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Dosimetry around metallic ports in tissue expanders in patients receiving postmastectomy radiation therapy: an ex vivo evaluation
Medical Dosimetry, Vol. 29, No. 1, pp. 49⫺54, 2004 Copyright © 2004 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/04/$–see front matter
doi:10.1016/j.meddos.2003.10.005
DOSIMETRY AROUND METALLIC PORTS IN TISSUE EXPANDERS IN PATIENTS RECEIVING POSTMASTECTOMY RADIATION THERAPY: AN EX VIVO EVALUATION JANAKI MONI, MARIA GRAVES-DITMAN, PAUL CEDERNA, KENT GRIFFITH, EDITHA A. KRUEGER, BENEDICK A. FRAASS, and LORI J. PIERCE Departments of Radiation Oncology, Radiation Oncology, Surgery, and Biostatistics, University of Michigan Medical School, Ann Arbor, MI (Received 30 March 2003; accepted 4 October 2003)
Abstract—Postmastectomy breast reconstruction can be accomplished utilizing tissue expanders and implants. However, in patients who require postoperative radiotherapy, the complication rate with tissue expander/implant reconstruction can exceed 50%. One potential cause of this high complication rate may be the metallic port in the tissue expander producing altered dosimetry in the region of the metallic device. The purpose of this study was to quantify the radiation dose distribution in the vicinity of the metallic port and determine its potential contribution to this extremely high complication rate. The absolute dosimetric effect of the tissue expander’s metallic port was quantified using film and thermoluminescent dosimetry (TLD) studies with a single beam incident on a metallic port extracted from an expander. TLD measurements were performed at 11 reproducible positions on an intact expander irradiated with tangential fields. A computed tomography (CT)-based treatment plan without inhomogeneity corrections was used to derive expected doses for all TLD positions. Multiple irradiation experiments were performed for all TLD data. Confidence intervals for the dose at TLD sites with the metallic port in place were compared to the expected dose at the site without the metallic port. Film studies did not reveal a significant component of scatter around the metallic port. TLD studies of the extracted metallic port revealed highest doses within the casing of the metallic port and no consistent increased dose at the surface of the expander. No excess dose due to the metallic port in the expander was noted with the phantom TLD data. Based upon these results, it does not appear that the metallic port in tissue expanders significantly contributes to the high complication rate experienced in patients undergoing tissue expander breast reconstruction and receiving radiation therapy. Strategies designed to reduce the breast reconstruction complication rate in this clinical setting will need to focus on factors other than adjusting the dosimetry around the tissue expander metallic port. © 2004 American Association of Medical Dosimetrists. Key Words: Metallic port, Tissue expanders, Postmastectomy radiotherapy.
Society of Plastic Surgery Information Service, 2001). The primary options for reconstruction include autogenous tissue techniques and tissue expansion followed by breast implant placement. Many patients either are not candidates for autogenous tissue reconstruction or prefer an expander/implant reconstruction. The expander/implant breast reconstruction typically requires 2 surgical procedures. The first stage of this 2 stage reconstruction involves creating a subcutaneous or submuscular (subpectoral) pocket at the site of the mastectomy, into which a tissue expander is placed. Initially, the expander resembles a deflated balloon. The tissue expander is then serially inflated with weekly postoperative percutaneous injections of sterile saline via a metallic port incorporated into the device. The gradually enlarging expander induces both stretch and growth of the overlying skin. Ultimately, there is sufficient tissue to perform the second-stage operation where the tissue expander is surgically removed and a permanent implant is placed into the pocket. The second stage procedure may be delayed if the patient requires postoperative radiotherapy. Typically, the tissue expansion is completed, and then the
INTRODUCTION Despite the increasing use of breast-conserving treatment for patients with breast cancer, some women still require mastectomy as their primary surgical management. Recently, 3 large trials have demonstrated a survival benefit associated with postmastectomy radiotherapy (PMRT).1–3 Controversies regarding the indications for such radiotherapy have been evaluated by the American Society of Therapeutic Radiology and Oncology, American College of Radiology, American Society of Clinical Oncology,4 and the National Institutes of Health.5 Each organization has generated consensus statements confirming the importance of PMRT in groups defined as having a high risk for locoregional recurrence following mastectomy. This has lead to increased referrals of patients for postmastectomy radiotherapy. Over 81,000 postmastectomy breast reconstructions are performed annually in the United States (American Reprint requests to: Janaki Moni, M.D., Department of Radiation Oncology, University of Michigan Medical School, 1500 E. Medical Center Drive, Ann Arbor, MI 48109. E-mail: [email protected] 49
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patient begins a 5 to 6-week course of radiation therapy. The volume of the expander is rarely adjusted during this time. However, the metallic port of the tissue expander is frequently, if not always, within the field of radiation. Once the radiation therapy has been completed, the size of the breast may be slightly adjusted through tissue expansion. The second-stage operative procedure is then performed at least 1-month postradiotherapy. Krueger et al.6 have recently shown that the use of radiation therapy in patients receiving the 2-staged expander/implant procedure significantly increases the risk of complications. These data have raised the possibility that an alteration of the radiation dose distribution around the tissue expander metallic port may contribute to the increased complication rate. To quantify the potential effect of the metallic port on the RT dose distribution, we performed film dosimetry and thermoluminescent dosimetry (TLD) studies, which we present below. MATERIALS AND METHODS The initial dose distribution studies were conducted using a metallic port extracted from an elastic silicone rubber tissue expander. Further measurements were then obtained using an intact elastic silicone rubber tissue expander (McGhan Medical Corporation, Santa Barbara, CA). All measurements were conducted using photons from a linear accelerator with a nominal accelerating potential of 6 MV (Clinac 2100CD, Varian Oncology Systems, Palo Alto, CA), the energy most commonly used to deliver PMRT. Dosimetric evaluation was conducted using film dosimetry and TLD, and the results were then compared to calculated estimates of the dose in the absence of the metallic filling port. Simple geometry To quantify the absolute effect of the metallic port on the dose distribution, film and TLD studies were conducted with a metallic port extracted from a tissue expander. Verification films (Kodak XV) were placed at varying distances from the extracted metallic port in a tissue-equivalent phantom constructed of sheets of plastic water. Specifically, the first film was placed directly under the metallic port and stacks of plastic water sheets separated subsequent films placed at 2, 7, 17 and 37 mm from the metallic port. The films were then processed to assess the dose gradient across the field in the shadow of the metallic port. Two sets of readings were taken, the first with the single incident beam perpendicular to the metallic port and the second with the beam parallel to the metallic port. TLD studies were performed using the single beam incident on the metallic port extracted from the tissue expander using TLD LiF-100 in packets of 2 chips each. The TLD chips were extracted and read using a manual TLD reader (Harshaw Bicron model 3500). TLDs were
Volume 29, Number 1, 2004
Fig. 1. Diagrammatic representation of metallic port in its housing and location of TLD chips.
irradiated to known doses of 1, 2, and 3 Gy at the same time to provide a calibration curve. The metallic port is contained in a disc-like housing measuring 3.5 cm in diameter and 1.5 cm in thickness. This can be opened to reveal a cup-shaped area that holds the metallic port within it (Fig. 1). TLD chip packets were positioned across the metallic port, as displayed in Fig. 2. TLD 1 was placed at the inner surface of the metallic port. TLD 2 was located at the inner surface of the implant where a cup-shaped area receives the metallic port. TLDs 3 through 5, were at the outer surface of the metallic port. TLDs 6 through 10 were on the superficial surface of the implant material, where it would abut against the skin or subcutaneous tissue in vivo. The metallic port was then immersed in a water phantom and irradiated to a dose of 2 Gy with a 10 ⫻ 10 cm field at 100 cm source-to-surface distance. Experiments were repeated using 1 of 2 perpendicular beams. The first beam was perpendicular to the flat surface and the second beam was in the plane of the metallic port. (Fig. 3). As shown in Fig. 3, TLDs 3 through 5 were at the greatest distance from the anterior beam and TLDs 5 and 9 were farthest from the lateral beam. Conversely, TLD 7 was closest to the anterior beam and TLDs 3 and 10 were closest to the lateral beam. Radiation experiments were performed 4 times from each direction, yielding 8 measurements per TLD location. These measurements were then compared to the dose estimates in water in the absence of the metallic port. Breast geometry Measurements were repeated on the intact expander inflated to full capacity and positioned on a phantom of plastic water sheets and surrounded by a rim of wet toweling as tissue-equivalent material. The average tissue thickness over the expander in vivo was determined by measuring the tissue thickness over the implant in the treatment planning CT scans of 6 patients (8 breast implants) previously treated with a tissue expander in
Metallic ports in tissue expanders ● J. MONI et al.
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Fig. 3. Diagrammatic representation of TLD studies - simple geometry: The perpendicular beam delivered 2 Gy to a 10 ⫻ 10-cm field at 100 cm SSD Gantry ⫽ 0. The parallel beam delivered 2 Gy to a 10 ⫻ 10-cm field at 100 cm SSD Gantry at 90.
expander was placed at the edge of the plastic water such that the setup simulated chest wall irradiation. The irradiation was performed using medial and lateral tangential fields with gantry angles 61° and 233° and field sizes of 14.5 ⫻ 19 cm using asymmetric jaws. Six 2-Gy treatments were delivered for a total of 12 TLD readings per point of measurement. The TLD results were verified using dose calculations performed with a convolution/ superposition algorithm based on the work of Mackie et al.7 using multiple energy components and an electron contamination term.8 The TLD readings were compared to the expected results at the equivalent locations as calculated by the CT-based treatment plan without inhomogeneity corrections. These calculated doses served to simulate doses that would have been received at corresponding locations by normal tissue in the absence of the metallic port.
Fig. 2. (a) Inner surface of metallic port against cup in implant material. Position of TLD 1. (b) Cup in inner surface of implant material. Position of TLD 2. (c) Outer surface of metallic port against fluid in implant. Position of TLDs 3 through 5. (d) Superficial surface of the implant facing the skin. Position of TLDs 6 through 10. These are the only TLDs clinically relevant, as they are closest to human tissue in vivo.
place. The thickness of tissue over the implant ranged from 0.4 cm to 3.9 cm, with an average of 1.2 cm and a median of 0.9 cm. In our setup, the expander was covered with 1 cm of wet toweling to represent the chest wall tissue in an average patient. This was further covered with 0.5 cm of tissue-equivalent bolus material to recreate the bolus used clinically when delivering PMRT. TLDs were placed as shown in Figs. 4 and 5. The
Statistics TLDs contained 2 sensors per unit. For both the simple and breast geometry experiments, the TLDs were
Fig. 4. Photograph of tissue expander irradiation setup to demonstrate TLD placement.
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Fig. 5. Diagrammatic representation of TLD placement on the intact tissue expander.
irradiated, removed, and read with the entire procedure replicated. The mean dose for the sensors per TLD was calculated, and the mean across replicates used for analysis. Ninety-five percent confidence intervals were constructed based upon the standard error across replicates and assuming a 2 sided t-distribution. Confidence intervals for the dose at TLD sites with the metallic port in place can be compared to the expected dose at the site without the metallic port. The dose delivered with the metallic port in place can be considered statistically different with p ⱕ 0.05, if the expected dose at the same site is not within the range of the confidence interval.
Fig. 6. Simple geometry—film dosimetry results. Diamonds: depth dose under center of metallic port; squares: depth dose under outer ring of metallic port; triangles: depth dose outside metallic port; and circles: calculated depth dose without metallic port.
RESULTS Simple geometry Figure 6 summarizes the results of the film studies. All doses are expressed as a percentage of the dose at dmax (the depth at which 100% of the dose is delivered) in the absence of the metallic port. The only increased doses have been noted at very close proximity to the metallic port (at 0 and 2 mm). With the beam perpendicular to the metallic port, the expected (calculated) value is 41% at 0 mm, 56% at 4 mm, 94% at 7 mm, 100% at 17 mm, and 91% at 37 mm. With the metallic port in place, the dose under the metallic port is approximately double the expected value at 0 mm (81%) and slightly less than double at 4 mm (88%). At 7 mm from the metallic port, there is no difference between the calculated dose without the metallic port and the measured dose around the metallic port. At 17 and 37 mm, a decrease in dose is noted directly under the center of the metallic port. At positions under the rim of the metallic port or outside the shadow of the metallic port, some increase in dose is seen at 0 and 2 mm. At 7 mm and beyond, there is no difference between observed and expected dose. With the beam parallel to the metallic port, there is no difference between any of the 4 values at 0 mm. At 2 mm, the dose under the metallic port rim and that outside the shadow of the metallic port is increased (79% and 82% vs. the calculated 56%). The dose under
the center of the metallic port is slightly lower than expected (53%). At 7, 17, and 37 mm, the observed dose under the rim and that outside the shadow of the metallic port is the same as the expected dose. However, there is a persistent decrease in the dose measured directly under the metallic port (65% to 71% vs. the expected 91% to 100%). For the direct anterior beam, the dose measured by TLDs varied from 79% to 111% (Fig. 7a), with the maximal dose being measured by TLDs 1 and 2. As seen in Fig. 1, TLDs 1 and 2 were located between the metallic port and the implant material. In both sets of measurements, the clinically-relevant TLDs 6 through 10 on the surface of the implant material received doses very similar to the expected dose.
Breast geometry The measurements for this geometry are summarized in Fig. 8. There was no statistically significant increase in dose noted due to the presence of the metallic port. At position 2, there was a slight decrease in dose noted, with the TLD readings averaging 102.5% (confidence interval 99.3 to 105.8) of the prescription dose and the calculated dose being 106.5% of prescription dose.
Metallic ports in tissue expanders ● J. MONI et al.
Fig. 7. Simple geometry—TLD measurements around metallic port in water. Diamonds: average of 4 irradiations; Squares: calculated dose in water without metallic port.
DISCUSSION Breast reconstruction following mastectomy can be successfully achieved using either autologous tissue or tissue expanders followed by implant placement. Previ-
Fig. 8. Breast geometry: Comparison of the average TLD readings at each location (shaded bars) with the dose calculated by the planning system at each respective location (solid bars). This plan presumes that the entire setup is water-equivalent material and estimates the dose that would have been delivered in the absence of the metallic port.
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ous studies of autologous tissue (i.e., transverse rectus abdominis myocutaneous flap, TRAM) breast reconstruction following radiotherapy have generally demonstrated good cosmetic results, with complication rates comparable to those observed in unirradiated cohorts. Complication rates associated with tissue expander/implant breast reconstruction in the presence of radiotherapy have significantly exceeded the rates reported in the absence of PMRT. Complications with alloplastic breast reconstruction in patients receiving PMRT include infection, extrusion, capsular contracture, and poor cosmetic outcome. In fact, 10% to 67% of all patients undergoing tissue expander/implant breast reconstruction and PMRT will not successfully complete their reconstruction.9 –12 The specific factors that contribute to this unacceptably high radiotherapy-associated complication rate have not been satisfactorily delineated. Certainly, the use of radiotherapy boluses, effect of total PMRT dose, the effect of fraction size, and the use of systemic therapies continue to be studied for their impact upon outcome. This report presented a detailed analysis of the potential effect of the in-dwelling tissue expander metallic port on radiotherapy dose distribution. Our results do not demonstrate a significant increase in tissue dose caused by the metallic port. The film dosimetry studies indicate that any increased dose is noted only in the immediate vicinity of the metallic port (⬍ 7 mm). This area is expected to represent the substance of the expander rather than normal tissue in the patient. Similar results were noted in the TLD studies using the extracted metallic port (simple geometry). The range of values obtained from TLDs placed at positions 6 through 10 (representing the expander-chest wall interface) coincided with the calculated doses expected at those points in the absence of the metallic port. TLD studies with the intact tissue expander (breast geometry) represent doses that can be expected in vivo at the expander-chest wall interface. The dose calculated by the CT-based treatment plan without inhomogeneity corrections represents the dose that one could expect at each point in the absence of a metallic port. The only TLD position that had readings that differed significantly from the calculated values was position 2. This position was in the direct shadow of the metallic port during treatment with the lateral tangential beam. A persistent decrease in dose under the shadow of the metallic port was observed in the film dosimetry studies. The lower dose noted at TLD 2 could be a result of a lower than expected dose from one of the tangential fields. However, the actual difference in calculated vs. measured dose averages 4%, which is well within clinically acceptable limits. One would not anticipate that this small decrease in dose could be responsible for an increased risk of recurrence. Therefore, it appears from our studies that the presence of a metallic port in the tissue expander at the time of radiation therapy does not significantly contribute to the high complication rate noted in tissue expander/
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implant breast reconstruction patients. It has been well documented that PMRT does contribute to the complication rate noted with alloplastic breast reconstruction.9,11–15 However, it does not appear that the presence of the metallic port in the tissue expander is a major contributor to this high complication rate. It is possible that the high morbidity in these patients may, in part, be due to the interaction between the capsule of the expander and the fibrotic response to radiation therapy in the tissues of the chest wall and the mastectomy flaps. Interstitial fibrosis has been noted in electron microscopic studies of the pectoralis major muscle in patients undergoing tissue expansion without the use of radiation therapy.16 Radiation therapy may very well aggravate this response, as fibrosis is known to be one of its common side effects.17 Further research is necessary to determine if this is indeed the case. In addition, technical factors such as use of bolus, radiation dose, timing of radiotherapy, and use of systemic therapies may also be factors. Studies are underway to assess their impact. CONCLUSIONS Radiation of the incorporated metallic port of a breast reconstruction tissue expander did not result in a consistent increase in dose to the surrounding tissues. Based upon these results, it does not appear likely that the presence of a metallic port contributes to the high complication rate or poor cosmetic result following postmastectomy radiotherapy of a tissue expander. Other technical factors should be considered when assessing the high complication rate in this patient population.
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REFERENCES 1. Overgaard, M.; Hansen, P.S.; Overgaard, J.; et al. Postoperative radiotherapy in high-risk premenopausal women with breast cancer who receive adjuvant chemotherapy. Danish Breast Cancer Cooperative Group 82b Trial. N. Engl. J. Med. 337:949 –55; 1997. 2. Overgaard, M.; Jensen, M.B.; Overgaard, J.; et al. Postoperative radiotherapy in high-risk postmenopausal breast-cancer patients
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given adjuvant tamoxifen: Danish Breast Cancer Cooperative Group DBCG 82c randomised trial. Lancet 353:1641–8; 1999. Ragaz, J.; Jackson, S.M.; Le, N.; et al. Adjuvant radiotherapy and chemotherapy in node-positive premenopausal women with breast cancer. N. Engl. J. Med. 337:956 –62; 1997. Recht, A.; Edge, S.B.; Solin, L.J.; et al. Postmastectomy radiotherapy: Clinical practice guidelines of the American Society of Clinical Oncology. J. Clin. Oncol. 19:1539 –69; 2001. Eifel, P.; Axelson, J.A.; Costa, J.; et al. National Institutes of Health Consensus Development Conference Statement: Adjuvant therapy for breast cancer, November 1–3, 2000. J. Natl. Cancer Inst. 13:979 –89; 2001. Krueger, E.A.; Wilkins, E.G.; Strawderman, M.; et al. Complications and patient satisfaction following expander/implant breast reconstruction with and without radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 49:713–21; 2001. Mackie, T.R.; Scrimger, J.W.; Battista, J.J. A convolution method of calculating dose for 15-MV x rays. Med. Phys. 12:188 –96; 1985. Fraass, B.A.; Bruinvis, I.A.D. Calculation of accurate dose distributions in the buildup region of conformal fields using a simple model for electron contamination. (Abstr.). Radiother. Oncol. 40: S9; 1996. Contant, C.M.; van Geel, A.N.; van der Holt, B.; et al. Morbidity of immediate breast reconstruction (IBR) after mastectomy by a subpectorally placed silicone prosthesis: The adverse effect of radiotherapy. Eur. J. Surg. Oncol. 4:344 –50; 2000. Forman, D.L.; Chiu, J.; Restifo, R.J.; et al. Breast reconstruction in previously irradiated patients using tissue expanders and implants: A potentially unfavorable result. Ann. Plast. Surg. 40:360 –3; 1998. Noone, R.B.; Frazier, T.G.; Noone, G.C.; et al. Recurrence of breast carcinoma following immediate reconstruction: A 13-year review. Plast. Reconstr. Surg. 93:96 –106; 1994. Rosato, R.M.; Dowden, R.V. Radiation therapy as a cause of capsular contracture. Ann. Plast. Surg. 32:342–5; 1994. Spear, S.L.; Onyewu, C. Staged breast reconstruction with salinefilled implants in the irradiated breast: Recent trends and therapeutic implications. Plast. Reconstr. Surg. 105:930 –42; 2000. Vandeweyer, E.; Deraemaecker, R. Radiation therapy after immediate breast reconstruction with implants. Plast. Reconstr. Surg. 106:56 –8; 2000. Victor, S.J.; Brown, D.M.; Horwitz, E.M.; et al. Treatment outcome with radiation therapy after breast augmentation or reconstruction in patients with primary breast carcinoma. Cancer 82: 1303–9; 1998. Gur, E.; Hanna, W.; Andrighetti, L.; Semple, J.L. Light and electron microscopic evaluation of the pectoralis major muscle following tissue expansion for breast reconstruction. Plast. Reconstr. Surg. 102:1046 –51; 1998. Fajardo, L.F.; Berthrong, M. Radiation injury in surgical pathology. Part I. Am. J. Surg. Pathol. 2:159 –99; 1978.
doi:10.1016/j.meddos.2003.10.005
DOSIMETRY AROUND METALLIC PORTS IN TISSUE EXPANDERS IN PATIENTS RECEIVING POSTMASTECTOMY RADIATION THERAPY: AN EX VIVO EVALUATION JANAKI MONI, MARIA GRAVES-DITMAN, PAUL CEDERNA, KENT GRIFFITH, EDITHA A. KRUEGER, BENEDICK A. FRAASS, and LORI J. PIERCE Departments of Radiation Oncology, Radiation Oncology, Surgery, and Biostatistics, University of Michigan Medical School, Ann Arbor, MI (Received 30 March 2003; accepted 4 October 2003)
Abstract—Postmastectomy breast reconstruction can be accomplished utilizing tissue expanders and implants. However, in patients who require postoperative radiotherapy, the complication rate with tissue expander/implant reconstruction can exceed 50%. One potential cause of this high complication rate may be the metallic port in the tissue expander producing altered dosimetry in the region of the metallic device. The purpose of this study was to quantify the radiation dose distribution in the vicinity of the metallic port and determine its potential contribution to this extremely high complication rate. The absolute dosimetric effect of the tissue expander’s metallic port was quantified using film and thermoluminescent dosimetry (TLD) studies with a single beam incident on a metallic port extracted from an expander. TLD measurements were performed at 11 reproducible positions on an intact expander irradiated with tangential fields. A computed tomography (CT)-based treatment plan without inhomogeneity corrections was used to derive expected doses for all TLD positions. Multiple irradiation experiments were performed for all TLD data. Confidence intervals for the dose at TLD sites with the metallic port in place were compared to the expected dose at the site without the metallic port. Film studies did not reveal a significant component of scatter around the metallic port. TLD studies of the extracted metallic port revealed highest doses within the casing of the metallic port and no consistent increased dose at the surface of the expander. No excess dose due to the metallic port in the expander was noted with the phantom TLD data. Based upon these results, it does not appear that the metallic port in tissue expanders significantly contributes to the high complication rate experienced in patients undergoing tissue expander breast reconstruction and receiving radiation therapy. Strategies designed to reduce the breast reconstruction complication rate in this clinical setting will need to focus on factors other than adjusting the dosimetry around the tissue expander metallic port. © 2004 American Association of Medical Dosimetrists. Key Words: Metallic port, Tissue expanders, Postmastectomy radiotherapy.
Society of Plastic Surgery Information Service, 2001). The primary options for reconstruction include autogenous tissue techniques and tissue expansion followed by breast implant placement. Many patients either are not candidates for autogenous tissue reconstruction or prefer an expander/implant reconstruction. The expander/implant breast reconstruction typically requires 2 surgical procedures. The first stage of this 2 stage reconstruction involves creating a subcutaneous or submuscular (subpectoral) pocket at the site of the mastectomy, into which a tissue expander is placed. Initially, the expander resembles a deflated balloon. The tissue expander is then serially inflated with weekly postoperative percutaneous injections of sterile saline via a metallic port incorporated into the device. The gradually enlarging expander induces both stretch and growth of the overlying skin. Ultimately, there is sufficient tissue to perform the second-stage operation where the tissue expander is surgically removed and a permanent implant is placed into the pocket. The second stage procedure may be delayed if the patient requires postoperative radiotherapy. Typically, the tissue expansion is completed, and then the
INTRODUCTION Despite the increasing use of breast-conserving treatment for patients with breast cancer, some women still require mastectomy as their primary surgical management. Recently, 3 large trials have demonstrated a survival benefit associated with postmastectomy radiotherapy (PMRT).1–3 Controversies regarding the indications for such radiotherapy have been evaluated by the American Society of Therapeutic Radiology and Oncology, American College of Radiology, American Society of Clinical Oncology,4 and the National Institutes of Health.5 Each organization has generated consensus statements confirming the importance of PMRT in groups defined as having a high risk for locoregional recurrence following mastectomy. This has lead to increased referrals of patients for postmastectomy radiotherapy. Over 81,000 postmastectomy breast reconstructions are performed annually in the United States (American Reprint requests to: Janaki Moni, M.D., Department of Radiation Oncology, University of Michigan Medical School, 1500 E. Medical Center Drive, Ann Arbor, MI 48109. E-mail: [email protected] 49
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patient begins a 5 to 6-week course of radiation therapy. The volume of the expander is rarely adjusted during this time. However, the metallic port of the tissue expander is frequently, if not always, within the field of radiation. Once the radiation therapy has been completed, the size of the breast may be slightly adjusted through tissue expansion. The second-stage operative procedure is then performed at least 1-month postradiotherapy. Krueger et al.6 have recently shown that the use of radiation therapy in patients receiving the 2-staged expander/implant procedure significantly increases the risk of complications. These data have raised the possibility that an alteration of the radiation dose distribution around the tissue expander metallic port may contribute to the increased complication rate. To quantify the potential effect of the metallic port on the RT dose distribution, we performed film dosimetry and thermoluminescent dosimetry (TLD) studies, which we present below. MATERIALS AND METHODS The initial dose distribution studies were conducted using a metallic port extracted from an elastic silicone rubber tissue expander. Further measurements were then obtained using an intact elastic silicone rubber tissue expander (McGhan Medical Corporation, Santa Barbara, CA). All measurements were conducted using photons from a linear accelerator with a nominal accelerating potential of 6 MV (Clinac 2100CD, Varian Oncology Systems, Palo Alto, CA), the energy most commonly used to deliver PMRT. Dosimetric evaluation was conducted using film dosimetry and TLD, and the results were then compared to calculated estimates of the dose in the absence of the metallic filling port. Simple geometry To quantify the absolute effect of the metallic port on the dose distribution, film and TLD studies were conducted with a metallic port extracted from a tissue expander. Verification films (Kodak XV) were placed at varying distances from the extracted metallic port in a tissue-equivalent phantom constructed of sheets of plastic water. Specifically, the first film was placed directly under the metallic port and stacks of plastic water sheets separated subsequent films placed at 2, 7, 17 and 37 mm from the metallic port. The films were then processed to assess the dose gradient across the field in the shadow of the metallic port. Two sets of readings were taken, the first with the single incident beam perpendicular to the metallic port and the second with the beam parallel to the metallic port. TLD studies were performed using the single beam incident on the metallic port extracted from the tissue expander using TLD LiF-100 in packets of 2 chips each. The TLD chips were extracted and read using a manual TLD reader (Harshaw Bicron model 3500). TLDs were
Volume 29, Number 1, 2004
Fig. 1. Diagrammatic representation of metallic port in its housing and location of TLD chips.
irradiated to known doses of 1, 2, and 3 Gy at the same time to provide a calibration curve. The metallic port is contained in a disc-like housing measuring 3.5 cm in diameter and 1.5 cm in thickness. This can be opened to reveal a cup-shaped area that holds the metallic port within it (Fig. 1). TLD chip packets were positioned across the metallic port, as displayed in Fig. 2. TLD 1 was placed at the inner surface of the metallic port. TLD 2 was located at the inner surface of the implant where a cup-shaped area receives the metallic port. TLDs 3 through 5, were at the outer surface of the metallic port. TLDs 6 through 10 were on the superficial surface of the implant material, where it would abut against the skin or subcutaneous tissue in vivo. The metallic port was then immersed in a water phantom and irradiated to a dose of 2 Gy with a 10 ⫻ 10 cm field at 100 cm source-to-surface distance. Experiments were repeated using 1 of 2 perpendicular beams. The first beam was perpendicular to the flat surface and the second beam was in the plane of the metallic port. (Fig. 3). As shown in Fig. 3, TLDs 3 through 5 were at the greatest distance from the anterior beam and TLDs 5 and 9 were farthest from the lateral beam. Conversely, TLD 7 was closest to the anterior beam and TLDs 3 and 10 were closest to the lateral beam. Radiation experiments were performed 4 times from each direction, yielding 8 measurements per TLD location. These measurements were then compared to the dose estimates in water in the absence of the metallic port. Breast geometry Measurements were repeated on the intact expander inflated to full capacity and positioned on a phantom of plastic water sheets and surrounded by a rim of wet toweling as tissue-equivalent material. The average tissue thickness over the expander in vivo was determined by measuring the tissue thickness over the implant in the treatment planning CT scans of 6 patients (8 breast implants) previously treated with a tissue expander in
Metallic ports in tissue expanders ● J. MONI et al.
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Fig. 3. Diagrammatic representation of TLD studies - simple geometry: The perpendicular beam delivered 2 Gy to a 10 ⫻ 10-cm field at 100 cm SSD Gantry ⫽ 0. The parallel beam delivered 2 Gy to a 10 ⫻ 10-cm field at 100 cm SSD Gantry at 90.
expander was placed at the edge of the plastic water such that the setup simulated chest wall irradiation. The irradiation was performed using medial and lateral tangential fields with gantry angles 61° and 233° and field sizes of 14.5 ⫻ 19 cm using asymmetric jaws. Six 2-Gy treatments were delivered for a total of 12 TLD readings per point of measurement. The TLD results were verified using dose calculations performed with a convolution/ superposition algorithm based on the work of Mackie et al.7 using multiple energy components and an electron contamination term.8 The TLD readings were compared to the expected results at the equivalent locations as calculated by the CT-based treatment plan without inhomogeneity corrections. These calculated doses served to simulate doses that would have been received at corresponding locations by normal tissue in the absence of the metallic port.
Fig. 2. (a) Inner surface of metallic port against cup in implant material. Position of TLD 1. (b) Cup in inner surface of implant material. Position of TLD 2. (c) Outer surface of metallic port against fluid in implant. Position of TLDs 3 through 5. (d) Superficial surface of the implant facing the skin. Position of TLDs 6 through 10. These are the only TLDs clinically relevant, as they are closest to human tissue in vivo.
place. The thickness of tissue over the implant ranged from 0.4 cm to 3.9 cm, with an average of 1.2 cm and a median of 0.9 cm. In our setup, the expander was covered with 1 cm of wet toweling to represent the chest wall tissue in an average patient. This was further covered with 0.5 cm of tissue-equivalent bolus material to recreate the bolus used clinically when delivering PMRT. TLDs were placed as shown in Figs. 4 and 5. The
Statistics TLDs contained 2 sensors per unit. For both the simple and breast geometry experiments, the TLDs were
Fig. 4. Photograph of tissue expander irradiation setup to demonstrate TLD placement.
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Volume 29, Number 1, 2004
Fig. 5. Diagrammatic representation of TLD placement on the intact tissue expander.
irradiated, removed, and read with the entire procedure replicated. The mean dose for the sensors per TLD was calculated, and the mean across replicates used for analysis. Ninety-five percent confidence intervals were constructed based upon the standard error across replicates and assuming a 2 sided t-distribution. Confidence intervals for the dose at TLD sites with the metallic port in place can be compared to the expected dose at the site without the metallic port. The dose delivered with the metallic port in place can be considered statistically different with p ⱕ 0.05, if the expected dose at the same site is not within the range of the confidence interval.
Fig. 6. Simple geometry—film dosimetry results. Diamonds: depth dose under center of metallic port; squares: depth dose under outer ring of metallic port; triangles: depth dose outside metallic port; and circles: calculated depth dose without metallic port.
RESULTS Simple geometry Figure 6 summarizes the results of the film studies. All doses are expressed as a percentage of the dose at dmax (the depth at which 100% of the dose is delivered) in the absence of the metallic port. The only increased doses have been noted at very close proximity to the metallic port (at 0 and 2 mm). With the beam perpendicular to the metallic port, the expected (calculated) value is 41% at 0 mm, 56% at 4 mm, 94% at 7 mm, 100% at 17 mm, and 91% at 37 mm. With the metallic port in place, the dose under the metallic port is approximately double the expected value at 0 mm (81%) and slightly less than double at 4 mm (88%). At 7 mm from the metallic port, there is no difference between the calculated dose without the metallic port and the measured dose around the metallic port. At 17 and 37 mm, a decrease in dose is noted directly under the center of the metallic port. At positions under the rim of the metallic port or outside the shadow of the metallic port, some increase in dose is seen at 0 and 2 mm. At 7 mm and beyond, there is no difference between observed and expected dose. With the beam parallel to the metallic port, there is no difference between any of the 4 values at 0 mm. At 2 mm, the dose under the metallic port rim and that outside the shadow of the metallic port is increased (79% and 82% vs. the calculated 56%). The dose under
the center of the metallic port is slightly lower than expected (53%). At 7, 17, and 37 mm, the observed dose under the rim and that outside the shadow of the metallic port is the same as the expected dose. However, there is a persistent decrease in the dose measured directly under the metallic port (65% to 71% vs. the expected 91% to 100%). For the direct anterior beam, the dose measured by TLDs varied from 79% to 111% (Fig. 7a), with the maximal dose being measured by TLDs 1 and 2. As seen in Fig. 1, TLDs 1 and 2 were located between the metallic port and the implant material. In both sets of measurements, the clinically-relevant TLDs 6 through 10 on the surface of the implant material received doses very similar to the expected dose.
Breast geometry The measurements for this geometry are summarized in Fig. 8. There was no statistically significant increase in dose noted due to the presence of the metallic port. At position 2, there was a slight decrease in dose noted, with the TLD readings averaging 102.5% (confidence interval 99.3 to 105.8) of the prescription dose and the calculated dose being 106.5% of prescription dose.
Metallic ports in tissue expanders ● J. MONI et al.
Fig. 7. Simple geometry—TLD measurements around metallic port in water. Diamonds: average of 4 irradiations; Squares: calculated dose in water without metallic port.
DISCUSSION Breast reconstruction following mastectomy can be successfully achieved using either autologous tissue or tissue expanders followed by implant placement. Previ-
Fig. 8. Breast geometry: Comparison of the average TLD readings at each location (shaded bars) with the dose calculated by the planning system at each respective location (solid bars). This plan presumes that the entire setup is water-equivalent material and estimates the dose that would have been delivered in the absence of the metallic port.
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ous studies of autologous tissue (i.e., transverse rectus abdominis myocutaneous flap, TRAM) breast reconstruction following radiotherapy have generally demonstrated good cosmetic results, with complication rates comparable to those observed in unirradiated cohorts. Complication rates associated with tissue expander/implant breast reconstruction in the presence of radiotherapy have significantly exceeded the rates reported in the absence of PMRT. Complications with alloplastic breast reconstruction in patients receiving PMRT include infection, extrusion, capsular contracture, and poor cosmetic outcome. In fact, 10% to 67% of all patients undergoing tissue expander/implant breast reconstruction and PMRT will not successfully complete their reconstruction.9 –12 The specific factors that contribute to this unacceptably high radiotherapy-associated complication rate have not been satisfactorily delineated. Certainly, the use of radiotherapy boluses, effect of total PMRT dose, the effect of fraction size, and the use of systemic therapies continue to be studied for their impact upon outcome. This report presented a detailed analysis of the potential effect of the in-dwelling tissue expander metallic port on radiotherapy dose distribution. Our results do not demonstrate a significant increase in tissue dose caused by the metallic port. The film dosimetry studies indicate that any increased dose is noted only in the immediate vicinity of the metallic port (⬍ 7 mm). This area is expected to represent the substance of the expander rather than normal tissue in the patient. Similar results were noted in the TLD studies using the extracted metallic port (simple geometry). The range of values obtained from TLDs placed at positions 6 through 10 (representing the expander-chest wall interface) coincided with the calculated doses expected at those points in the absence of the metallic port. TLD studies with the intact tissue expander (breast geometry) represent doses that can be expected in vivo at the expander-chest wall interface. The dose calculated by the CT-based treatment plan without inhomogeneity corrections represents the dose that one could expect at each point in the absence of a metallic port. The only TLD position that had readings that differed significantly from the calculated values was position 2. This position was in the direct shadow of the metallic port during treatment with the lateral tangential beam. A persistent decrease in dose under the shadow of the metallic port was observed in the film dosimetry studies. The lower dose noted at TLD 2 could be a result of a lower than expected dose from one of the tangential fields. However, the actual difference in calculated vs. measured dose averages 4%, which is well within clinically acceptable limits. One would not anticipate that this small decrease in dose could be responsible for an increased risk of recurrence. Therefore, it appears from our studies that the presence of a metallic port in the tissue expander at the time of radiation therapy does not significantly contribute to the high complication rate noted in tissue expander/
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Medical Dosimetry
implant breast reconstruction patients. It has been well documented that PMRT does contribute to the complication rate noted with alloplastic breast reconstruction.9,11–15 However, it does not appear that the presence of the metallic port in the tissue expander is a major contributor to this high complication rate. It is possible that the high morbidity in these patients may, in part, be due to the interaction between the capsule of the expander and the fibrotic response to radiation therapy in the tissues of the chest wall and the mastectomy flaps. Interstitial fibrosis has been noted in electron microscopic studies of the pectoralis major muscle in patients undergoing tissue expansion without the use of radiation therapy.16 Radiation therapy may very well aggravate this response, as fibrosis is known to be one of its common side effects.17 Further research is necessary to determine if this is indeed the case. In addition, technical factors such as use of bolus, radiation dose, timing of radiotherapy, and use of systemic therapies may also be factors. Studies are underway to assess their impact. CONCLUSIONS Radiation of the incorporated metallic port of a breast reconstruction tissue expander did not result in a consistent increase in dose to the surrounding tissues. Based upon these results, it does not appear likely that the presence of a metallic port contributes to the high complication rate or poor cosmetic result following postmastectomy radiotherapy of a tissue expander. Other technical factors should be considered when assessing the high complication rate in this patient population.
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