STUDY OF DOSIMETRIC EFFECTS DUE TO METALLIC HETEROGENEITY OF TISSUE EXPANDERS IN POST-MASTECTOMY RADIOTHERAPY

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1 2009 International Nuclear Atlantic Conference - INAC 2009 Rio de Janeiro,RJ, Brazil, September27 to October 2, 2009 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: STUDY OF DOSIMETRIC EFFECTS DUE TO METALLIC HETEROGENEITY OF TISSUE EXPANDERS IN POST-MASTECTOMY RADIOTHERAPY Débora M. Trombetta 1,2, Simone C. Cardoso 2, Alessandro Facure 3, Luiz A. R. da Rosa 4 and Ademir X. da Silva 1 1 Programa de Engenharia Nuclear PEN/COPPE Universidade Federal do Rio de Janeiro Centro de Tecnologia, Bloco G Rio de Janeiro, RJ dtrombetta@con.ufrj.gov.br 2 Departamento de Física Nuclear Instituto de Física Universidade Federal do Rio de Janeiro Centro de Tecnologia, Bloco A-3º. andar Rio de Janeiro, RJ simone@if.ufrj.br 3 Comissão Nacional de Energia Nuclear - CNEN/RJ Rua Gal Severiano, 90 - Botafogo Rio de Janeiro - RJ - Brasil 4 Instituto de Radioproteção e Dosimetria -IRD Av. Salvador Allende, s/nº Recreio dos Bandeirantes Rio de Janeiro ABSTRACT Breast cancer is the most common cancer among women. In each year approximately 20% of the new cases of cancer in women are breast cancers. Despite the increase in the use of breast-conserving treatment, some women still require mastectomy as their primary surgical management. A large part of these women, whom undergo a mastectomy, realize a breast reconstruction afterwards. The most common options for reconstruction include autogenous tissue techniques and tissue expansion followed by breast implant placement. Many trials have demonstrated a survival benefit associated with potmastecmomy radiotherapy mainly if the treatment starts right after the mastectomy. In such case patients whom were realizing the breast reconstruction using tissue expanders can be irradiated using this. These patients posses a material with high atomic number within the irradiated area, so this metallic port becomes an heterogeneity which can modify the calculated doses distribution for the treatment. The study was due through the quantification of the relative transmission of 6MV and 15MV radiotherapy beam, making use of computer simulations with Monte Carlo method implemented by the MCNPX code. The results show that the presence of this metallic heterogeneity changes the transmission of the beam, causing a reduction up to 13% in the geometric shadowed region. According to dosimetric protocols, which recommend that the reduction in the dose should be bellow of 5%, the difference found in the study could be significant.

2 1. INTRODUCTION According to a study realized by the National Institute of Cancer (INCA- Brazil), 49,400 new cases of breast cancer in 2008 are expected, with an estimated risk of 51 cases to 100 thousand women [1]. The treatment of breast cancer involves different types of surgeries and therapies that are dependent on the clinical signs and histological disease. The technique of conservative surgery for breast cancer in early stage has been prioritized by doctors, but a large number of patients are still subjected to mastectomy. Most women who are submitted to mastectomy then perform a breast reconstruction surgery. There are many techniques for breast reconstruction and it mainly depends on the amount of tissue removed by the surgery and their location. The most common are those that make use of the own breast tissue, repositioning it to filling the empty space winch is caused by the removal of cancer. In cases of great resections, without sufficient breast tissue to perform the reconstruction, distant flaps or breast implants are used. Adjustable breast tissue expanders with an integral injection site may be used for breast reconstruction. These devices are designed to accept a needle so that saline can be injected to expand the prosthesis both during and after the operation. The tissue expander is intended for temporary subcutaneous implantation to develop surgical flaps and additional tissue coverage. It is constructed from silicone elastomer and consists of an expansion envelope with a textured surface, and a metallic site integrated injections site, containing a rare-earth, permanent magnet for an accurate injection system, detectable through 60mm of phantom tissue. When a magnetic finder external-locating device is passed over the surface of the tissue being expanded, its magnet indicates the location of the metallic injection site. To use breast implants is necessary for patients are used when the quantity of the patient's skin is not enough for the placement of final prosthesis of silicone, in this case a temporary tissue expander is placed in the patient's breast during the surgical procedure of mastectomy, in order to inflate the skin and prepare it to receive the permanent prosthesis. The breast reconstruction with temporary tissue expander makes use of a silicone s bag with a door made of Samarium-Cobalt and Titanium on its surface. The silicone bag is inflated with saline solution periodically through the metal, allowing the distension of skin and pectoral muscle, creating a space that allows subsequent replacement by a permanent silicone implant. The second stage, with the inclusion of the final implant is performed after three to six months [2]. A growing number of studies have shown that post-mastectomy radiotherapy has benefits associated with patient s survival [3-5]. Radiotherapy in this case is used with the purpose of destroying cancer cells that remain after surgery. This type of cancer treatment aims to deliver the greatest possible amount of radiation to the irradiated volume, and cause the least possible damage to healthy tissue. The radiotherapy treatment planning considers the breast composed of soft tissue. However, patients using a tissue expander have a high atomic number material (titanium) in the irradiated area, a heterogeneity that possibly changes the distribution of the treatment s calculated dose. Some works present results of dose measurements in the vicinity of these heterogeneities, using different types of detectors and experimental arrangements. In 2004, Moni et al [2] measured the dose in the vicinity of the metal using films and thermoluminescent dosimeters (TLD's). The experimental results were compared to the calculations of doses without taking into consideration the presence of the

3 heterogeneity and it was observed an increase of the dose values up to 40%. In 2005 Naomi et al [6] did statistical analysis in some hospitals with the objective of verifying the effects produced by the heterogeneity in the field s profile. The survey showed that 22% of patients achieved excellent results in irradiation of the planned area, and the other 78% had the treatment impaired. In 2006, Damast et al [7] reported attenuation of the beam by 22% due to the presence of heterogeneity. Observing the literature related to this work [2-9], it s possible to see that all assessments of doses and attenuation of the beam were experimental, using different detectors and showing discordant results among them. The objective of this study was therefore to quantify the attenuation in dose as a result of the radiation beam passing through the magnet. This was achieved by the Monte Carlo method 2. MATERIALS AND METHODS 2.1. MCNP The evaluation of the effects produced by metallic heterogeneities during breast radiotherapy was performed using the MCNPX code [10]. The use of computer simulations to calculate radiation transport has become a basic tool in radiological protection and dosimetry. The MCNP is based on the Monte Carlo method for radiation transport and the code has the ability to handle complex geometries in three dimensions and a variety of options for data input. The X version of the code MCNP [10], developed at Los Alamos National Laboratory (USA), is currently one of the most widely used computational codes in the simulations of radiation s transport concerning neutrons, photons, electrons and charged particles such as protons, deuterons, alpha particles, etc Modeling For the simulations, the breast was considered as being a prism of rectangular base, with dimensions of 30 cm x 30 cm x 15 cm. This type of regular geometry was used to allow the examination of the effects caused by the presence of the heterogeneity only, without the interference of the geometry. As the breast is entirely composed of soft tissue, it can be simulated as consisting only of water in the first approximation. The metallic artifact (Figure 1) present in the tissue expanders, and the one simulated in this work with the code MCNP, has the form of a disk of samarium-cobalt (SmCo5) encapsulated in titanium (Ti), with 3.5cm of diameter and 0.35cm thick [11]. a)

b) c) Figure 1. (a) An illustration of the tissue expander described. (b and c) The metallic artifact simulated model. SmCo5 is represented in pink and titanium is represented is gray.

To simulate the irradiation of the breast, the photon spectra generated by a linear accelerator of electrons was approximated by a divergent punctual source of 6 or 15MV photons, with spectrum

4 b) c) Figure 1. (a) An illustration of the tissue expander described. (b and c) The metallic artifact simulated model. SmCo5 is represented in pink and titanium is represented is gray. (b) Frontal view ; (c) Side view. To simulate the irradiation of the breast, the photon spectra generated by a linear accelerator of electrons was approximated by a divergent punctual source of 6 or 15MV photons, with spectrum described by Daryoush and Rogers [12]. The source was positioned at 100 cm from the surface of the breast simulator, generating a 10 cm x 10 cm beam on its surface. The potential accelerators of 6 and 15MV were chosen because, its major use in breast radiotherapy and its uses by most of the experimental references in the literature [2-9]. a) b) Figure 2. Irradiation geometry for two positions of the heterogeneity in relation to the beam incidence: (a) Perpendicular and (b) Parallel. To simulate the experiment [9], detector spheres were arranged in two transversals planes on direction of beam incidence, in two distances of the heterogeneity: 2.2 and 5.2 cm. The same conditions of irradiation used in reference, were used here: source-surface distance of 100cm,

5 field of irradiation of 10 cm x 10 cm, photon beams of 6 and 15MV, and heterogeneity located at 0.5 cm depth. The energy deposited was integrated in spheres of water with 0.5 cm radius. Since, in general, the treatment of breast cancer is developed with previous and tangential fields, two extreme positions of the metallic artifact with relation to the incidence direction of the beam were considered: parallel and perpendicular as is shown in Figure 2. Results for the beam attenuation due to the presence of heterogeneity were obtained, and these values were compared with the values of attenuation of the beam in the absence of the heterogeneity. For this assessment, the relative transmission (RT) of the beam was calculated by the following formula: (1) 3. RESULTS Figures 3 and 4 show the results for the relative transmission curves, obtained by simulations, for the two depths considered: 2.2 and 5.2 cm. Each graph presents the heterogeneity position with relation of the incidence direction of the beam: perpendicular and parallel, respectively, for 6 and 15 MV beams. The vertical lines represent the geometric shadow of the disc. The graphs show coincident curves for the two depths, which was already expected, since the two depths are separated only by a 3.0 cm layer of water, which is not enough to change significantly the attenuation pattern for 6 and 15 MV beams. The decrease in the transmission on the central axis of the graph is also visible in the shadow of the heterogeneity. The transmission curve for parallel position of the heterogeneity appears with greater variation in attenuation, which is due to the major beam path within the material of high Z, in this position. a) b) mm 52mm mm 52mm Relative Transmission (%) Relative Transmission (%) Distance (cm) Distance (cm)

6 c) d) mm 52mm mm 52mm Relative Transmission (%) Relative Tranmission (%) Distance (cm) Distance (cm) Figure 3. Transmission curve obtained by simulation with MCNP for the 6 and 15MV spectrum respectively. (a, c) Perpendicular position; (b, d) Parallel position. Table 1 shows the attenuation data calculated by MCNP, in comparison with experimental data from literature [9]. Table 1. Comparison between the results obtained by MCNP for the attenuation of the beam, and experimental data from reference [10] ATTENUATION (6MV) Depth Parallel MCNPX Perpendicular MCNPX Parallel Reference [9] Perpendicular Reference [9] 22 mm 22% 7% 22% 6% 52 mm 22% 7% 21% 4% ATTENUATION (15MV) Depth Parallel MCNPX Perpendicular MCNPX Parallel Reference [9] Perpendicular Reference [9] 22 mm 21% 6% 16% 3% 52 mm 21% 6% 16% 4%

7 . From Table 1 it can be observed that the results of the simulations were consistent with the reference experiment for the 6 MV beam, except for the transmission at perpendicular position, at the depth of 5.2cm. For the 15 MV beam there were major differences. It is worth emphasizing that the experimental reference doesn t mention the uncertainty in the measurements, and the uncertainty given by the simulation is below 1% of the measurement. 3. CONCLUSIONS It can be concluded that the presence of the metallic heterogeneity can change the beam attenuation, and it causing a reduction of up to 13% in the region of geometric shadow of the disc. This difference may be important for radiotherapy, where the limit set by dosimetry protocols is below 5% for differences in absorbed dose measurements and calculated by the treatment planning system [13]. REFERENCES 1. Estimativas 2008: Incidência de Câncer no Brasil. Instituto Nacional de Câncer, Coordenação de Prevenção e Vigilância de Câncer, Secretaria de Atenção à Saúde do Ministério da Saúde, (2007). 2. J. Moni, P. Cederna, K. Griffith, et al., Dosimetry around metallic ports in tissue expanders in patients receiving postmastectomy radiation therapy: an ex vivo evaluation, Medical Dosimetry, 29, pp (2004). 3. M. Overgaard, P.S. Hansen and J. Overgaard et al., Postoperative radiotherapy in highrisk postmenopusal women with breast cancer who receive adjuvant chemotherapy. Danish Breast Cancer Cooperative Group 82b Trial, N. Engl. J. Med. 337, pp (1997). 4. M Overgaard, M.B. Jensen and J, Overgaard et al., Postoperative radiotherapy in highrisk postmenopausal breast-cancer patients given adjuvant tamoxifen, Danish Breast Cancer Cooperative Group DBCG 82c Randomises Trial, Lancet 353, pp (1999). 5. J. Ragaz, I.A. Olivotto and J.J. Spinelli et al., Locoregional radiation therapy in patients in high-risk breast cancer receiving adjuvant chemotherapy 20-year, results of the British Columbia randomized trial, J. Natl. Cancer Inst. 97, pp (2005). 6. Naomi R., Eric A., George H., et al., Immediate Breast Reconstruction can Impact Postmastectomy Irradiation, American Journal of Clinical Oncology, 28(5), pp (2005). 7. Damast S., Beal K., Losasso T.J. et al, Do metallic ports in tissue expanders affect postmastectomy radiation delivery?, International Journal of Radiation Oncology, 66(1), pp (2006). 8. Rosemary C.A. e Andrew M., Investigation into dosimetric effect of a MAGNA-SITE tissue expander on post-mastectomy radiotherapy, Medical Physics, 32(6), pp (2005).

8 9. Beryl L. McCormick, Do metallic ports in tissue expanders affect postmastectomy radiation delivery?, International Journal Radiation. Oncology and Biology, Phys, 66, pp (2006). 10. X-5 Monte Carlo Team MCNP: A general Monte Carlo N-Particle Transport Code. Version 5, Los AlamosNational Laboratory Report LA-CP, pp (2003). 11. Samarium Cobalt Magnet: SmCo5, (2007). 12. Daryoush, S. B. and Rogers, D. W. O. Monte Carlo calculation of nine megavoltage photon beam spectra using the BEAM code, Medical Physics 29, pp (2002). 13. ICRU Report No. 24, Determination of Absorbed Dose in Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy (1976)

DOSIMETRIC STUDY OF I-125 SEEDS USED IN PROSTATE BRACHYTHERAPY

DOSIMETRIC STUDY OF I-125 SEEDS USED IN PROSTATE BRACHYTHERAPY 2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR ABEN ISBN: 978-85-99141-04-5 DOSIMETRIC STUDY OF I-125