- 學位論文
以蒙地卡羅模擬法評估核醫檢查中乳房劑量：假體研究
Estimation of breast dosimetry in nuclear medicine examinations using Monte Carlo Simulation：a phantom study
摘要
女性乳房是屬於輻射敏感的器官之一，因此近年來評估女性受檢者在接受放射性檢查中乳房所接受的輻射劑量是一個很重要的議題。隨著核子醫學在腫瘤影像診斷的蓬勃發展，核子醫學檢查對女性乳房造成的輻射劑量也日漸受到重視。為了評估受檢者的體內劑量，美國核醫學會的醫學體內劑量委員會(MIRD)發展一系列的劑量評估文獻以提供參考。然而，該方法在乳房的劑量評估中只能計算固定大小與成份的乳房，同時也僅能評估左側和右側乳房總和的平均劑量，而無法針對個別的左右乳房進行個別的劑量評估。因此本研究針對核醫檢查中乳房劑量進行評估並且改變不同乳房參數，探討不同核醫藥物對個別乳房劑量之影響。本研究建立兩種ORNL擬人假體(Phantom A: 163cm/58kg, Phantom B: 170cm/70kg)，模擬放射性核種18F與99mTc，並應用乳房攝影中收集之乳房體積與成份等資訊，建立數學乳房假體，改變乳房形狀、大小、成分及腺體分佈，並且選用本實驗室發展之SimDose模擬系統進行蒙地卡羅模擬。射源器官分別選用乳房、心臟、肺臟、肝臟和胸腺進行乳房劑量之評估。 當乳房體積相同且射源器官體積不變時，分別以半橢球和半扁球之乳房假體進行乳房形狀改變之比較，其結果發現乳房S-value之比值介於0.94到1.01之間，數值相當接近。然而，當不同擬人假體之S-value比較時，乳房之S-value會有明顯的變化。此外依據台灣女性乳房攝影數據為參考，改變半扁球之乳房假體半徑(5-9公分)，評估不同乳房大小之乳房劑量。兩種模擬核種之結果顯示隨著乳房半徑5公分增加到9公分時，對18F而言左側乳房之自我吸收S-value從3.573×10-4 (mGy/MBq‧s) 減少到 0.569×10-4 (mGy/MBq‧s)，而Tc-99m的S-value 由2.675×10-5 (mGy/MBq‧s) 減少到0.475×10-5 (mGy/MBq‧s)。其他射源器官對乳房之S-value影響也因為器官之位置不同而有不同程度之影響。針對左側及右側乳房之劑量評估，當射源器官為心臟時，F-18模擬兩種假體之結果，心臟對於左側乳房的劑量沉積為右側乳房的劑量沉積的1.61倍和1.73倍；Tc-99m模擬兩種假體之結果，心臟對於左側乳房的劑量沉積為右側乳房的劑量沉積的1.64倍和1.77倍。當射源器官為肝臟時，F-18模擬兩種假體之結果，肝臟對於右側乳房的劑量沉積為左側乳房的劑量沉積的2.78倍和3.19倍；Tc-99m模擬兩種假體之結果，肝臟對於右側乳房的劑量沉積為左側乳房的劑量沉積的3.13倍和3.88倍。由上述結果可以發現將兩側乳房劑量分開評估是必要的。此外使用五種不同乳房腺體含量分別模擬乳房之S-value和腺體之S-value並將兩種結果進行比較。其結果發現當乳房為10%腺體含量時，F-18在兩種假體中乳房之S-value和腺體之S-value之比值為0.923到1.020；Tc-99m在兩種假體中乳房之S-value和腺體之S-value之比值為0.956到1.113。當腺體分佈範圍改變時，由結果可知改變腺體分佈之範圍對於腺體之S-value大約有5%到8%之影響。 就結論而言，乳房大小的改變會造成S-value之顯著變化，而形狀、成分及腺體分佈的變化所造成的影響較小，本研究提供了一套評估核醫檢查中乳房劑量的新方法。
關鍵字
乳房劑量並列摘要
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.