個人化核醫劑量評估系統之發展:蒙地卡羅方法結合外部劑量量測

接受核子醫學診斷與治療的患者，每年都有很大量的成長，而且核子醫學檢查對患者所造成的約定有效劑量與局部電腦斷層的有效劑量相近，因此劑量的評估是一個重要的議題。目前最常用來計算放射劑量的工具是由醫學內部放射劑量協會所發展的系統(MIRDOSE)或器官層級內部劑量評估:指數模型 (Organ Level INternal Dose Assessment/EXponential Modeling; OLINDA/EXM) 第一版，利用數學參考人假體及內部放射性核種活度評估個人劑量。該方法利用標準人假體與模型化組織計算劑量，並不適用於不同個體的劑量評估。我們發展並驗證之個人化劑量系統(SimDOSE)能夠由單光子發射電腦斷層/電腦斷層(SPECT/CT)或正子發射電腦斷層/電腦斷層(PET/CT)提供之放射核種濃度影像中評估身體劑量。其中所有器官的時間活度曲線都需要在整個診斷或治療的過程中經由造影而得，然而這種掃描規則基本上很難對於每一個患者常規執行。因此我們又提出一個新的方法用來評估各器官的時間活度曲線，此方法分為二個步驟:第一步驟、利用SimDOSE軟體，由核醫影像中判斷各個源器官，並計算各個源器官到體表之S值。第二步驟、在核醫檢查的過程中，在不同時間點，於患者的體表的明顯標誌處放置熱發光劑量計，並量測其結果。熱發光劑量計的劑量受到所有器官、組織內的放射性核種發出輻射曝露而得，因此可根據熱發光劑量計之數目寫出聯立方程式，其中S值(參數)與熱發光劑量計之值(結果)為已知，各器官之累積活度為未知。解出此聯立方程式，則可得到各器官在量測時間內之累積活度。再利用SimDOSE得到之各器官間之S值，則全身劑量就可以被計算。我們依據醫學內部放射劑量協會第19號報告之器官活度，利用美國橡樹林國家實驗室(ORNL)假體模擬並計算氟-18之氟化葡萄糖注射後15、30、45、60、120、180、240和300分鐘八個時間點的劑量。實驗時，我們將80顆TLD放置於假體表面，並計讀各個時間點的劑量。再利用TLD計讀結果去評估假體各器官所受劑量。結果顯示各器官之累積活度與各器官總吸收劑量率之累積平方誤差之百分比分別落在4%與1.5%，說明這個方法是有效的。這個提出之方法能用來評估個人化核醫劑量，應該也能監控腫瘤用放射性藥物時間活度曲線，及核醫放射核種治療劑量。最後,我們使用假體實驗來驗證此方法之可行性。一個類似NEMA假體，其中插入三個小的圓柱體，分別在圓柱內注入鎝99m之過鎝酸鹽(其活度以劑量校正儀量測確定)。給藥之後，在假體外放上三到四組的熱發光劑量計量測三十分鐘。經計讀儀量測電量，最後再校正成吸收劑量。在經過修正因子的修正後，針對四種給藥條件，百分累積平方誤差分別為5.8%, 6.1%, 3.5%, 4.2%。參考活度與我們提出的方法得到的活度，呈現出很好的相關性，而上述修正因子(約30%)的誤差應該是來自於熱發光劑量計絕對劑量校正時所產生。此假體實驗之前期結果，證明了此方法在實際使用上的可行性，但仍有待改善。總結來說，此篇論文提出了一個新的結合蒙地卡羅的方法與體外劑量量測的方法，用以得到時間活度曲線的資訊，而不需要透過正子發射斷層或單光子發射電腦斷層的連續掃描。

關鍵字

時間活度曲線；核子醫學劑量；熱發光劑量計；醫學體內輻射劑量方法； SimDOSE

並列摘要

There are a growing number of patients undergoing nuclear medicine examination each year. It is estimated that the committee effective doses of patients contributed from nuclear medicine examinations are as high as those contributed from sectional computed tomography (CT) scan. Thus, the estimation dose of nuclear medicine examinations is an important issue. At present, the Medical Internal Radiation Dose Committee system (MIRDOSE) and OLINDA/EXM v1.0 (Organ Level INternal Dose Assessment/EXponential Modeling) are commonly toolkits for calculating radiation dose to selected organs and whole body from internally administered radioisotopes. The MIRDOSE calculates the dose based on referenced human body and modeled tissue organs. They are not suitable for individual dose estimation. In this dissertation, we developed and validated a patient-specific dosimetry system namely SimDOSE that enabled the estimation of the body dose based on the static nuclear image. The time-activity curves (TAC) of all source organs are required before one can estimate the body dose during the period of examination. However, TAC is known to be difficult to obtain for each individual. Therefore, we propose a novel method to estimate the total dose from the dose measured outside the body without resorting to the TAC information. The method involves two steps. First, to compute the S values on the body surface for each source organ based on nuclear medicine image using SimDOSE. Second, to measure the dose externally from the TLD placed on the body surface during nuclear examination. Since the doses in TLD are contributed by the radiations from all source organs, they can be expressed by simultaneous equations with the S values as known variables and the cumulative activities of source organs unknown. Solving the simultaneous equations, the cumulative activities of all source organs can be obtained and subsequently the total body dose can be computed. The ORNL mathematical phantom with TAC adapted from MIRD Report 19 was simulated at eight time point (15, 30, 45, 60, 120, 180, 240 and 300 min). Eighty TLDs were placed on the phantom and the TLD readings were employed to estimate the doses at various organs. The percent sum of square errors in the estimation of cumulative activity and organ dose rate were within 4% and 1.5%, respectively. The results demonstrate the effectiveness of this method. The proposed method can be used to estimate patient-specific dose of nuclear medicine examination in PET/CT and should be used to monitor the TAC of radiopharmaceutical in tumor for the treatment dose in nuclear medicine therapy. At last, a phantom study was performed to prove the feasibility of the proposed method. A NEMA-like phantom with three cylinder inserts was administered with 99mTc pertechnetate (which activity was measured by dose calibrator). After administration, three or four sets of TLDs were placed on the surface of the phantom under 30 min exposure of radioisotope. The TLD readings (nC) were calibrated to be the absorbed dose. With a correction of factor that activity measured by dose calibrator divided by that calculated by our proposed method, the results of PSSE for the four experiments are 5.8%, 6.1%, 3.5%, 4.2%, respectively. The trend between references and our method are quite correlative and the factor (about 30%) should result from the absolute dose measurement of TLD. Preliminary results of the phantom experiments have proved the feasibility of the proposed method and yet there are still rooms for improvement. In conclusion, a novel method has proposed in the dissertation and it can provide TAC information by using Monte Carlo method combined with external dose measurement without sequential scans by PET or SPECT.

並列關鍵字

time activity curve ； nuclear medicine dosimetry ； SimDOSE ； TLD ； MIRD method

參考文獻

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個人化核醫劑量評估系統之發展:蒙地卡羅方法結合外部劑量量測

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