations from a treatment planning system would have been useful, we found
that performing such calculations is quite complex and
beyond the scope of this clinically focused article.
Thompson et al. recently reported 23.4 –29.5% attenuation from the presence of the metallic port oriented parallel
to a single 6-MV beam at distances close to the magnet
(depth 0 –1 cm) in an ex vivo study (11). In a clinical setting,
we estimate a distance of 2–5 cm between the metallic port
and the targeted tissue. The results of our film dosimetry
phantom experiment show that at these depths, the shadow
cast by the Magna-Site present in the tissue expander can
lead to areas of substantial underdose in the phantom when
irradiated with a single incident 6-MV beam (22% attenu-
● S. DAMAST et al.
309
ation in the parallel orientation at 22-mm depth). With
15-MV beams, there is less of a dose decrease distal to the
magnet (16% attenuation in the parallel orientation at
22-mm depth). The in vivo films measuring exit dose, one
side at a time, during 1 patient’s chest wall treatment with
15-MV tangent beams, concur with the phantom film dosimetry using 15 MV.
It is important to recognize that these film experiments
predict changes in dose only during treatment with a single
beam, and do not translate into an equivalent dose decrease
in a true clinical setting, when patients are treated with two
tangent beams. This was the rationale for the TLD portion
of the experimental design, in which the TLDs were left on
the patients’ TE-reconstructed breasts through an entire
treatment with both 15-MV tangent beams.
The in vivo TLD measurements show acceptable variation in RT estimated dose to the skin. The use of in vivo
thermoluminescent dosimeters in breast cancer patients has
been validated previously (16 –20). There are several explanations why the consistent underdosing that was seen in the
film dosimetry was not seen as significantly in the TLD
experiments. One reason is with both beams on, the different beam angles can compensate for any areas of underdose
in the direct shadow of the magnet seen with just one beam.
Another explanation may be due to the physical limitations
of this experiment. TLDs were positioned by localizing the
Magna-Site on the digitally reconstructed radiographs and
by palpation. It is likely that these TLD chips were not
aligned perfectly with the Magna-Site in each patient, and
therefore dose measured by the detectors may represent rays
traveling parallel to the magnet rather than rays passing
directly through the magnet. The magnet in the center of the
Magna-Site is just 2.7 mm thick, and the width of a standard
TLD chip is 3 mm. An error in placement of the TLD chips
by just 1–2 mm may be enough to offset alignment with the
magnet, and therefore mask any effects on underdosing. In
fact, the TLD studies did not show a dramatic dose variation, except in one case, which showed doses as low as 74%
of the prescription dose. This one case probably had TLD
chips aligned directly over the Magna-Site These results
suggest that even with both beams on, there may still be an
effect on dose from the metallic port.
The clinical significance of the underdosing seen in the
shadow of the magnet, at distances 2–5 cm, is unknown. It
can be argued that the volume of tissue that is potentially
underdosed is small given the tiny width of the Magna-Site.
Furthermore, some of the underdosed volume may be occupied by the saline solution that is within the tissue expander. However, because of potential concerns, we have
changed our practice at this institution when irradiating
breast cancer patients who have undergone mastectomy and
have a temporary tissue expander in place. For such patients, we routinely use a 1.0 –1.5 cm bolus and 15 MV for
the chest wall RT to decrease the effect of attenuation from
the Magna-Site as much as possible. Although there is still
beam attenuation in the shadow of the magnet with the
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higher energy radiation, it is markedly less than the attenuation seen with a 6-MV beam.
Despite reports of increased complication rates in patients
radiated with tissue expanders (21–23); to date, there have
been no complications reported in the 6 patients in this
study.
CONCLUSION
Dosimetry effects from the presence of the Magna-Site is
a potential complication of postmastectomy breast radiation
therapy for which there are only a few studies published in
Volume 66, Number 1, 2006
the literature. Our investigation with single beam film dosimetry shows that the Magna-Site may attenuate a standard
6-MV photon beam by as much as 22% and a 15-MV beam
by 16%. In a clinical setting, with tangent beam treatment,
this attenuation effect may still be evident, although to a
lesser degree. Nevertheless, if it is necessary to treat a
patient with a tissue expander in place, we recommend
treating with a 15-MV beam with compensating bolus.
Although we still feel it is preferable to treat with a permanent implant, it is not unreasonable to treat with a tissue
expander in place since the volume of tissue affected by
attenuation from the Magna-Site is likely quite small.
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