Using Monte Carlo Method for Evaluation of kvp & mas variation effect on Absorbed Dose in Mammography

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1 Using Monte Carlo Method for Evaluation of kvp & mas variation effect on Absorbed Dose in Mammography Poster No.: C-2078 Congress: ECR 2011 Type: Authors: Keywords: DOI: Scientific Exhibit F. Salmani Rezaei, S. A. H. Feghhi, S. M. R. Aghamiri, A. Salmani Rezaei, A. Ebrahimi; Tehran/IR Breast, Radioprotection, Mammography, Dosimetry /ecr2011/C-2078 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 13

2 Purpose One of the best ways of stopping the breast cancer is diagnosis in primary stages of its creation. According to global statistics, one of every 8 women will suffer the breast cancer, and one of every 35 patients who are suffering this disease will die [1]. Mammography with X-Ray is one of the most important inventions for diagnosis and control of breast cancer. But ionizing radiation risk for patients who receive these rays is always the most important risk in mammography. Therefore according to this effect calculating or measuring the absorbed dose should be mentioned. Generally there are two dosimetry methods in mammography: Dosimetry by using phantom and in vivo dosimetry. Breast tissue is included of fat tissue and glandular tissue with different percentages. Mean Glandular Dose (MGD) is the criterion of breast absorbed dose which is suggested by ICRP. MGD is calculated by the following equation [2]. MGD=D X ESE ( 2) (2) In the above equation MGD is in mgy, X ESE (Entrance Skin Exposure) in Rontgen and D is conversion factor in mgy/r. An ionization chamber which is filled of air measures XESE based on kvp, mas and beam quality. D is defined tentatively or with computerized simulation method and depends on beam quality (kvp and HVL), target, filter, breast thickness and composition. It is given in references No. 3 and 4 as some tables. Methods and Materials In this research, MCNP4C code has been used for particles transportation and dosimetry calculation. The evaluation of patient's dose during the imaging is one of this code's possibilities. The first step of dose calculation is the X-Ray spectra simulation. 1- X-Ray Spectra As the voltage is applied between the hot cathode and anode in the X-Ray tube, the electrons accelerate from cathode to anode's focal field. When the electrons strike the target, X-Ray photons with zero energy to maximum applied energy will be produced. Therefore X-Ray spectra includes characteristic and bremsstrahlung radiation. When the electron strike the target, MCNP code pursues the electrons in the target until they stop because of finishing their kinetic energy. In order to achieve the error less than 2%, electrons with 10 degrees incident angle have been pursued. 2- Breast Dose Calculation Page 2 of 13

3 Since the total breast dose with 100% glandular tissue is equal to mean glandular dose, the breast density is considered as 1.4 g/cm3. The breast is modeled as a semi cylinder with 16 cm diameter and its height is equal to compressed breast thickness. Each code input includes 3 parts: cell card, surface card and data card. In cell card, all the mammography device components such as Mo, Be and Al filters (which the spectra has to pass them) and also compressor and compressed breast are defined. In surface card, all surfaces of mentioned cells are defined. In data card the source is defined as a spherical with angular distribution and energy distribution according to spectra of each kvp. In this card also the cells materials and number of histories are defined. By transporting photons, the error reached to less than 2%. Tally is used to calculate the breast dose. In the European Protocol it was recommended that TLDs are positioned at the upper inner quadrant of the woman's breast where the risk of obscuring clinical important details is less [6, 7, 8]. According to Reference No. 7 and for further accuracy TLDs have been arranged in breast cell in 2 or 4 cm distance from the chest wall as is illustrated in figure No Simulated Mammography Device In order to simulate the mammography device, the specification of mammography device of Taleghani hospital has been used. This machine which is Finland's product is SOPHIE model and offers digital mammography. The X- Ray tube includes a Molybdenum target with a 0.03-mm thick Molybdenum or a mm thick Rhodium or a 0.5- mm thick Al filter in which the automatic mode is set on Mo/Mo. Figure 2 shows the device. Images for this section: Page 3 of 13

4 Fig. 1: TLD arrangement (top view & section) Page 4 of 13

5 Page 5 of 13

6 Fig. 2: The used mammography device Page 6 of 13

7 Results Figure 1 shows the X-Ray spectra of 30 kev energy for Mo/Mo target/filter. This spectra average energy is 16.9 kev which is obtained by statistical analysis. In reference No. 9 this value has been obtained 16.8 kev by IPEM and 17 kev by MCNP code. Since the range of mammography energy is kev, X-Ray spectra of 23, 27 and 30 kev energy for Mo/Mo target/filter is shown in figure 2. The peaks of all three curves are similar to each other; because the peak point depends on the target material. By increasing the energy level, the number of emitted photons will increase; which lead to further area under curve and beam penetrability. Figure 3 shows the values of MGD for a 4-cm thick breast in different kvps when mas is equal to 50. According to figure 4, simulation results show that there is a linear relationship between the Mean Glandular Dose and kvp. In this figure Mean Glandular Dose curve versus kvp for a 4-cm thick breast is approximated with a linear function in the form of y=0.127x Increasing the kvp will increase the beam quality (i.e. more penetrability) and beam quantity (i.e. more output), consequently the MGD increases. In every exposure the beam output rate is determined by mas. According to figure 5, simulation results show that there is a linear relationship between the Mean Glandular Dose and mas. Increasing mas will lead to increase output which causes to further dose. relationship between the Mean Glandular Dose and mas. Increasing mas will lead to increase output which causes to further dose. Images for this section: Page 7 of 13

8 Fig. 1: X-Ray spectra of 30 kev energy for Mo target and 0.5-mm thick Be and mm thick Mo filters. Page 8 of 13

9 Fig. 2: X-Ray spectra of 23, 27 and 30 kev energy for Mo/Mo target/filter Page 9 of 13

10 Fig. 3: MGD values for a 4-cm thick breast in different kvp Page 10 of 13

11 Fig. 4: The relation between MGD and kvp for a 4-cm thick breast Fig. 5: The relation between MGD and mas for a 4-cm thick breast Page 11 of 13

12 Conclusion Generally, according to simulation for a 4-cm thick breast the Mean Glandular Dose varies between 1.38 to 1.76 mgy, which is acceptable based on existing standards. In addition, the results show that the dose increases linearly with kvp; thus the Mean Glandular Dose can be predicted for the allowable range of kvps which are not examined during mammography. Also the absorbed dose increases if mas increases because of the increase in output rate. References 1] From website: many_people_get_breast_cancer_5.asp [2] S. Their, "MAMMOGRAPHY", the Professional Medical Journal, INV-33, pp From website: [3] X. Wu, E. L. Gingold, G. T. Barnes, D. M. Tucker, "Normalized Average Glandular Dose In Molybdenum Target-Rhodium Filter And Rhodium Target-Rhodium Filter Mammography", Journal of Radiology, pp , [4] C. M. Plott "Mammography Dosimetry:Average Glandular Dose As A Function Of X-ray Equipment, Imaging Technique, Breast Composition, And Breast Thickness", Department Of Nuclear Engineering, Texas A&M University, [5] J. M. Boone, "Glandular Breast Dose For Monoenegetic And High-Energy X-ray Beams: Monte Carlo Assessment", Journal of Radiology, Vol. 213 No. 1, pp , [6] H. M. Warren-Forward & L. Duggan, "Towards in vivo TLD dosimetry in mammography", The British Journal of Radiology, 77, , [7] J. Zoetelie, M. Fitzgerald, W. Leitz, M. Sabel, "European protocol on dosimetry in mammography", Luxembourg: European Commission, [8] D. R. Dance, C. L. Skiner, G. Carlsson"Breast dosimetry", Appl Radiat Isot, Vol. 50, pp , [9] M. R. Ay, M. Shahriari, S. Sarkar, M. Adib, H. Zaidi, "Monte Carlo simulation of x-ray spectra in diagnostic radiology and mammography using MCNP4C", Journal of Phys. Med. Biol., Vol. 49, pp , Page 12 of 13

13 Personal Information F. Salmani Rezaei 1, S. A. Feghhi 1, S. M. Aghamiri 1, A. Salmani Rezaei 2, A. Ebrahimi 2 1-Department of Nuclear Engineering, Shahid Beheshti University, Tehran, Iran. 2-Department of Electrical Engineering, University of Tehran, Tehran, Iran. Page 13 of 13