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. 305 306 I. J. Radiation Oncology ● Biology ● Physics 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 1.0 308 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 1, 2006 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 310 I. J. Radiation Oncology ● Biology ● Physics 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. 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