Abstract
The female breast is a radiosensitive organ; therefore, assessing the absorbed dose of female breast is a very important issue when under radiological examination. The diagnosis of Nuclear Medicine in oncology was more and more popular in recent years, and the absorbed dose of female breast was gradually taken seriously. To assess internal dose, MIRD (Medical Internal Radiation Dose Committee) has developed a series of dose assessment literatures for internal dosimetry. Nevertheless, the current programs only assess the dose of the breast which is fixed in size and composition. MIRD only provided the mean dose of the sum of the left breast and the right breast, they didn’t assess individual breast dose. Therefore, the purpose of this study is to explore the effects on individual breast dose in Nuclear Medicine by changing the relevant parameters of breasts phantom. This study constructed two sizes of ORNL phantoms (Phantom A: 163cm/58kg, Phantom B: 170cm/70kg) and used two radionuclides: 18F and 99mTc. We use information of the breast volume and composition collected from mammography information to establish mathematical breast phantoms. Breast phantoms with various breast sizes, shapes, compositions, and the glandular distribution were created. SimDose, a Monte Carlo simulation code developed by our laboratory, was used to estimate breast dose in the phantom. The breasts, the heart, the lung, the liver and the thymus are selected as source organs to assess dose to each breast. With volumes of both breasts remain fixed, we used semi-ellipsoid breast and semi-oblate spheroid models to compare the S-values of different breast shapes. In addition, referring to mammographic data of Taiwanese women, we changed the radius of the semi-oblate spheroid model (5-9 cm) to assess the breast dose for different breast sizes. Breasts with five glandular contents were simulated to compare the S-values of breasts and glands. Finally, we evaluated gland doses in breast for different glandular distributions. Results showed the S-value ratios of the breast ranged from 0.94 to 1.01, and the difference caused by breast shapes was insignificant. However, the difference between S-values of breast was significant when different adult phantoms were compared. Therefore, this study considered that distance between organs was important factors in dose estimation. The S-values of left breast self-absorption decreased from 3.573×10-4 (mGy/MBq‧s) to 0.569×10-4 (mGy/MBq‧s) for F-18 and 2.675×10-5 (mGy/MBq‧s) to 0.475×10-5 (mGy/MBq‧s) for Tc-99m with breast radius vary from 5cm to 9cm. The effect of S-value of breast from other source organs caused different effects depending on the locations of source organs. For F-18 simulation with heart as source organ, the deposited doses of the left breast are 1.61 and 1.73 times for phantom A and B, respectively, larger than that of the right breast. For Tc-99m simulation, the doses to the left breast are 1.64 and 1.77 times for phantom A and B, respectively, larger than that of the right breast. For F-18 with liver as source organ, the deposited doses of the right breast are 2.78 and 3.19 times for phantom A and B, respectively, larger than the doses of the left breast. Using Tc-99m simulation, the doses to the right breast are 3.13 and 3.88 times for phantom A and B, respectively, larger than the doses to the left breast. The results indicated that assess organ dose of left breast and right breast separately is necessary. When the glandular content is 10%, the S-value ratio between the gland and breast for phantoms A and B ranged from 0.923 to 1.020 for F-18 and ranged from 0.956 to 1.113 for Tc-99m. Our results indicated that the differences in S-values of the gland self-absorbed dose to be only 5% to 8% for different glandular distributions. We conclude that the breast size has significant impact on the S-value of the breast while the breast shape, compositions, and the glandular distribution cause less change in the S-value. Therefore, this study provided the new method to assess dose of breast in Nuclear Medicine.